Journal of Physiology - Paris 98 (2004) 171–189

www.elsevier.com/locate/jphysparis

Multisensory contributions to the 3-D representationof visuotactile peripersonal space in humans:evidence from the crossmodal congruency task

Charles Spence a,*, Francesco Pavani b, Angelo Maravita b, Nicholas Holmes a

a Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford OX1 3UD, UKb Institute of Cognitive Neuroscience, University College London, Alexandra House, 17 Queen Square, London WC1N 3AR, UK

Abstract

In order to determine precisely the location of a tactile stimulus presented to the hand it is necessary to know not only which part

of the body has been stimulated, but also where that part of the body lies in space. This involves the multisensory integration of

visual, tactile, proprioceptive, and even auditory cues regarding limb position. In recent years, researchers have become increasingly

interested in the question of how these various sensory cues are weighted and integrated in order to enable people to localize tactile

stimuli, as well as to give rise to the ‘felt’ position of our limbs, and ultimately the multisensory representation of 3-D peripersonal

space. We highlight recent research on this topic using the crossmodal congruency task, in which participants make speeded ele-

vation discrimination responses to vibrotactile targets presented to the thumb or index finger, while simultaneously trying to ignore

irrelevant visual distractors presented from either the same (i.e., congruent) or a different (i.e., incongruent) elevation. Crossmodal

congruency effects (calculated as performance on incongruent) congruent trials) are greatest when visual and vibrotactile stimuli are

presented from the same azimuthal location, thus providing an index of common position across different sensory modalities. The

crossmodal congruency task has been used to investigate a number of questions related to the representation of space in both normal

participants and brain-damaged patients. In this review, we detail the major findings from this research, and highlight areas of

convergence with other cognitive neuroscience disciplines.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Multisensory integration; Peripersonal space; Body image; Attention

1. Introduction

For many years, both scientists and philosophers

have been interested in the question of how the brain

derives common representations of external space across

different sensory modalities (such as vision, touch, and

audition), given that sensory information is coded at the

peripheral receptor level in a variety of different frames

of reference [45,93,121,130]. Some of the most impres-

*Corresponding author. Tel.: +44-1865-271364; fax: +44-1865-

310447.

E-mail address: charles.spence@psy.ox.ac.uk (C. Spence).

0928-4257/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jphysparis.2004.03.008

sive advances toward a resolution of this question have

emerged from contemporary neuroscience, particu-larly single-cell neurophysiology [139]. For example,

researchers have demonstrated the existence of multi-

sensory neurons in several areas of the cat and monkey

brain, including the putamen, superior colliculus, ven-

tral premotor cortex, and parietal area 7b, that represent

visual and tactile stimuli in approximate spatial register

[35,36,41,42,94,114,140]. Many of the cells in these areas

that are responsive to tactile stimuli on the hand alsohave visual receptive fields (RFs) in the region around

the hand. More importantly, the visual RFs of these

neurons appear to follow the hand around as the arm is

placed in different postures (see [34] for a review). A

growing body of research in both normal human par-

ticipants [68,133], and in patients with selective brain-

damage [22,73], now suggests the existence of similar

Fig. 1. Schematic view of a participant adopting both the uncrossed (a)

and crossed hands (b) postures. Participants held a foam cube in each

of their left and right hands. Two vibrotactile stimulators (shaded

rectangles) and two visual distractor lights (filled circles) were

embedded in each cube, by the thumb and index finger. Participants

made speeded elevation discrimination responses (by raising the toe or

heel of their right foot), in response to vibrotactile targets presented

172 C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189

multisensory representations of 3-D peripersonal spacein humans as well. 1

One paradigm that we have used extensively in recent

years to investigate the representation of visuotactile

space in humans is the crossmodal congruency task

[132,138]. Participants are typically required to make

speeded elevation discrimination responses to a series of

targets in one sensory modality (most frequently touch),

while simultaneously trying to ignore irrelevant di-stractors presented in another sensory modality (typi-

cally vision). The crossmodal congruency task has been

shown in a number of studies to provide a robust

experimental index of common spatial location across

different sensory modalities. We have used the task in a

wide variety of experimental situations to investigate the

multisensory representation of visuotactile space in both

normal participants [132,138], and in brain-damagedpatients [135], and also to investigate the consequences

of prolonged tool-use on the boundaries of peripersonal

space and the body schema [52,81].

In this review, we first describe the basic crossmodal

congruency effect, before going on to highlight the re-

sults of a number of experiments that have used the

crossmodal congruency task to investigate the conse-

quences of posture change on the representation of vi-suotactile space. In subsequent sections, we illustrate

how the crossmodal congruency task is currently being

used to address increasingly sophisticated questions

regarding the representation of 3-D peripersonal space.

Although this review is primarily designed to highlight

the major findings to have emerged from research on the

crossmodal congruency task itself, we also draw paral-

lels to the findings made by researchers in other disci-plines, such as primate neurophysiology and cognitive

neuropsychology.

either from the ‘top’ by the index finger of either hand, or from the

‘bottom’ by either thumb respectively. Maximal crossmodal congru-

ency effects were always reported for visual distractors placed closest to

the location of the vibrotactile target (i.e., on the same foam cube), no

matter whether the hands were held in an uncrossed or crossed pos-

ture.

2. The crossmodal congruency task

In a typical crossmodal congruency study, partici-

pants are required to hold two foam cubes, one in eitherhand (see Fig. 1a for a schematic illustration). A vi-

brotactile target stimulus and a visual distractor are

presented randomly and independently from one of the

1 Interestingly, while the involvement of bimodal visuotactile

neurons in brain areas such as those reported by Graziano, Gross,

and colleagues have been suggested to explain human behaviour in a

variety of normal and patient studies [22,68,135,136], the involvement

of such areas has not, to our knowledge, been demonstrated directly in

humans previously. Recent neuroimaging data from this laboratory

[77] has provided some of the first empirical evidence that the same

network of neural structures appears to be involved in the multisensory

representation of limb position in humans as reported previously in

primates, namely the IPS and inferior frontal gyrus (corresponding to

the VIP-F4 circuit in monkeys) [116].

four possible stimulus locations on each trial. We typi-

cally use vibrotactile target stimuli consisting of pulsed

white noise (i.e., three 30 ms bursts of white noise each

separated by 20 ms silent intervals) presented to one of

the vibrators, while visual distractors consist of the

pulsed illumination of one of the four LEDs.Participants are required to make a series of speeded

elevation discrimination responses, deciding whether

vibrotactile target stimuli are presented from the index

finger or thumb of either hand (i.e., ‘‘above’’, at the

index finger; or ‘‘below’’, at the thumb), while simulta-

neously trying to ignore the visual distractors presented

at approximately the same time. Although these di-

stractors are just as likely to be presented from the same

Direction of tactile attention

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C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189 173

elevation as the target, as from a different elevation,participants are normally much worse (i.e., both slower

and less accurate, see Fig. 2a) at discriminating the

elevation of the vibrotactile targets when the visual di-

stractors are presented from an incongruent elevation

(i.e., when the vibrotactile target is presented from the

top and the visual distractor from the bottom, or vice

versa) than when they are presented from the same

(congruent) elevation (i.e., either both up or bothdown).

Crossmodal congruency effects are calculated as the

difference in performance between incongruent and

congruent distractor trials for a particular pair of dis-

tractor LEDs. 2 Although we typically focus on the

crossmodal congruency effects present in the reaction

time (RT) data, similar effects are normally reported in

the error data as well. Often, therefore, we combinethese two measures of performance into a single measure

known as inverse efficiency (IE)––where the inverse

efficiency score equals the mean RT for a particular

condition divided by the proportion of correct responses

for that condition [135,144].

While the magnitude of any effects reported in a

particular experimental situation decline with practice,

significant crossmodal congruency effects still occur evenafter participants have performed many hundreds of

trials [81,138]. The very existence of crossmodal con-

gruency effects highlights a fundamental problem of

crossmodal selective attention in humans: Specifically,

people cannot ignore what they see, even if they are

instructed to respond only to what they feel (see [134] for

a similar failure of audiovisual selective attention).

Crossmodal congruency effects have also been ob-served when the role of the two stimulus modalities is

reversed; That is, when participants are instructed to

respond to the elevation of the visual stimuli, while

attempting to ignore the elevation of the vibrotactile

stimuli [147,148]. However, the crossmodal congruency

effects elicited by vibrotactile distractors on visual ele-

Fig. 2. Modulation of the crossmodal congruency effect as a function

of spatial, temporal, and attentional manipulations [138]. (a) The

crossmodal congruency effect is greatest when the target and distractor

stimuli are presented from approximately the same spatial location,

and declines as the distance between the target and distractor stimuli

increases (here the azimuthal distance between the target and distrac-

tor was 63� in the Far condition). (b) Crossmodal congruency effects

are large when the onset of the visual distractors precedes the onset of

the vibrotactile targets by 30 ms, with the effect declining in magnitude

if the onset of the target and distractor occur simultaneously, or if the

onset of vibrotactile targets occurs before that of the visual distractors.

(c) The magnitude of the crossmodal congruency effect was, however,

unaffected by whether the participant knew in advance to which hand

the vibrotactile target would be presented (focused attention), versus

when the target was presented unpredictably to either hand on each

trial (divided attention). The bars show the crossmodal congruency

effects in the RT data, while the line plots show the crossmodal con-

gruency effects present in the error data. Error bars indicate the

standard error of the mean.

2 One interesting question that has yet to receive an empirical

answer concerns the nature of the effect of the visual distractor stimuli

on vibrotactile elevation discrimination responses. That is, we do not

know whether the presentation of the visual distractors improves (in

terms of speed and accuracy) vibrotactile elevation discrimination

responses on congruent trials, impairs performance on incongruent

distractor trials, or whether both effects may co-occur when perfor-

mance is compared to that in a no-distractor baseline condition. We

are currently initiating a series of experiments to address this issue.

However, it seems probable that visual distractors may influence

vibrotactile discrimination responses not just in terms of congruency

effects, but also in terms of crossmodal attentional cuing and/or

crossmodal alerting effects [68,136]. It is worth noting though that our

use of the crossmodal congruency paradigm does not depend upon a

precise and exact understanding of the causes of the effect, since we use

it simply as an index of common location across different sensory

modalities.

174 C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189

vation discrimination responses tend to be somewhatsmaller in magnitude than for the prototypical case of

vibrotactile targets and visual distractors (see [70,86] for

similar results from somewhat different experimental

paradigms). This asymmetrical pattern of crossmodal

congruency effects may reflect an underlying difference

in the relative salience of the vibrotactile and visual

stimuli used in our previous studies, and/or some sort of

inherent bias of our participant’s attentional resourcestoward the visual modality [106,137]. In another series

of experiments, Merat and colleagues have shown that

vibrotactile distractors elicit a small, but reliable,

crossmodal congruency effect when participants are re-

quired to discriminate the elevation of auditory targets

(up to approximately 80 ms when the two stimuli are

presented at same azimuth; [91]).

There are at least two possible explanations for thecrossmodal congruency effect, one in terms of a multi-

sensory ‘perceptual’ integration effect (i.e., spatial ven-

triloquism), and the other in terms of competition at the

level of response selection between the target and dis-

tractor. According to the spatial ventriloquism account,

the perceived location of the vibrotactile target in our

prototypical study might be ventriloquized toward that

of the incongruent visual distractor (see [8,14]). Whenthe visual distractor is placed at a different elevation

from the vibrotactile target, but still close to it (i.e., on

the same hand), the latter may be mislocalized toward

the former. Such a spatial ventriloquism effect, should it

occur, might lead to errors in participant’s responses, or

simply to their finding it harder (and therefore taking

more time) to discriminate the correct elevation of the

target on incongruent trials (ventriloquism would pre-sumably, if anything, facilitate the perception of the

‘correct’ location of the target on congruent trials).

Alternatively, according to the response competition

account, the crossmodal congruency effect may reflect

the consequences of competition between the response

tendencies elicited by the target and distractor on

incongruent trials. Presumably the presentation of both

stimuli will prime the response(s) associated with theelevation at which they are presented. Given that the

distractor will prime the incorrect response on incon-

gruent trials, this might lead to a slowing of responses,

attributable to the time taken to overcome the incon-

gruent (i.e., ‘inappropriate’) response tendency. In fact,

the slowest responses in crossmodal congruency exper-

iments are usually reported on trials where the visual

distractor is presented from the same side as the vibro-tactile target, but at an incongruent elevation [138]. By

contrast, performance of congruent trials might be ex-

pected to show some degree of response facilitation,

since the target and distractor stimuli would both prime

the same, ‘correct’, response [85].

We have attempted to demonstrate the contribution

of spatial ventriloquism to the crossmodal congruency

effect by conducting an unspeeded version of theexperiment, in which participants were not permitted to

respond until at least 750 ms after the onset of the target

and distractor. The importance of response accuracy

over response speed was also repeatedly stressed to the

participants. If response compatibility is responsible for

the crossmodal congruency effect then one might expect

that there should be virtually no residual crossmodal

congruency effect, given that participants in this un-speeded version of the task presumably had sufficient

time to resolve any potential response conflict. The re-

sults demonstrated a small but significant increase in

errors specifically when the visual distractor was pre-

sented from an incongruent elevation on the same side

(and not when the distractor was presented from either

position on the opposite side), suggesting some role for

spatial ventriloquism in the crossmodal congruency ef-fect. We do, however, believe that the majority of the

crossmodal congruency effect probably reflects response

conflict effects instead. Although the very existence of

the crossmodal congruency effect is by itself of theoret-

ical interest, our continued use of this paradigm has

been motivated by the fact that crossmodal congruency

effects are typically both large in magnitude (in com-

parison to many other behavioural effects), and modu-lated by the spatial separation between the target and

distractor stimuli.

2.1. Spatial modulation of the crossmodal congruency

effect

Crossmodal congruency effects are normally largest

when the target and distractor stimuli are presentedfrom the same azimuthal location (i.e., when the dis-

tracting lights are situated by the hand receiving the

vibrotactile target), and decline as the visual distractor

and vibrotactile target hand are moved further and

further away from each other [132,138] (see Fig. 2a).

Over the last few years, we have conducted many

studies investigating the consequences of a number of

basic postural manipulations using the crossmodalcongruency task [132,138]. These studies have shown

that the crossmodal congruency effect elicited by a

particular pair of visual stimuli falls off as the hand

receiving the vibrotactile target is moved further away

from them. Even when the hands are crossed over the

midline (see Fig. 1b), visual distractors by the current

target hand position still elicit the largest crossmodal

congruency effects; this, despite the fact that the afferentsignals from the vibrotactile targets presented to the

crossed hand project, at least initially, to the opposite

cerebral hemisphere with respect to the visual distrac-

tors. Such results support claims that placing a hand by

a light may ensure that the relevant location within a

bimodal visuotactile topographic representation is

stimulated by the light [22,73].

C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189 175

The findings reported so far are consistent with thehand-position dependent modulation of the visual RF

of bimodal visuotactile neurons reported previously in

primates [35,36]. Interestingly, however, the spatial

modulation of the crossmodal congruency effect does

not appear to depend on participants actually being able

to see their hands, as it has even been shown to occur

when the hands remain out of view, as when participants

perform the crossmodal congruency task in completedarkness [138]. This result demonstrates that proprio-

ceptive and tactile cues regarding limb position can, by

themselves, provide sufficient information to code a

particular light source as being either close to, or far

from, an unseen hand (see [68] for similar results). Once

again, these behavioural findings are consistent with the

known primate neurophysiology [37,96], though see

[21,74] for neuropsychological evidence that the limb-position dependent modulation of crossmodal visuo-

tactile extinction, shown in certain parietal patients,

appears to depend upon their being able to see their own

hand and arm.

2.2. Does spatial attention modulate the crossmodal

congruency effect?

Spence et al. [138, Experiment 1] have shown that the

magnitude of crossmodal congruency effects are greater

when visual distractors lead vibrotactile targets by 30

ms, than when the two stimuli are presented simulta-

neously, or when vibrotactile targets are presented

shortly before (30 ms) visual distractors (mean cross-

modal congruency effects of 72, 59 and 46 ms respec-

tively; see Fig. 2b). It is for this reason (i.e., to maximizethe size of the crossmodal congruency effect) that we

normally present visual distractors shortly (i.e., 30 ms)

before vibrotactile targets in our crossmodal congruency

experiments.

This experimental design is in some sense similar to

that of crossmodal studies of exogenous spatial atten-

tion (see [71,129] for detailed discussion of the difference

between exogenous and endogenous spatial attention).For example, in a typical crossmodal cuing study, a

spatially non-predictive cue in one modality is presented

to the left or right of fixation, and followed shortly

afterward by a target stimulus in a different modality

(see [127] for a review). Several studies have shown that

the presentation of a spatially non-predictive visual cue

facilitates elevation discrimination responses for vibro-

tactile targets presented from the same (as opposed tothe opposite) side of fixation for several hundred milli-

seconds after cue onset [16,67,68]. Typically, crossmodal

cuing effects evidence themselves as a facilitation of

target discrimination responses of around 20–30 ms

when the target is presented from the cued, as opposed

to the uncued, side.

Given such findings, it would seem likely that theonset of the visual distractor shortly before the vibro-

tactile target in our crossmodal congruency studies

would also have led to a shift of ‘tactile’ attention to the

side of the visual distractor [127,128]. However, while

maximal facilitation would be expected to accrue at the

particular location of the visual stimulus (i.e., distrac-

tor), it is likely that the other location (i.e., elevation) at

the same azimuthal position would also be facilitated toaround the same extent, given the relatively spatially-

unfocused nature of any crossmodal shifts of attention

(see [127] on this point). Consequently, any exogenous

shift of spatial attention elicited by the visual distractor

would not be expected to have much of an effect on the

magnitude of crossmodal congruency effects, since it

should primarily result in a reduction of the overall RT

when the target and distractor are presented from thesame side, without differentially affecting performance

on congruent as opposed to incongruent trials.

One might also wonder whether there is any role for

endogenous spatial attention in modulating crossmodal

congruency effects. However, somewhat surprisingly,

Spence et al. [138, Experiment 1] reported that the

magnitude of crossmodal congruency effects is relatively

unaffected by the direction of endogenous spatialattention to one hand or the other (see Fig. 2c). That is,

crossmodal congruency effects were just as large when

participants knew in advance which hand the vibrotac-

tile target would be presented to (and so could pre-

sumably direct their spatial attention in advance to just

the target hand), as when the target was presented

unpredictably to either hand on each trial (and where

spatial attention presumably had to be divided equallybetween the two hands).

Spence et al.’s [138, Experiment 1] results stand in

marked contrast to the results of a number of other

studies of endogenous spatial attention, where elevation

discrimination responses for both vibrotactile and visual

targets (presented individually, and in the absence of

any distractors) have been shown to be facilitated by the

direction of endogenous spatial attention to a particularexpected target side/hand [13,133]. Presumably, while

directing attention to one hand or the other can speed-

up overall response latencies to stimuli presented by (or

to) that hand, it has little effect on the pattern of

crossmodal congruency effects, since performance on

both congruent and incongruent distractor trials will be

facilitated to the same extent (just as for the exogenous

case described above).

3. Using the crossmodal congruency task to investigate the

representation of 3-D peripersonal space

One of the key findings to emerge from the re-

search discussed up to this point has been that

176 C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189

larger crossmodal congruency effects are found whenvibrotactile targets and visual distractors are pre-

sented from the same azimuthal position, and that

the magnitude of these effects falls off as the sepa-

ration between the target and distractor increases

[132,138]. As such, the spatial modulation of the

crossmodal congruency effect provides a reliable

index of whether vibrotactile and visual stimuli are

perceived (functionally) to occupy the same spatiallocation or not. The results reported so far highlight

important parallels between human performance on

the crossmodal congruency task, and the findings of

a number of primate neurophysiological studies [35–

37,96], a link that is also seen in the studies reported

below.

Although the crossmodal congruency task is only

one of a range of experimental paradigms currentlyavailable to researchers to investigate the spatial co-

registration of stimuli across different sensory modali-

ties, it has the benefit over many of these other

tasks (such as the crossmodal exogenous spatial ori-

enting paradigm; [127]) of eliciting large and robust

behavioural effects. Moreover, the crossmodal con-

gruency task also has the advantage of being seem-

ingly robust to the specific distribution of spatialattention (both exogenous and endogenous) to one

hand or the other. The crossmodal congruency effect

is, therefore, ideal for use when investigating more

subtle questions related to multisensory spatial repre-

sentation, and when testing patient populations, as we

shall see.

3 Many researchers have made a conceptual distinction between the

terms ‘body image’ and ‘body schema’. Body image is typically used

when referring to a conscious representation of the body, or of bodies

in general, and may depend primarily on semantic information. By

contrast, body schema is typically meant to refer to an unconscious

representation of the body and its position and movement in space.

The body schema is thought to be derived primarily from proprio-

ceptive, kinaesthetic, and possibly visual information arising from the

body itself (for further discussion and reviews, see [7,17,28,29,48,55]).4 Note that other researchers have elicited visual capture effects

using either mirrors (with phantom limb patients) [109,110,122] or

prisms [44,46,152] as well (see also [12,55]).

3.1. Visual and proprioceptive contributions to tactile

localization

Having characterized the crossmodal congruency ef-

fect and, more specifically, having demonstrated the

reliability and robustness of the crossmodal congruency

task as an indicator of common location across vision

and touch, we next went on to investigate the relative

contributions of visual and proprioceptive cues to thelocalization of tactile stimuli in 3-D peripersonal space.

Our research was stimulated by a fascinating report

published in Nature by Botvinick and Cohen [11] that

highlighted the potential importance of visual cues in

determining perceived limb position. Participants in

their study sat at a table, with their left hand placed on

the table occluded from view behind a screen. A life-

sized rubber left hand and arm (an ‘alien’ limb) wasplaced in a plausible posture in front of the participant.

The experimenter stroked the rubber arm in full view of

the participant, while the participant’s own left hand

was stroked synchronously, but out of sight, behind the

occluding screen. Botvinick and Cohen reported that

participants rapidly started to ‘incorporate’ the left

rubber arm into their own body image. 3 In fact, after 10min of stroking, participants would agree strongly with

questionnaire statements such as ‘I felt as if the rubber

hand were my hand’, and, more importantly for present

purposes, ‘It seemed as if I were feeling the touch of the

paintbrush in the location where I saw the rubber hand

touched.’

Botvinick and Cohen [11] also reported in a second

experiment that after stroking, participants would sys-tematically misreach when trying to point with their

right hand under the table to the position of the unseen

left hand resting on the table-top. Critically, the mag-

nitude of this intermanual pointing error was correlated

with the perceived duration of the rubber hand illusion

during stroking. These results suggest that the partici-

pants in Botvinick and Cohen’s study really perceived

their left hand as if it was displaced toward the locationof the rubber arm (i.e., they experienced a ‘visual cap-

ture’ of felt limb position). By contrast, the illusion did

not occur (and pointing remained veridical) under con-

ditions where the real and fake left arms were stroked

asynchronously. These results highlight the prominence

of visual over proprioceptive cues in determining per-

ceived limb position (and possibly also tactile localiza-

tion) when the senses are artificially placed in conflict.Similar reports of the visual capture of proprioception

by fake or alien limbs/digits have also been reported in

several other studies over the years [95,141,151]. 4

Importantly, however, the interpretation of many

of these studies is limited by the possibility that experi-

menter expectancy effects, task demands, and/or

response biases may have contributed to (or even

determined) the participant’s performance [15,58,97,120]. That is, participants in these studies may simply

have responded in the manner in which they thought the

experimenter wanted them to respond, rather than be-

cause they actually experienced ‘visual capture’ per se.

Furthermore, given that participants in many of these

studies were not made aware that the body part they saw

might not be a part of their own body, they may simply

have responded on the basis of what they saw, becausethey had no reason to doubt the veridicality of their

(normally accurate) visual perception (cf. [154]). A final

C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189 177

limitation to the interpretation of all of these previousstudies is that while they may provide evidence (the

above caveats notwithstanding) for the visual capture of

proprioceptive sensation, this does not necessarily imply

any consequences for the localization of tactile sensation

per se (as implied by a literal interpretation of the

everyday expression––an ‘out-of-body’ experience).

Pavani et al. [104] attempted to overcome these lim-

itations by examining the consequences of the rubberhand illusion for specifically tactile perception using the

crossmodal congruency task. Participants were required

to wear a pair of rubber washing-up gloves, and to hold

two foam cubes on each of which were mounted two

vibrators. Participants could not see their own hands as

they were hidden below an opaque screen (see Fig. 3).

Pavani et al. reported that the magnitude of the cross-

modal congruency effect elicited by the visual distractorsincreased when a pair of rubber arms (actually stuffed

rubber washing-up gloves) were placed in a plausible

posture in front of their participants (and on top of the

occluding screen), apparently ‘holding’ the visual di-

stractors. Using a questionnaire similar to that devised

by Botvinick and Cohen [11], Pavani et al. also found

that the magnitude of the increase in the crossmodal

congruency effect was correlated with subjective reportsof the vividness of the rubber hand illusion, as indexed

by participant’s agreement with the statements: ‘I felt as

if the rubber hands were my hands’, and ‘It seemed as if I

were feeling the vibration in the location where I saw the

rubber hands’.

Fig. 3. Schematic view of the experimental set-up in Pavani et al.’s

[104] ‘rubber hand’ experiments, highlighting the location of the vi-

brotactile stimulators (indicated by the arrows) on the foam cubes held

by the participant below an occluding screen, and the visual distractor

lights (open circles on the upper cubes) held by the rubber hands were

present and aligned with the participant’s own hands. Note that in

some conditions (not shown), the rubber arms were placed at 90� to the

participant’s own arms (i.e., in a posture that the participant could not

possibly adopt).

In contrast to Botvinick and Cohen’s [11] study, Pa-vani et al.’s [104] results were obtained in the absence of

any stroking of either the real or rubber hands, thus

showing that synchronized multisensory stimulation is

not a prerequisite for the visual capture of propriocep-

tive and tactile sensation, at least under conditions

where proprioceptive acuity is relatively low (i.e., in the

elevation dimension; see [150]). Interestingly, Pavani et

al. [104, Experiment 2] also reported that crossmodalcongruency effects were unaffected by the presence of the

rubber arms if they were placed in an implausible pos-

ture for the participants (i.e., by placing them at 90� withrespect to the participant). The latter result enabled

Pavani et al. to rule out any interpretation of their

apparent visual capture results in terms of either in-

creased saliency of the lights when placed in close

proximity to what may have been a visually intriguingpair of rubber hands (i.e., ruling out the possibility that

participants may simply have been directing more of

their attention to the modality of the distractors when

the rubber hands were present), and also that placing

fingers and thumbs by the lights might result in the lights

eliciting a greater propensity to initiate a finger (up) or

thumb (down) response (i.e., an enhanced semantic re-

sponse activation account).It is important to note that the visual capture effect in

Pavani et al.’s [104] study occurred despite the fact that

participants were made aware of the sensory ‘deception’

taking place. That is, the experimenter pointed out to

participants both the existence and construction of the

rubber arms, and participants could also see the rubber

hands being positioned in front of them at the start of

each block of trials when they were present. The par-ticipants were therefore clearly aware that the limbs they

saw were not part of their own body, hence making

Pavani et al.’s visual capture results even more dramatic.

Finally, it is worth noting that the fact that the illusion

still occurred under these transparent conditions, sup-

ports the cognitive impenetrability of the visual capture

phenomenon [8]. It appears, therefore, that people do

not have direct access to the absolute localization oftactile stimuli in space, but instead their perception re-

flects only the consequences of the multisensory inte-

gration of the available sensory cues (be they visual,

proprioceptive, tactile, or even perhaps, auditory 5

[23,98,99,119,153]).

Remarkably, Erin Austen and colleagues at the

University of British Columbia [4] have shown that

limbs bearing even less resemblance to the human form(a pair of ‘Frankenstein-like’ green hairy arms) can also

elicit visual capture, resulting in the mislocalization of

vibrotactile stimuli (though see [3,38,108]). It is, of

5 As when we hear sounds indicating that our limbs have made

contact with a particular surface.

178 C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189

course, possible that more pronounced visual captureeffects might have been demonstrated in Austen et al.’s

study if participants were not made aware of the

deception (as happened in previous studies where

appendages with a far greater likeness to the human

form were used; though see [154] for evidence that

people do not find it as easy to recognize their own limbs

as the phrase: ‘I know it like the back of my own hand’

would suggest; see also [151]). It is an interesting ques-tion for future research to try and identify those factors

that are critical for eliciting this form of ‘identification’

with a visually-presented limb, as this may help to

provide important constraints for the design of virtual

haptic and remote teleoperation systems in the years to

come [51,84].

Pavani et al. [104] argued that the increased magni-

tude of crossmodal congruency effects reported in thealigned rubber hand condition of their experiment could

be attributed to the ‘apparent’ perception of vibrotactile

targets as being close to the distractor lights. In other

words, that tactile (and not just proprioceptive) stimuli

were mislocalized toward the apparent visual location of

a limb (really a stuffed rubber washing-up glove). In a

more recent study, Walton and Spence [148] repeated

Pavani et al.’s study, but simply reversed the role of thevisual and vibrotactile stimuli. That is, participants now

had to respond to the elevation of the visual ‘target’

stimuli, while attempting to ignore the elevation of the

vibrotactile ‘distractor’ stimuli. Once again, the majority

of the participants in this study experienced the rubber

hand illusion when the rubber hands were placed on top

of the occluding surface in alignment with their own

hands. However, the occurrence of the rubber armillusion now led to a reduction in the magnitude of the

crossmodal congruency effect that was correlated with

responses on the post-experiment questionnaire (i.e.,

those participants who experienced the illusion most

strongly showed the greatest reduction of the magnitude

of the crossmodal congruency effect; the opposite pat-

tern of results to those reported by [104]).

Walton and Spence’s [148] results would appear tosupport the view that placing the hands near the visual

stimuli may make them more task-relevant, perhaps

because this leads to the lights producing more activa-

tion in a topographic representation of visuotactile

space [22,73]. Given that the magnitude of the cross-

modal congruency effect has been shown to depend on

the relative intensity and discriminability of the target

and distractor stimuli [85], this would have the effect ofincreasing the crossmodal congruency effect in Pavani et

al.’s [104] study, where the relative saliency of the di-

stractors would have been increased, but reducing them

in Walton and Spence’s study, where the relative

saliency of the targets were now increased. This modu-

latory effect of rubber hands on the crossmodal con-

gruency effect does appear to depend, though, on the

rubber arms being in a posture that is compatible (i.e.,plausible) with the participant’s own body.

Neurophysiological data on the phenomenon of vi-

sual capture of perceived limb position was reported in a

study by Michael Graziano [33,38] in which a monkey’s

own right arm was hidden below an occluding screen

while a taxidermied monkey’s right arm was placed

above the occluding screen in front of the monkey. The

visual response of bimodal premotor neurons with vi-sual RFs centred on the observed fake arm were shown

to be reduced when the position of the fake arm was

changed from an uncrossed to a crossed posture while

the monkey’s own arm remained in an uncrossed pos-

ture below the occluding screen [33]. Moreover, the

postural response of parietal neurons was enhanced by

the vision of a stuffed rubber arm with a posture com-

patible to that of the hidden arm [38]. Similarly, Farn�eet al. [26] have also demonstrated, in a group of uni-

lateral spatial neglect patients, that the presentation of a

rubber limb by a visual stimulus can modify the extent

to which the visual stimulus extinguished the patient’s

perception of an ipsilesionally-presented tactile stimulus.

These visual capture effects only occurred when the

rubber hands were placed in alignment with the patient’s

own body, and not when positioned at 90� to them (justas in Pavani et al.’s [104] study, in normal participants).

3.2. Indirect perception of the limbs

In a further series of experiments, we have explored

whether crossmodal congruency effects can be modu-

lated through a more abstract understanding of the

source of visual stimuli in a scene [82]. In particular,whether a spatial re-coding of distant visual stimuli

would make them equivalent to near visual stimuli in

terms of the crossmodal congruency effects that they

elicit. To this end, participants were presented with a

mirror reflection of their own limbs (while any direct

view of their hands was occluded by means of an opaque

screen). Such indirect reflected visual information con-

veys bivalent information about the true spatial positionof lights placed by the hands. Visual stimuli originating

close to one’s body (e.g., a comb stroking one’s hair),

but only visible via a mirror reflection, give a visual

impression suggesting an object situated behind (or

through) the mirror, at least in terms of initial retinal

projections (see [39] for an interesting account on mirror

reflections from a psychological perspective). However,

our familiarity with reflecting surfaces (in addition tothe synchronous proprioceptive and somatosensory

information, in the example given above), provides us

with an immediate and exact notion of the true spatial

location of the comb. The experimental question ad-

dressed in Maravita et al.’s study was whether visual

stimuli that appear in the mirror to occupy a position in

far space (i.e., outside peripersonal space) would be

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ii

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grue

ncy

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Fig. 4. (a) Schematic view of the experimental set-up in Maravita

et al.’s [82] experiments on the effects of viewing visual stimuli indi-

rectly via a mirror reflection on crossmodal congruency effects. Par-

ticipants held one foam cube in either hand below an opaque screen (i).

A semi-reflecting mirror (ii) was placed on one side of an open opaque

box (iii) facing the participant. Depending on the ambient illumina-

tion, participants could either see their own hands reflected in the

mirror, or else see through the mirror to reveal the contents of the box.

Lines drawn from the distractor lights on each sponge held by the

observer and crossing the mirror, suggest the position of the virtual

image produced on the mirror by such objects, as observed by the

participant. This horizontal distance is equivalent to double the dis-

tance of the foam cubes from the mirror (60 cm) plus the distance

between the participant and the foam cubes themselves (30 cm). The

foam cubes and distractor lights in the Far condition were placed at

the exact same position as the position of these virtual images in the

Mirror condition. (b) The crossmodal congruency effect in the three

experiments (E1, E2, and E3) conducted by Maravita et al. [82] was

significantly larger for the Mirror condition (white bars) than for the

Far condition (black bars). The bars show the results from the analysis

of the reaction time data (the error bars represent the standard error of

the mean), while the numbers within each column highlight the error

rates for that condition for each of the three experiments. Crossmodal

congruency effects reflect the difference between incongruent and

congruent trials (see text for details).

C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189 179

treated as originating in near or peripersonal space if theparticipants were made aware that what they are look-

ing at was a mirror reflection of near peripersonal space

(albeit seen indirectly). We assessed this by asking

whether the apparent visual conflict generated by mirror

reflections could be modulated, in a similarly automatic

fashion, by the obligatory spatial effect of visual di-

stractors assessed by the crossmodal congruency task.

In a series of experiments, Maravita et al. [82] sys-tematically varied the position of the visual distractors,

so as to obtain two stimulation conditions, while par-

ticipants made speeded vibrotactile elevation discrimi-

nation responses. In one condition, the visual distractors

(LEDs) were placed near the hands (Mirror condition),

occluded from direct view by an opaque screen, and

were observed as their reflection in a mirror placed 90

cm from the participant. In this situation, the retinalprojection produced by the visual distractors (virtual

mirror image) was equivalent to that of stimuli placed at

a distance double that between the real stimulus and the

mirror (plus the distance between the observer’s eyes

and the stimulus itself; see Fig. 4a). In a second condi-

tion, visual distractors were located far away from the

hands, inside a box (Far condition). Participants now

observed these stimuli by looking through the mirror(actually a semi-reflecting mirror). Visual distractors in

this condition were specifically positioned so as to pro-

duce a retinal projection that was exactly equivalent to

the virtual mirror image produced by the distractors in

the previous Mirror condition. Now, the only clue to

indicate any difference between the actual positions of

the distractors in the two conditions was the partici-

pant’s knowledge about the experimental setting.We made sure that participants always knew when a

mirror reflection of a stimulus near the hand was ob-

served or, when the stimulus was actually located far

from the hand, inside the box. A preliminary training

session was included prior to each block of experimental

trials in which participants were repeatedly required to

move the finger that was stimulated tactually to rein-

force this. This corresponded to a visual input of themoving finger in the Mirror condition, but the partici-

pants received no such visual feedback when they were

looking at the pair of rubber hands placed inside the

box. Given our finding that the largest crossmodal

congruency effects are reported when visual distractors

are situated close to the vibrotactile target stimuli

[104,132,138], we expected a larger crossmodal congru-

ency effect in the Mirror condition, where visualdistractors were physically located close to the partici-

pant’s hands that received the vibrotactile targets, than

in the Far condition, where visual objects were genuinely

placed outside peripersonal space.

Consistent with our expectations, the participants in

our study showed larger crossmodal congruency effects

from the visual distractors in the Mirror condition than

from the visual distractors in the box in the Far condi-

tion (see Fig. 4b). This result was replicated in a secondexperiment when participants observed rubber hands

grasping the sponges inside the box in the Far condition

(actually washing gloves stuffed with a metallic frame

180 C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189

and padded with cotton in order to give them the real-istic look of a real hand inside the glove; similar to those

used in Pavani et al.’s [104] study), as opposed to their

own hands in the mirror. In a final experiment, the

participants responded by releasing spatially-compatible

response buttons that were embedded in the foam cubes

they were holding, instead of by releasing foot-pedals as

in the majority of our previous studies. This meant that

participants were given some visual feedback regardingtheir own hand movements in the Mirror condition

while they were actually performing the crossmodal

congruency task (and not just in the period preceding

each test block). Critically, in the Far condition of this

final experiment, participants saw the hands of a sex-

matched experimenter holding the foam cubes inside the

box. The experimenter also made responses on each

trial, consistent with those of the participant (thoughslightly delayed). Nevertheless, even though participants

could see a pair of real hands placed by the foam cubes

in the Far condition performing the same task, the vi-

sual distractors in the Mirror condition still resulted in a

larger crossmodal congruency effect than those pre-

sented in the Far condition inside the box. These results

demonstrate that visual stimuli seen distally via a mirror

reflection are correctly coded as originating in near-space when they are presented from cubes held by the

participant (but out of direct view; see also [63,64,

90,117]).

In a similar vein, Maravita et al. [79] have also shown

that a visual stimulus seen indirectly in a mirror reflec-

tion placed close to one of the hands of a right-hemi-

sphere brain-damaged patient elicits greater crossmodal

extinction for tactile stimuli presented simultaneously tothe other hand than when the ‘same’ visual stimulus is

positioned and seen in far space. Once again, vision of

the hand seen in the mirror must have activated a rep-

resentation of peripersonal space and not extrapersonal

space, as was suggested by the retinal image provided by

the stimulus in the mirror. Maravita et al.’s [79,82] re-

sults appear to show that knowledge of the experimental

situation, Mirror or Far condition, is sufficient forpeople to ‘‘re-map’’ the position of the visual distractors

to their actual location (within near, peripersonal space

in the Mirror condition, and in far space in the Far

condition). So it would appear that while crossmodal

congruency effects may be ‘‘cognitively impenetrable’’,

they can still be modulated by abstract knowledge

concerning the real location of visual distractors, and

not just by physical changes of the positions of thehands relative to visual distractors.

We are currently attempting to extend this line of

research to look at people’s ability to adapt to other

kinds of indirect information about limb position as, for

example, when the hand is seen in a video monitor (cf.

[60], for neurophysiological data on this; and

[19,20,27,145], for human data), and the consequences

and determinants of this for the sense of ownership oridentification with a limb as one’s own [5,6,30,31,

75,100,118,123,142,143,146].

3.3. Tool-use and the body schema: consequences for

peripersonal space

Thanks to the evolutionary liberation of the hands

from locomotion, humans can efficiently use tools inorder to extend the range of actions that they can per-

form [102]. Think, for example, of the croupier’s rake,

the blacksmith’s hammer or the surgeon’s knife. In fact,

humans frequently use tools to perform a variety of

different tasks in their everyday lives; Everything from

eating using cutlery to writing with pens and other

implements, from gardening with rakes, shovels, and

hoes, to playing sport with a variety of racquets, sticks,poles, and other equipment. Tool-use has become such

an integral part of modern life that there are actually few

activities left that we perform without them, and this

raises fascinating and important questions about how

sensory information arriving at the somatosensory epi-

thelia can be modulated and spatially re-coded by tool-

use. In particular, how are visual and somatosensory

information integrated in tool-use, and can functionalperipersonal space (that region of visible space around

the body that is reachable with our hands [9,107,

113,115]) be extended dynamically by active tool-use?

The scientific study of tool-use is one area that is cur-

rently becoming increasingly popular with cognitive

neuroscience researchers ([57,59–62], see [53] for a re-

view).

In the classic neurology literature, the so-called bodyschema is constructed by continuous input from

somatosensory and proprioceptive afference [29,48].

This ‘‘schema’’ is an on-going and constantly updated

internal representation of the shape of the body, and of

the position of the body in space both in respect to the

external world, and in relation to its own parts [7,34].

Somatosensory signals are both perceived and localized

with reference to this representation and these functionscan be impaired as a consequence of sensory disruption

following selective brain damage [7,17,28], or peripheral

deafferantation (e.g., phantom limbs; [89]). In many

cases, it has been argued that tools are actually assimi-

lated into the ‘body schema’ or ‘body image’

[7,62,101,102,153,156] (see also Footnote 3 on this

distinction).

Phantom phenomena, in particular, provide remark-able evidence in support of the plasticity of the body

image, and its extension by inanimate objects or ‘tools’

[12,65,88,101,103,110,123]. Many amputees feel pain in

their missing limb [89], and over time, their phantom

and its associated pain retracts, ‘telescoping’ toward the

stump. The wearing of a prosthetic limb, however, can

suddenly relieve pain and restore the phantom to its

75 cm

75 cm(a)

(b)

Fig. 5. Schematic view of the experimental set-up used by Maravita et

al. [81] to investigate the possible modification of the body schema

elicited by extended tool use. The position of the vibrotactile stimu-

lators is indicated by the grey triangles by participant’s hands, while

the circles at the distal tip of the tools represent visual distractors

(LEDs). (a) shows the Uncrossed-tools condition, while (b) shows the

Crossed-tools condition. In Maravita et al.’s Experiment 1, the tools

consisted of toy-golf clubs, while plastic sticks were used in their

Experiment 2. The alignment of the tools was changed actively by the

participant after every four trials in Experiment 1, and changed by the

experimenter after every 48 trials in Experiment 2. (Redrawn from

[81].)

C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189 181

previous length, ‘fleshing out’ the artificial limb ([89],[111, p. 221]). 6 Several accounts from primate studies,

as well as from normal participants and brain-damaged

human patient populations suggests that the manipula-

tion of tools, and other external objects that frequently

come into contact with our bodies (such as rings worn

on the hand) can also become incorporated into the

body schema [2,18,48,59]. Certain patterns of brain

damage can also lead to specific deficits in the ability touse or manipulate tools. For example, Paillard [102]

reports that while some patients with left parietal brain

damage can describe a tool and correctly define its

function, they may also, at the same time, be unable to

manipulate it correctly.

Pioneering primate neurophysiological data in this

area has demonstrated that the multisensory integration

of visual and somatosensory inputs can be critically af-fected by the use of tools. In monkeys trained to use

tools, the emergence of bimodal visuotactile cells was

reported during recording from the medial bank of the

intraparietal sulcus [59]. Many of the cells in this area

had overlapping RFs, and would respond both to tactile

or proprioceptive stimulation of, for example, the hand,

and to visual stimuli (especially the sight of a food re-

ward) approaching the hand. Immediately after tool-use, the visual RF of these bimodal cells approved to be

elongated along the length of the tool, such that visual

stimuli approaching the tip of the tool were as effective

at driving the neurons as stimuli approaching the hand

[59]. Iriki et al. speculated that the use of a tool could

plastically extend the representation of the hand in the

body schema, so that now even distant stimuli activate

those multisensory neurons coding for stimuli presentednear the body (i.e., an extension of peripersonal space to

incorporate all stimuli that are now accessible by the

manipulation of the tool, and not just by the hand).

More recently, Maravita and colleagues [81] have

demonstrated behaviourally that the body image can be

modified by extended tool-use: The prolonged wielding

of golf-club-like sticks resulted in changes in the pattern

of crossmodal congruency effects elicited by visual di-stractors placed at the end of tools (see also [52]). Par-

ticipants in their study were required to make speeded

elevation discrimination responses with their right foot

to vibrotactile target stimuli presented from vibrators

attached to the proximal ends of two tools, one held in

either hand. As in our previous studies, participants

rested their index fingers and thumbs on these vibrators

in an upper/lower arrangement (see Fig. 5). However, incontrast to our previous studies, the upper and lower

visual distractors were now placed at the far end of each

6 The crossmodal congruency task may provide an ideal tool to

examine empirically the perceptual reality of this retraction and

extension of (physically non-existent) phantom limbs.

tool. On some trials, participants were required to keep

the tools in an uncrossed posture (Uncrossed- orStraight-tools condition, Fig. 5a), while on other trials

they were required to cross the tools over the midline

(Crossed-tools condition, Fig. 5b). Although there were

visual distractors and vibrotactile stimulators on each

side of space in both conditions, the relative spatial

relationship between the pairs of visual distractors and

the vibrotactile targets connected by each tool changed

when the tools were crossed over. While each hand was‘connected’ by the tool with distractors on the same side

of space in the Uncrossed-, or Straight-tools condition,

each hand was ‘connected’ with distractors on the

opposite side of space in the Crossed-tools condition.

The question Maravita et al. addressed was whether

reaching with the tool to distractors on the opposite side

of space could reduce, or even invert, the usual pattern

of crossmodal congruency effects (whereby visual di-stractors on the same side as vibrotactile targets usually

produce larger crossmodal congruency effects than those

on the opposite side), such that we would find larger

crossmodal congruency effects for opposite side than for

same side distractors. A reversal of this kind would be

predicted if one believed that by extending the hand’s

action space via the tool, vibrotactile stimuli at the hand

182 C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189

and visual distractors on the opposite end of the toolwould now share a common multisensory representation

and possibly show larger crossmodal congruency effects

(see [138], discussed earlier, for related results with

crossed hands).

The results confirmed our prediction by showing that

the typical pattern of larger crossmodal congruency ef-

fects for same side distractors demonstrated in the

Straight-tools posture, was reversed by crossing thetools (Fig. 6a). Interestingly, however, while this pattern

of results was found in the first experiment, where par-

ticipants actively switched between the two postures

after every 4 trials, no such reversal of crossmodal

congruency effects was reported in a second experiment

(see Fig. 6b) when the participant’s posture was swit-

ched passively by the experimenter after only every 48

trials, instead. Under such conditions, the pattern ofcrossmodal congruency effects remained very similar for

the two postures. These results suggest that the tool-

based spatial re-mapping of the crossmodal congruency

effect requires both frequent and active use of the tools

(cf. [59,102]).

In order to clarify the precise effects of practice fur-

ther, Maravita et al. [81] also compared the results from

the earlier and later parts of the experimental session.Interestingly, the critical spatial reversal of the cross-

modal congruency effect with crossed tools was only

present in the second part of the experiment (i.e., blocks

3–5; see Fig. 6d), and not in the first part (i.e., in the first

two experimental blocks; see Fig. 6c), presumably due to

the prolonged practice with the tools participants had

had by this stage. This result could be compatible with a

modification of the representation of the hand in the

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Fig. 6. The pattern of crossmodal congruency effects demonstrated in Marav

congruency effects (the error bars represent the standard error of the mean

(uncrossed versus crossed). For all panels, the white bars represent performa

vibrotactile targets, while the black bars represent performance when the ta

values shown within each column highlight the mean crossmodal congruency

blocks) in which the alignment of the tools was changed actively by the partici

in which the alignment of the tools was changed by the experimenter only aft

blocks of trials (respectively) in Experiment 1.

body schema, which followed the prolonged use of thetool. Note here that the majority of previous studies

investigating the use of tools in patients incorporated an

adaptation phase at the start of their experiments in

which participants were required to use the tool for a

while––for example, to retrieve some sort of distal ob-

jects with a rake [25,83].

These results suggest that the crossmodal congruency

effect is prone to modulation due to changes of spatialcoding following tool-based modification of crossmodal

integration. The crossmodal congruency effect elicited

by the visual distractors in this task depended not only

upon the spatial proximity between the target stimuli

and the distractors, but also upon a ‘‘functional’’

proximity in terms of action space (for logically related

reports in brain-damaged patients, see [1,9,25,80,

83,105]). Once a region of space, distant from the hand,is reached by a tool it might become equivalent to a

near, peripersonal source of stimulation for visuotactile

congruency effects. Our results with the crossmodal

congruency task therefore converge nicely with the re-

sults from single-cell studies in monkeys who have been

taught to use tools [59], and the results of human studies

using a tactile temporal order judgment task ([156], see

also [112]).Our most recent research into the tool-use dependent

modulation of visuotactile peripersonal space has ad-

dressed the question of whether peripersonal space is

literally ‘extended’ along the shaft of actively-wielded

tools, or whether instead just the space around the tip of

the tool is modified [52]. To do this, we presented visual

distractors not only at the tips of two actively-wielded

hand-held tools, but also halfway along the shaft of the

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Straight Crossed

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(d)

(b)

ita et al.’s [81] tool-use study. The data represent the mean crossmodal

) in terms of the RT data as a function of the alignment of the tools

nce when the visual distractors were presented on the same side as the

rget and distractor were presented from different sides. The numerical

effect in terms of the error data. (a) The results for Experiment 1 (all

pant after every four trials. (b) The results for Experiment 2 (all blocks)

er every 48 trials. (c) and (d) show the results for the first and last four

C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189 183

tool, and next to the hands. We found that visual di-stractors presented at the tip of the tool held in the right

hand produced much larger crossmodal congruency ef-

fects for simultaneous right hand vibrotactile targets,

than for identical targets presented to the left hand. This

same-tool effect also occurred for visual distractors

presented on the tool held in the left hand. The effect of

visual distractors in the middle of the tool shafts, how-

ever, was not selective for the hand holding the tool.Two further experiments supported this finding by

demonstrating that target-directed motor tasks per-

formed in both peripersonal space (pushing a button

with the detached handle of the tool) and extrapersonal

space (pointing to a distant target with a laser-pointer)

also did not result in significant differences between di-

stractors on the same-side versus on opposite-sides to

the vibrotactile targets. This result seems to suggest thatvisuotactile peripersonal space is not simply ‘extended’

by active tool-use, but rather that only the functional

part of a hand-held tool is incorporated into periper-

sonal space. We are currently extending this line of re-

search to investigate whether the active wielding of a

long tool is sufficient for this effect to occur, or whether

instead the part of the tool used to perform a task is

critical for the observed modulations of peripersonalspace. We also hope to look at the link between the

wielding of real physical tools that are directly manip-

ulated by a person, and the remote control of tools, such

as when we use a mouse to move a cursor across a

computer monitor [56].

4. Using the crossmodal congruency task to investigate theneural underpinnings of 3-D perispersonal space

The behavioural results from the crossmodal con-

gruency studies reviewed so far are consistent with the

existence in humans of visuotactile representations of 3-

D peripersonal space that are updated as posture

changes, and that can adapt to incorporate into peri-

personal space visual stimuli that would normally beconsidered to be in far space instead [43]. However, it is

not clear whether the maintenance of an accurate rep-

resentation of visuotactile space as posture changes re-

lies on cortical structures (such as ventral premotor

cortex and parietal area 7b), sub-cortical structures

(such as the putamen), or both, since bimodal visuo-

tactile neurons with tactile RFs on the hand and visual

RFs that follow the hands as they move have been re-ported in all of these structures [35,36].

4.1. The representation of visuotactile space in the split-

brain

Spence et al. [135] attempted to address this question

by testing a split-brain patient on the crossmodal con-

gruency task. For split-brain patients (as for normalparticipants), the left hemisphere controls the right hand

and receives direct visual projections from the right vi-

sual field. Similarly, the right hemisphere controls the

left hand and receives direct visual projections from the

left visual field. In most situations, neural signals

resulting from visual and tactile stimuli in the same

spatial location will project, at least initially, to the same

hemisphere (i.e., the right hand and the right visual fieldproject to the left hemisphere; and the left hand and left

visual field project to the right hemisphere). It is unclear

though what happens when a hand is crossed over into

the opposite hemifield. For instance, if the right hand of

a split-brain patient is placed in the left field, will visual

events in the left visual field map onto the tactile RFs of

the right hand, as they apparently do in the intact hu-

man brain? If this normal remapping does not occur,then bimodal cells in cortical structures in one hemi-

sphere (such as the ventral premotor cortex, parietal

area 7b, or both)––which are disconnected from similar

structures in the opposite hemisphere of split brain pa-

tients––would appear to be crucial for remapping the

visual RF onto the tactile RF when the hand crosses the

midline. Conversely, if this normal remapping does oc-

cur in the split brain, then bimodal cells in sub-corticalstructures (e.g., putamen or superior colliculus)––which

are shared between the disconnected hemispheres––

would appear to be implicated.

Spence et al. [135] therefore compared the perfor-

mance of a split-brain patient (J.W.) with that of two

healthy age-matched neurologically normal control

participants on the crossmodal congruency task. At the

time of testing, J.W. was a 45 year-old patient, whosecorpus callosum had been completely sectioned in 1979

(with the anterior commissure left intact) in order to try

to cure his intractable epilepsy (see [125], for a more

detailed description of J.W.’s neurological status). All

three participants made elevation discrimination re-

sponses with their right foot to vibrotactile targets pre-

sented to the thumb or index finger of the right hand,

thus ensuring that both their perception of the vibro-tactile target, and the initiation of their elevation dis-

crimination response were controlled by the same left

cerebral hemisphere. The participants held a foam cube

in their right hand in one of three different postures,

while their left arm rested passively in their laps. The

visual distractor stimuli were presented from two foam

cubes, one presented to either side of fixation (see Fig. 7

for a schematic illustration of the postures adopted, andthe crossmodal congruency results obtained).

Visual inspection of Fig. 7a and b shows that the

magnitude of the crossmodal congruency effects elicited

by the visual distractors on the right cube were modu-

lated by the relative position of the right hand: More

specifically, crossmodal congruency effects from the

right distractor lights were more pronounced when the

Fig. 7. Schematic view of the foam cubes (represented by open rect-

angles) and postures adopted by the normal control participants and

by the split-brain patient J.W., in Spence et al.’ [135] study (a–c)

showing the direction of fixation (dotted line) in the different posture

conditions. The locations of the vibrotactile targets, which were always

presented to the right hand, are indicated by the letter ‘T’. Mean

crossmodal congruency effects elicited by visual distractors are shown

numerically next to the cube on which they were situated. (The absence

of any values next to certain cubes shows that no distractor lights were

attached to that particular cube.) Crossmodal congruency effects rep-

resent a difference score: Performance on incongruent–distractor trials

(i.e., trials on which the vibrotactile target and visual distractor ap-

peared at different elevations) minus performance on congruent–dis-

tractor trials (i.e., trials on which the target and distractor were

presented from the same elevation), measured in terms of inverse

efficiency (average response time divided by the proportion correct for

each condition [144], which combines speed and accuracy into a single

measure to allow comparisons among conditions uncontaminated by

possible speed-accuracy trade-offs (see text for details).

184 C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189

right hand held the cube on which they were mounted,and decreased when the participants grasped a more

eccentrically positioned cube with their right hand in-

stead. However, the most interesting result for present

purposes occurred when participants moved their right

hand across the midline into the left hemifield (see Fig.

7c): For the two control participants, crossmodal con-

gruency effects were now greater for distractor lights on

the left cube (now held by the crossed right hand) thanfor lights on the right cube, again replicating our pre-

vious findings. By contrast, the right distractor lights

always interfered more than those on the left for the

split-brain patient J.W., no matter whether his right

hand was placed in an uncrossed or a crossed posture.

This result suggests a failure to remap visuotactile space

appropriately when his right hand crosses into the left

hemispace.Before accepting this conclusion, however, an alter-

native interpretation for J.W.’s performance needs to be

ruled out: Namely, that there may actually have been

nothing wrong with his ability to remap visuotactile

space across the midline per se; Instead, his problems

might be explained simply in terms of his responding left

hemisphere not ‘seeing’ the left visual distractors, which

were initially projected to his ‘non-responding’ righthemisphere. (Note that the left visual distractors never

actually elicited any noticeable crossmodal congruency

effects for vibrotactile targets presented to J.W.’s righthand, no matter what the posture adopted.) Although a

range of previously-published behavioural data made

such an explanation seem unlikely [32,54,76], we

thought it preferable to examine this issue more directly

in the context of the crossmodal congruency task.

To this end, we conducted a number of further pos-

tural manipulation experiments on J.W. in the years

following our original study. In one such study, wefound that left visual distractors can elicit significant

crossmodal congruency effects for vibrotactile targets

presented to the right hand, no matter whether the right

hand was placed in the right or left hemispace (see [136,

Fig. 2, for details]). These results presumably reflect the

behavioural consequences of the residual connections

between cortical and sub-cortical visual areas that re-

main intact in J.W., rather than cortico–cortico con-nections [10,47,76]. However, the crucial point is that

even though the distractor lights on the left could

interfere with right hand elevation discrimination per-

formance, there was still no evidence of a remapping of

visuotactile space when J.W.’s right hand crossed into

left hemispace.

These results, therefore, demonstrate that J.W.’s

problem is not simply with seeing lights presented ipsi-lateral to the responding hemisphere (i.e., on the left),

but more specifically with a failure to maintain an

accurate representation of visuotactile peripersonal

space across the two hemifields. Spence et al.’s [135,136]

results indicate that cross-cortical (or more strictly

speaking, trans-commissural) connections are crucial for

the maintenance of an up-to-date representation of

peripersonal visuotactile space, at least when the righthand crosses the midline. It would be interesting in fu-

ture studies to investigate whether a similar pattern of

results would be obtained if J.W. were required to make

elevation discrimination responses with his left foot for

vibrotactile targets presented to the left hand (cf.

[49,50]).

4.2. Disrupting the representation of visuotactile space

with repetitive transcranial magnetic stimulation

The pattern of results obtained with the split-brain

patient J.W. supports the view that performance on the

crossmodal congruency task may index a relatively high-

level (i.e., cortical) representation of visuotactile space.

However, given that J.W. has by now been tested on a

near-daily basis for much of the last 20 years, it isimportant that converging findings are sought from

other cognitive neuroscience methodologies to back up

the claims made on the basis of this rather unique pa-

tient. To this end, we are currently investigating whether

we can elicit the abnormal pattern of crossmodal con-

gruency effects demonstrated by J.W. in a relatively

normal population of Oxford undergraduates: We are

C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189 185

using repetitive transcranial magnetic stimulation(rTMS) to disrupt activity in a region corresponding

approximately to the angular gyrus and the posterior

parts of the intraparietal sulcus. Although this research

is still at an early stage, our preliminary results suggest

that performance on the crossmodal congruency task in

our normal participants can also be selectively impaired

when they adopt a crossed hands posture (rather than

an uncrossed posture) and rTMS is applied over theregion of the angular gyrus and posterior parts of the

intraparietal sulcus (rather than over primary visual or

somatosensory areas, or when sham rTMS is applied to

the back of the neck; [126,149]). The pattern of cross-

modal congruency effects observed while applying rTMS

therefore provide converging evidence to support the

critical importance of cortical structures (and presum-

ably also cross-cortical or cross-commissural connec-tions) in maintaining an up-to-date representation of

visuotactile space.

Some indirect evidence in favour of a cortical

involvement in the representation of visuotactile peri-

personal space also comes from an extensive body of

research on neurological patients suffering the clinical

phenomenon of crossmodal extinction (see [73] for a

recent review). In these patients, a tactile stimulus to thecontralesional side of the body (e.g., the hand or neck) is

often ‘extinguished’ by a concurrent stimulus presented

in a different sensory modality (e.g., a light or a sound),

particularly when it is presented near the corresponding

body part on the ipsilesional side (e.g., the other hand,

or the other side of the neck). However, such a difference

in the magnitude of visuotactile (or audiotactile)

extinction as a function of the vicinity of the ipsilesionalstimulus to the body part (the ‘near’ vs. ‘far’ compari-

son), has not been consistently observed in all patients

[72,87]. This suggests that under some circumstances,

the cortical lesion of these patients (typically involving

frontal, temporal and/or parietal areas of the right

hemisphere (e.g., [22,73,83]) can disrupt the normal

multisensory representation of peripersonal space,

resulting in a similar degree of multisensory extinctionregardless of the proximity of the ipsilesional stimuli.

Although a clear anatomical definition of brain regions

responsible for crossmodal extinction, and for the ex-

trapersonal/peripersonal phenomena has so far been

complicated by the extensive nature of the brain-damage

often presented by these patients, high-resolution struc-

tural and functional neuroimaging could now help to

shed light on the neural basis of multisensory repre-sentation of peripersonal space (e.g., [77]).

4.3. Future studies using the crossmodal congruency task

It will be particularly interesting in future research to

compare the pattern of performance on the crossmodal

congruency task highlighted in the split-brain patient

J.W. with that of other patients, such as those withparietal brain damage exhibiting the phenomenon of

crossmodal extinction [22,73]. We also hope to extend

our research to investigate the neural underpinnings of

the crossmodal congruency effect and the multisensory

perception of limb position more directly, using func-

tional Magnetic Resonance Imaging (fMRI; cf.

[77,78,92]).

In future studies, it may also prove to be particularlyinformative to investigate the extent to which other

tasks, such as making saccadic responses to sudden

onset visual and tactile stimuli, may rely more on sub-

cortical, rather than cortical, representations of visuo-

tactile space [24,40–42]. A growing body of behavioural

evidence is now leading many researchers to suggest that

performance on certain behavioural tasks may reflect

the competing influences of multiple representations ofvisuotactile space (or of the body image/schema

[34,56,66,69,131,153]), hence possibly resulting in a dif-

ferent pattern of behavioural results, even when normal

participants place their limbs in unusual postures

[124,155].

5. Conclusions

Hopefully, it should be clear by now that variations

in the magnitude of the crossmodal congruency effect

provide both a reliable and a robust index of common

spatial position across different sensory modalities, in

particular, vision and touch. Over a number of studies,

we have shown that visual distractors interfere with

speeded elevation discrimination responses to vibrotac-tile target stimuli presented to the thumb or index finger

of either hand, even when participants are repeatedly

instructed to ignore what they see. Crossmodal con-

gruency effects are maximal when vision and touch are

presented from the same spatial location at approxi-

mately the same time, and fall off as the relative spa-

tiotemporal separation between target and distractor

stimuli increases [132,138]. The maximal crossmodalcongruency effects elicited by visual distractors follow

the hands when they move through space, even when

they cross the midline in normal participants [132,

135,138].

In the last few years, we have used the crossmodal

congruency task to investigate the flexibility of the body

representation (body schema), as highlighted by the

apparent displacement of the limbs seen in the ‘rubberhand’ illusion [104,148], and the changes in peripersonal

space that can occur following extended tool-use [52,81].

These results are consistent with the extant neurophys-

iology of the visuotactile representation of 3-D peri-

personal space seen in primates [33,34,59,96]. The

crossmodal congruency task can also be used to eluci-

date disturbances to the visuotactile representation of

186 C. Spence et al. / Journal of Physiology - Paris 98 (2004) 171–189

space seen following specific brain damage, such as thesectioning of the corpus callosum in split-brain patients

[135,136]. The growing understanding of the factors that

govern whether or not particular distal events will be

functionally incorporated into the body schema to ex-

tend the boundary of what constitutes peripersonal

space may also have a number of important applications

for the future design and implementation of teleopera-

tion and virtual haptic reality systems [51,84].Taken together, we believe that the results of the

crossmodal congruency studies conducted to date

highlight the utility of the paradigm itself for investi-

gating the relative contributions of visual, tactile, and

proprioceptive inputs to the multisensory representation

of 3-D peripersonal space in both normal and patient

populations. In the years to come, we hope to be able to

combine neurophysiological, electrophysiological, neu-ropsychological, and neuroimaging data with behavio-

ural data from normal participants on this task in order

to try and bridge the gap between the rich body of

published single-cell neurophysiological data, and the

human perceptual experiences with which we are all

familiar (cf. [34]). For it is only by adopting this con-

verging methodologies approach that we will make sig-

nificant inroads toward resolving the questionsregarding the multisensory representation of space that

have long vexed both scientists and philosophers

[121,130].

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