does target viewing time influence perceived reachability?
TRANSCRIPT
Intern. J. Neuroscience, 117:1331–1339, 2007Copyright C© 2007 Informa HealthcareISSN: 0020-7454 / 1543-5245 onlineDOI: 10.1080/00207450600936809
Brief Communication
DOES TARGET VIEWING TIME INFLUENCEPERCEIVED REACHABILITY?
CARL GABBARDDIALA AMMAR
Texas A&M UniversityCollege Station, Texas, USA
This study examined the influence of target viewing time on perceived (estimatesof) reachability. Right-handed participants were asked to judge the simulatedreachability of midline targets using their dominant limb in viewing conditionsof 150 ms, 500 ms, 1 s and 2 s. Responses were compared to actual maximumreach. In reference to percent error, interestingly, the 150 ms condition revealed theleast error at peripersonal targets and the most inaccuracy with distal (extrapersonal)targets. This condition was also distinct with a significant overestimation bias—acommon observation in earlier studies. However, with increasing viewing time thisbias was reduced. These data provide evidence that 150 ms is effective for estimatingreach within one’s general peripersonal workspace. However, with judgments distalfrom that point, more time enhanced accuracy, with 500 ms and 1 s being optimal.Overall results are discussed relative to perceptual effectiveness in programmingreaching movements.
Keywords imagined movements, motor imagery, perceived reachability, reaching
Reaching requires a complex set of perceptual to motor transformations. Oneof the initial steps is to derive a perceptual estimate of the object’s distance andlocation relative to the body. From a Gibsonian view (1979), the detection ofthe affordance for a particular mode of reaching entails perceiving whether the
Received 26 May 2006.Address correspondence to Carl Gabbard, TAMU 4243 College Station, TX 77843-4243, USA.
E-mail: [email protected]
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reach action will fit in the existing layout of the environment. This means thatan individual must be able to perceive critical reach distances, beyond whicha particular reach action is no longer afforded. Is the object close enough toreach while seated, or do I need to stand up to contact the object? Arguably,this estimate forms the initial cognitive basis of the motor program, that is,the cognitive level of action processing. The study of imagined versus actualmovement affords an attractive approach in the quest to identify the specificmechanisms and relationship involved in action processing. Broadly speaking,imagined movement, also known as motor imagery, is defined as a dynamicstate during which an individual mentally simulates a specific motor action(Decety, 1996; Jeannerod, 2001).
Ever since Jeannerod (1994) made one of the initial arguments thataction planning and motor preparation can be studied effectively using motorimagery, a multitude of studies have followed. For example, several reportssuggest that imagined and actual movement share overlapping neurocognitivenetworks resulting in a high correlation between real and imagined movementperformance (e.g., Gentilluci et al., 2004; Glover et al., 2005; Sabate et al.,2004; Sheng et al., 2004; Stinear et al., 2006). One form of imagined movement,which is the focus of the work reported here, is perceived reachability. Althougha considerable body of evidence suggests that humans are very accurate atestimating reachable distances, even from childhood (see Viguier et al., 2001for a review), several reports confirm an overestimation bias (Carello et al.,1989; Fischer, 2005; Heft, 1993; Robinovitch, 1998; Rochat & Wraga, 1997).That is, individuals exhibit a general tendency to perceive that they can reachobjects that are actually out of reach.
An interesting observation from this literature is the amount of time thatparticipants were allowed to view the target (stimulus duration). Of the studiesnoted, none varied this parameter and in each case, no time constraint wasspecified. In two recent studies reported by the authors’ lab using the samegeneral set up described for the present experiment (Figure 1), participantsviewed the target for 150 ms (Gabbard, Ammar et al., 2005a; Gabbard et al.,2005b). In support of previous studies, a significant overestimation bias atmidline locations was observed. In a subsequent study in which the authorsexamined use of monocular and binocular cues with the same setup using 1s target exposure, a significant reduction in the overestimation bias was noted(Gabbard et al., 2005c). This comparison peaked their interest concerning thepossible influence of viewing time on estimates of reach.
This study examined the effect of different target exposures on perceivedreachability in one’s personal workspace (peripersonal space) and locations
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VIEWING TIME 1333
Figure 1. The experimental set-up. (See Color Plate IV at end of the issue.)
beyond one’s absolute reach (extrapersonal space). From one perspective, thisstudy explores an aspect of action planning involving an element of body-scaling under the constraint of time. The initial prediction was that, due to reach-ing experience in peripersonal workspace, estimation error would be less than inextrapersonal space. Furthermore, it was expected that with greater perceptualdemands with shorter viewing times, participants would make more errors. Onthe other hand, at what point does more time provide no additional utility?
METHOD
Participants
Participants were 29 (15 male and 14 female) right-handed university studentsbetween the ages of 19 to 23 years. All participants were screened using aquestionnaire to ensure that none had a history of past or present sensorimotorimpairment. For this study, only participants identified as strong right-handers(i.e., those for whom all items scored in that lateral direction using theLateral Preference Inventory questionnaire [Coren, 1993]) were included inthe investigation. All participants signed informed consent forms approved bythe Institutional Review Board before beginning the experiment and were naı̈veto the hypotheses under investigation.
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Apparatus
A general illustration of the testing set-up used to solicit perceived andactual reaching behavior is shown in Figure 1. Visual images (targets) weresystematically projected onto a table surface at the individual’s midline (90◦)position. Two basic image template presentations were used; one measuredactual maximum reach and the other involved single point random presentationof imagined targets. Imagined stimuli were programmed for random appearancefrom a 14 cm line consisting of seven 2 cm diameter (penny size) round whiteimages, touching at the rims. The table was constructed on a sliding bracketframe, allowing it be moved back and forward for adjustment to the participant;table height was fixed at 74 cm. Participants sat in an adjustable ergonomicschair fixed to the floor, aligned with the midline of the table and projectedimage midline, and set at a fixed height of 44 cm from the top of the seat panto the floor. Table and chair height were adopted from Carello et al. (1989)and Heft (1993). To aid in establishing actual reach limitations (a 1-df action),a commercial seatbelt system was modified and secured to the back of thechair. The room was darkened with the exception of light from the computermonitor and white visual images projected onto the table programmed with agray background surface.
Procedure
Participants were initially told that they would be asked to make judgmentsrelative to whether a target (projected image) was within reach. After beingsystematically positioned in the chair, actual reach was determined; maximumextension of middle finger to pull back a penny using a 1-df reach (Carelloet al., 1989). A 1-df reach involved a comfortable effort of the hand,forearm, and upper arm acting as a single functional skeletal unit. A secondexperimenter observed the subject to ascertain that restrictions for the 1-dfmovement were not violated; in such a situation, the trial was repeated. Basedon the single maximum reach, seven imagined target presentations (2 cmdiameter-penny size) were randomly programmed with “4” representing actualreach complemented with three images sites above (referred to as extrapersonalspace) and three below (peripersonal space) touching at the rims (Figure1). Peripersonal space in this context was defined as the area within one’spersonal workspace for a 1-df reach, with the absolute critical boundary beingtarget 4 (maximum reach). Extrapersonal space refers to targets outside thisboundary (see Carello et al., 1989 and Mark et al., 1997 discussions of critical
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boundaries for reaching). That is, peripersonal space included targets (1–4),while extrapersonal space represented targets (5–7).
For imagined responses, participants were asked to kinesthetically “feel”themselves executing the movement with the right limb—therefore being moresensitive to the biomechanical constraints of the task (Johnson et al., 2001).For all trials, the dominant (right) hand was placed within a drawn box onthe table close to the torso at midline. In order to facilitate the “focus” ofimagined hand use, the right hand was placed on the table edge at the midline,and the left (non-use) limb placed in the participant’s lap, resting on the upperthigh. Following three practice trials, data collection began with a 5 s “Ready!”signal—that was immediately followed by a central fixation point lasting 3s, at the end of which the participant heard a first tone. The image appearedimmediately thereafter and lasted for the prescribed target exposure. A secondtone then provided the signal for the participant to respond immediately witha “Yes” or “No” in reference to whether the stimulus was “reachable” or not.No feedback was available about the accuracy of performance. Initial pilotwork confirmed that participants respond (after instruction and practice trials)within 200 ms; as verified by response time. Stimulus presentation was givenin random order with participants receiving five trials at each of the seven sites.Four viewing time conditions were given: 150 ms, 500 ms, 1 s, and 2 s; conditionorder was counterbalanced between participants. The participants completed140 total trials across all conditions. This setup affords accurate measurementof actual and imagined reaching responses relative (scaled) to each participant.
Statistical Analyses
For measures of accuracy, chi-square procedures were used to compare thefour conditions in regard to total error, distribution of error across targets,and between target responses. Analysis of variance (ANOVA) procedures withDuncan’s post hoc tests were employed to determine estimates of error in termsof mean bias in cm; mean bias represented the general direction of error (i.e.,over- or underestimation). That is, from actual reach (target 4), each of thethree higher and lower (closest to the body) targets was 2 cm apart. Data weregiven a positive or negative sign and then summed to provide a signed mean.Zero on the y-axis represented no bias, whereas a minus value represented anunderestimation and above zero an overestimation. For example, if a participantnoted that target 5 was reachable (“yes”) when in fact it was not, it was anoverestimation. As appropriate, post hoc analyses, using Duncan’s MultipleRange tests were performed (p <.05).
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0
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Figure 2. Total error for viewing time conditions. (See Color Plate V at end of the issue.)
RESULTS
In reference to error for total trials, chi-square results indicated that participants’responses to the four viewing time conditions did not differ (ps >.05). Therange of error was 17% (2s) to 21% (1s), with a mean of 19%. However,when viewing error in reference to reachable (peripersonal) workspace (targets1–4) and extrapersonal space (targets 5–7), an interesting difference was noted(Figure 2). That is, in the 150 ms condition participants exhibited the least errorin peripersonal space (ps <.05) and the most inaccuracy in extrapersonal space.All comparisons with 150 ms were statistically significant with the exceptionof 500 ms (p = .14).
Although accuracy for total error was relatively high, attention was focusedon what is arguably a more insightful view—where the errors occurred. Thereader should keep in mind that there were seven target locations, with “4”representing the participant’s actual maximum reach. Incorrect responses atthe three targets above actual (5–7) indicated an overestimation, whereasan incorrect response at any of the lower targets (1–4) was considered anunderestimation. For example, if a participant noted that target 5 was reachable(‘yes’) when in fact it was not, it was an overestimation. Interestingly, a similarprofile to total error was observed showing that participants in the 150 mscondition produced the least error in peripersonal space and the most at thedistal locations (Figure 3). Chi-square results for time within targets revealedsignificant differences at targets 3, 4, 5, and 6 (ps <.05). All other comparisons
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Figure 3. Distribution of error across targets.
were statistically nonsignificant (ps >.05). In consideration of all conditions,as one might expect, most errors occurred around targets 4 and 5 (about 50%).
In reference to mean error (estimation bias), a Target Location XCondition (viewing time) ANOVA revealed an overall effect of Condition,F(3, 29) = 11.79, p < = .01. Follow-up procedures indicated that participantsoverestimated with 150 ms (.42 cm) and with increasing viewing time reducedthe bias. That is, participants were more accurate with 500 ms (.09) and 1 s(–.12); these conditions were significantly different from 150 ms and 2 s (–.24),but similar to each other. Furthermore, there was an obvious effect of TargetLocation, F(6, 29) = 34.40, p < = .01, however, Target Location × Conditioninteraction did not reach the level of significance, p = .81.
DISCUSSION
This experiment examined the influence of target viewing time on perceivedreachability in peripersonal and extrapersonal workspace. Although nodistinction across time exposures was found in regard to total (combined)workspace error, a significant difference emerged in reference to specificworkspace. That is, the 150 ms condition produced the least error in peripersonalspace and the most in extrapersonal space (Figure 2). As a general observationin view of combined responses, less error occurred with targets 1–3 and the mosterror around the critical boundary (targets 4 and 5). As initially predicted, itseems reasonable to assume that this outcome may be a reflection of experiencein one’s personal workspace with more difficulty estimating distance around
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the absolute boundary. In regard to the time element, the authors envisaged thatwith more viewing exposure there will be less error (with for example, 150ms versus 2 s). Apparently, for peripersonal space, 150 ms exposure providessufficient information for effective response in estimating reachability. Thisobservation in itself may represent a testament to the perceptual efficiency ofthis aspect of the action processing system.
However, in regard to extrapersonal space, the prediction of time wasmore apparent as evidenced in Figure 3 and the results of mean bias. Thatis, the 150 ms condition was distinct by showing a significant overestimationbias. From these data an interesting trend emerged—with increasing viewingtime, the overestimation bias was reduced in the remaining conditions. Morespecifically, responses in the 500 ms and 1 s conditions were closer to accurate,with participants underestimating (slightly) in the 2 s condition. In essence, forextrapersonal space, these data suggest that target exposure over 1 s provides noadditional usefulness for response. However, given the small range of error incomparison to actual target accuracy, less than one-half cm across conditions,the interpretation of these results is, arguably, less than definitive. In essence,the overall findings of this study should be considered in view of percentageerror as well as estimation bias.
In summary, these data provide evidence that the relatively short viewingtime of 150 ms is effective for perceived reachability of one’s general1-df workspace. However, with judgments beyond that point, more timeenhances accuracy, with 500 ms and 1 s being optimal in the contextof this experiment; additional time does not appear to enhance response.One implication of this outcome, at least for adults, is evidence of theperceptual effectiveness underlying the programming of reaching movementsin peripersonal workspace; that is, the ability to perceive reachable distancesbased on relatively brief exposures of visual information.
REFERENCES
Carello, C., Grosofsky, A., Reichel, F. D., Soloan, H. Y., & Turvey, M. T. (1989).Visually perceiving what is reachable. Ecological Psychology, 1, 27–54.
Coren, S. (1993). The lateral preference inventory for measurement of handedness,footedness, eyedness, and eardness: Norms for young adults. Bulletin of thePsychonomic Society, 31, 1–3.
Decety, J. (1996). The neurophysiological basis of motor imagery. Behavioural BrainResearch, 77, 45–52.
Fischer, M. H. (2005). Perceived reachability: The role of handedness and hemifield.Experimental Brain Research, 160, 283–289.
Int J
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rosc
i 200
7.11
7:13
31-1
339.
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nloa
ded
from
info
rmah
ealth
care
.com
by
Uni
vers
ity o
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n 10
/25/
14. F
or p
erso
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se o
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VIEWING TIME 1339
Gabbard, C., Ammar, D., & Rodrigues, L. (2005a). Handedness effects on mentallysimulated reaching. Human Movement Science, 24(4), 484–495.
Gabbard, C., Ammar, D., & Rodrigues, L. (2005b). Perceived reach in hemispace. Brainand Cognition, 58(2), 172–177.
Gabbard, C., Ammar, D., & Rodrigues, L. (2005c). Visual cues and perceivedreachability. Brain and Cognition, 59(3), 287–291.
Gentilluci, M., Roy, A. C., & Stefanini, S. (2004). Grasping an object naturally or witha tool: Are these tasks guided by a common motor representation ExperimentalBrain Research, 157(4), 496–506.
Gibson, J. (1979). The ecological approach to visual perception. Boston: HoughtonMifflin.
Glover, S., Dixon, P., Castiello, U., & Rushworth, M. F. (2005). Effects of an orientationillusion on motor performance and motor imagery. Experimental Brain Research,166, 496–506.
Heft, H. (1993). A methodological note on overestimates of reaching distance: Distin-guishing between perceptual and analytical judgments. Ecological Psychology, 5,256–271.
Jeannerod, M. (1994). The representing brain: Neural correlates of motor intention andimagery. Behavioural Brain Sciences, 17, 187–245.
Jeannerod, M. (2001). Neural simulation of action: A unifying mechanism for motorcognition. NeuroImage, 14, S103–S109.
Johnson, S., Corballis, P., & Gazzaniga, M. (2001). Within grasp but out of reach:Evidence for a double dissociation between imagined hand and arm movementsin the left cerebral hemisphere. Neuropsychologia, 39, 36–50.
Mark, L. S., Nemeth, K., Gardner, D., Dainoff, M. J., Duffy, M., & Grandt, K. (1997).Postural dynamics and the preferred critical boundary for visually guided reaching.Journal of Experimental Psychology: Human Perception and Performance, 23,1365–1379.
Robinovitch, S. N. (1998). Perception of postural limits during reaching. Journal ofMotor Behavior, 30(4), 352–358.
Rochat, P., & Wraga, M. (1997). An account of the systematic error in judgingwhat is reachable. Journal of Experimental Psychology: Human Perception andPerformance, 23(1), 199–212.
Sabate, M., Gonzales, B., & Rodriquez, M. (2004). Brain lateralization of motorimagery: Motor planning asymmetry as a cause of movement lateralization.Neuropsychologia, 42, 1041–1049.
Sheng, L., Latash, M. L., & Zatsiorsky, V. M. (2004). Effects of motor imagery on fingerforce responses to transcranial magnetic stimulation. Cognitive Brain Research,20, 273–280.
Stinear, C. M., Byblow, W. D., Steyvers, M., Levin, O., & Swinnen, S. P.(2006). Kinesthetic, but not visual imagery modulates corticomotor excitability.Experimental Brain Research, 168, 157–164.
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or p
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