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Unconscious inhibition and facilitation at theobjective detection threshold: Replicable
and qualitatively different unconsciousperceptual effects
Michael Snodgrass*, Howard Shevrin
University of Michigan, Ann Arbor, MI 48105, USA
Received 27 July 2004; accepted 27 June 2005
Abstract
Although the veridicality of unconscious perception is increasingly accepted, core issues remain
unresolved [Jack, A., & Shallice, T. (2001). Introspective physicalism as an approach to the science
of consciousness. Cognition, 79, 161196], and sharp disagreement persists regarding fundamental
methodological and theoretical issues. The most critical problem is simple but tenaciousnamely,how to definitively rule out weak conscious perception as an alternative explanation for putatively
unconscious effects. Using a direct task and objectively undetectable stimuli, the current experiments
demonstrate clearly reliable unconscious perceptual effects, which differ qualitatively from weakly
conscious effects in fundamental ways. Most importantly, the current effects correlate negatively
with stimulus detectability, directly rebutting the exhaustiveness, null sensitivity, and exclusiveness
problems [Reingold, E., & Merikle, P. (1988). Using direct and indirect measures to study perception
without awareness. Perception & Psychophysics, 44, 563575; Reingold, E., & Merikle, P. (1990).
On the inter-relatedness of theory and measurement in the study of unconscious processes. Mind and
Language, 5, 928)], which all predict positive correlations. Moreover, the current effects are
entirely bidirectional [Katz, (2001). Bidirectional experimental effects. Psychological Methods, 6,
270281)] and radically uncontrollable, including below-chance performance despite intentions to
facilitate. In contrast, weakly conscious effects on direct measures are unidirectional, facilitative, and
potentially controllable. Moreover, these qualitative differences also suggest that objective and
subjective threshold phenomena are fundamentally distinct, rather than the former simply being a
Cognition 101 (2006) 4379www.elsevier.com/locate/COGNIT
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cognition.2005.06.006
* Corresponding author. Address: Department of Psychiatry, University of Michigan, 2101 Commonwealth,
Suite B, Ann Arbor, MI 48105, USA. Tel.: C1 734 936 8703; fax: C1 734 764 3506.
E-mail address: [email protected] (M. Snodgrass).
http://www.elsevier.com/locate/COGNIThttp://www.elsevier.com/locate/COGNIT -
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weaker version of the latter [Merikle, P., Smilek, D., & Eastwood, J. (2001). Perception without
awareness: Perspectives from cognitive psychology. Cognition, 79, 115134]. Accordingly, it is
important to distinguish between rather than conflate these methods. Further, the current effectsreinforce recent work [e.g. Naccache, L., Blandin, E., & Dehaene, S. (2002). Unconscious masked
priming depends on temporal attention. Psychological Science, 13, 416424] demonstrating that
unconscious effects, although not selectively controllable, are nonetheless mediated by strategic and
individual difference factors, rather than being immune to such influences as long thought.
q 2005 Elsevier B.V. All rights reserved.
The history of unconscious perception has been characterized by a boom and bust cycle of
critical acceptability (Greenwald, 1992; Jack & Shallice, 2001). Currently, unconscious
perception enjoys broad acceptance (see e.g., the March, 2001 special issue on consciousness
in Cognition). This latest swing of the pendulum is perhaps related to the growing acceptance
of the study of consciousness in general, and in particular to a greater willingness to regard
participants subjective phenomenology as a legitimate subject of scientific inquiry. Although
we applaud these developments and ourselves argue for the reality of unconscious perception
(Shevrin, 1968; Snodgrass, Shevrin, & Kopka, 1993a,b), we agree with Jack and Shallice
(2001, p. 163) that the fundamental methodological problems in this area remain unresolved.
Consequently, the emerging positive consensus lacks a firm foundation, and may simply
perpetuate the boom and bust cycle unless these difficulties are satisfactorily addressed. The
most critical problem is deceptively simple and surprisingly tenaciousnamely, definitively
ruling out weak conscious perception as an alternative explanation for putatively unconscious
effects. If such alternative explanations remain viable, models that postulate only a single,
conscious perceptual process are more parsimonious and hence to be preferred. Indeed, using
this reasoning, several critiques have recently appeared (Dulany, 1997; Perruchet & Vinter,
2002) which deny that unconscious perception exists at all.In this paper, we present evidence for exceptionally reliable unconscious perceptual
effects obtained under stimulus exposure conditions widely held to be maximally
stringentnamely, wherein participants cannot detect even the mere presence versus
absence of the relevant stimuli. Notably, the current effects reflect unconscious perceptual
influences on a direct task, as opposed to the usual strategy of seeking unconscious effects
on indirect tasks. Further, it will emerge that these effects reflect qualitative differences
which: (1) provide particularly strong evidence against alternative weak conscious
perception accounts; (2) suggest that the two primary methods for assessing consciousness
(i.e. objective versus subjective threshold approaches; see below) may index fundamentally
distinct phenomena; and (3) underscore the importance of strategic factors in mediating
even completely unconscious effects, long thought immune to such influences. Beforeproceeding to the current paradigm and findings, we first briefly discuss the relevant
methodological and theoretical issues in order to clarify the basis for these implications.
1. Methodological and theoretical issues in demonstrating unconscious perception
Attempts to demonstrate unconscious perceptual influences typically rely on some
version of the dissociation paradigm (Erdelyi, 1985, 1986). Usually, performance on two
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tasks is compared; one intended to index conscious perception, the other unconscious
perception. In its classic and most common form, the dissociation paradigm seeks to obtain
effects on the unconscious perception index despite null sensitivity on the consciousperception index, thus justifying inferences for unconscious perception. The latter is
typically a basic, direct perceptual discrimination task (e.g. detection), whereas the former
is often an indirect task requiring more complex processing (e.g. semantic priming).Direct
tasks inform participants of the stimulus manipulation and ask them to perform intentional
judgments concerning the relevant aspects of the ostensibly subliminal stimuli. For
example, presence/absence detection tasks straightforwardly ask participants to discern
whether or not a stimulus was just presented. In contrast, indirect tasks do not inform
participants of the relevant stimulus manipulation, but instead assess noninstructed effects
of the ostensibly subliminal stimuli, usually on subsequent processing of other stimuli, as
in semantic priming effects.1 Further, although typical, direct versus indirect comparisons
are not essential. Rather, the fundamental requirement of the dissociation paradigm is that
the conscious perception index be exhaustively sensitive (Reingold & Merikle, 1990) to all
conscious perception that could contribute to (putatively) unconscious perception index
effects. If so, either direct or indirect tasks can be used to index unconscious perception (cf.
Merikle & Reingold, 1990). If not, unconscious perception cannot be validly inferred no
matter how it is indexed.
2. But how should awareness be assessed?
Everyone agrees that conscious perception is positively related to stimulus intensity.
Strong stimuli are clearly visible; as stimulus intensity is reduced (by varying stimulus
duration and/or the intensity of masking), visual percepts become fainter and less distinct.
Finally, when stimuli are sufficiently weak, conscious perception seems eliminated. But
how, exactly, should one define the threshold for consciousness? It turns out that there are
two basic alternatives.
2.1. Subjective threshold approaches
The most obvious and intuitively appealing way to index phenomenal awareness is to
manipulate stimulus intensity and simply ask participants to indicate when they can no
longer see (or hear, etc.) the relevant stimuli. This stimulus intensity is then taken to
indicate the threshold for consciousness. Because such methods focus on participants
self-reports of their phenomenal states rather than their behavioral performance, they arecalled subjective threshold approaches (Cheesman & Merikle, 1984, 1986). Notably,
under such conditions, participants not only exhibit robust indirect effects but moreover
1 The other, less commonly employed species of indirect task involves asking participants to make judgments
about irrelevant (i.e. nonmanipulated) aspects of the ostensibly subliminal stimuli. For example, subliminal mere
exposure paradigms (e.g. Kunst-Wilson & Zajonc, 1980) participants make judgments about pairs of stimuli, one
previously presented, one new. Under direct instructions, participants attempt to discern which stimulus was
previously presented; under indirect instructions, they are asked which of the two stimuli they prefer.
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perform substantially above chance on direct discrimination tasks (e.g. forced-choice
identification) as well. Indeed, such above-chance performance on direct tasks is the
canonical subjective threshold effect, and is how such effects were initially discovered (cf.Adams, 1957).
With the development of signal detection theory (SDT; e.g. Green & Swets, 1966),
however, it became clear that that subjective thresholds might simply reflect response
criterions applied to a single, continuously varying conscious process, rather than indexing
a dichotomous conscious/unconscious boundary. From this perspective, subjective
threshold effects can be plausibly interpreted in terms of weak conscious perception
(see, e.g. Green & Swets, 1966, pp. 335337; Holender, 1986; Macmillan, 1986;
Macmillan & Creelman, 1991, pp. 112, 239, 255; Merikle, 1982), because denials of
awareness may reflect very low confidence rather than a complete absence of awareness.
Importantly, the SDT criterion artifact critique does not question or reject participants
self-reports, but views them as resulting from both perceptual and decision processesrather than being unmediated readouts, as it were, of participants phenomenal states.
Indeed, a core finding of SDT is that perceptual sensitivity (d0) and the response criterion
(c) are independent, separable processes, and hence that for any given level of sensitivity
participants can voluntarily shift their response criterion depending on the exigencies of
the experimental situation. Accordingly, participants can and do respond differently to the
same stimuli depending upon where they place their criterion. With all this in mind,
subjective threshold approaches face serious problems in ruling out alternative weak
conscious perception explanations. At the same time, the SDT critique does not demand
this skeptical conclusion; it remains possible that such stimuli really are phenomenally
unconscious.
2.2. Objective threshold approaches
To minimize the plausibility of alternative weak conscious perception accounts,
some investigators prefer to arrange stimulus conditions such that participants not
only deny awareness but perform at chance (i.e. d0Z0), not above, on direct
discrimination tasks (e.g. Draine & Greenwald, 1998; Naccache & Dehaene, 2001).
This stimulus intensity is then taken as the threshold for consciousness. Such methods
are called objective threshold approaches (Cheesman & Merikle, 1984) because they
rely on direct task performance rather than self-report.2 Not surprisingly, objective
threshold conditions are more stringent than subjective threshold conditionsthat is,
the former require even weaker stimulus intensities than the latter. However, although
these paradigms are clearly less vulnerable to SDT-based criterion artifact concerns,
they nonetheless still suffer from the null sensitivity problem (Macmillan, 1986;
Reingold & Merikle, 1990). Namely, even if null sensitivity is apparently obtained,
intrinsic measurement error means that participants true, underlying sensitivity might
actually exceed zero.
2 Of course, indirect tasks (e.g. priming effects) can be objective as well; in this context, the objective versus
subjective distinction is understood to refer to subtypes of direct tasks.
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Some progress on the null sensitivity problem has recently been made by Greenwald
and associates (e.g. Greenwald, Klinger, & Schuh, 1995) regression approach. This
method regresses indirect performance onto direct performance, and the y-intercept istaken as the point estimate of the indirect effect when direct d0Z0. However, because
direct performance still contains measurement error, it is not clear that this really solves
the null sensitivity problem. In particular, when the slope and the means of the direct and
indirect measures are positive, which is often the case, y-intercepts are overestimated.
Although Klauer, Greenwald, and Draine (1998) have proposed corrective procedures,
they may not be sufficient (see Dosher, 1998; Miller, 2000; but see also Klauer &
Greenwald, 2000). Further, the validity of the regression method depends on whether
using the regression equation for predictive purposes (i.e. to estimate y-intercepts) is
justified at all. Doing so is clearly valid when the direct and indirect measures are related
(although measurement error is still problematic), but not when they are unrelated. Given
that these measures are typically unrelated in regression method experiments (e.g. Draine
& Greenwald, 1998), the associated y-intercepts may be invalid (cf. Dosher, 1998; Merikle
& Reingold, 1998; Snodgrass, Bernat, & Shevrin, 2004a), leaving the null sensitivity
problem essentially intact.
2.3. So how can alternative weak conscious perception accounts be definitively ruled out?
To deal with this nagging concern, many investigators attempt to obtain converging
evidence by demonstrating qualitative differences between conscious and putatively
unconscious effects. Although intuitively appealing, however, qualitative differences are
difficult to interpret because they may simply indicate an additional conscious process (cf.
Dulany, 1997; Erdelyi, 1986; Holender, 1986). We return to this problem later; for now,
we discuss further implications of subjective versus objective threshold approaches.
3. The theoretical tension between subjective and objective threshold approaches
The very idea of objective threshold effects raises important theoretical and
methodological questions. Namely, if subjective threshold approaches are valid, the
robustly above-chance performance on direct tasks obtained under such conditions is due
to unconscious influences. If so, reducing stimulus intensity to the point that direct task
performance is at chance, as objective threshold methods require, should seriously reduce
or even eliminate not only conscious but unconscious perceptual influences as well. In
short, objective threshold effects should be intrinsically weak or absent, especially ondirect tasks. Accordingly, it would seem that objective threshold approaches must make
the strong assumption that direct measures are sensitive only to conscious perceptual
influences (cf. Reingold & Merikle, 1990viz. the exclusiveness problem). Conversely, if
reliable objective threshold effects are indeed obtainable, this would seem to indicate that
direct tasks actually are insensitive to unconscious influences. If this is true, then
subjective threshold effects on direct tasks must be due to conscious rather than
unconscious influences, which imply that subjective threshold methods are invalid. In a
nutshell, subjective and objective threshold methods appear to make conflicting
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assumptions about unconscious influences on direct tasks, which in turn suggests that the
two approaches cannot both be valid.
To resolve this dilemma, Merikle and Daneman, 2000) and Merikle, Smilek, andEastwood 2001) have recently suggested that both subjective and objective threshold
approaches have produced reliable results, and that the latter are simply more conservative
versions of the former. This amounts to the claim that putatively objective threshold
effects are, in reality, weak subjective threshold effectsthat is, not really objective
threshold effects at all. In other words, Merikle and associates explain objective threshold
effects by invoking the null sensitivity problem, arguing that true direct task performance
in objective threshold paradigms actually does exceed chance, despite obtained null
sensitivity. They further imply that subjective and objective threshold paradigms produce
similar (i.e. not qualitatively different) effects, although they provide no specific support
for this claim.
If Merikle and associates are correct, there would be good reasons to simply abandon
objective threshold approaches (cf. Merikle & Reingold, 1998). For example, whereas
subjective threshold effects are easily obtainable, some argue that reliable, genuine
objective threshold effects are difficult (e.g. Greenwald, 1992; Holender, 1986) or even
impossible (Merikle & Reingold, 1998) to obtain, or are intrinsically very short-lived and
thus obtainable only under highly speeded conditions (Draine & Greenwald, 1998). If
objective threshold approaches indeed suffer from these limitations, it would support the
contention that they are unnecessarily conservative, making it more difficult to study
unconscious perception. Further, one could argue that objective threshold approaches are
inherently wrongheaded in that they emphasize behavioral performance, whereas
subjective threshold approaches appear to deal straightforwardly with phenomenal
experience itself (Jack & Shallice, 2001; Merikle & Daneman, 2000; Merikle et al., 2001).
With all these considerations in mind, one might conclude that objective thresholdapproaches are at best superfluous and at worst obscure the very phenomenon they seek to
elucidate.
In this paper, we present evidence for reliable unconscious perceptual influences on
direct task performance under objective threshold conditions, extending the paradigm first
used by Snodgrass et al. (1993a) and replicated by Van Selst and Merikle (1993). We
present both new experiments and meta-analyses of the original and new experiments.
Contrary to the position outlined in the preceding paragraph, the current findings will
suggest that objective threshold effects are reliably obtainable, are not very short-lived,
and are not merely weak, overly conservative subjective threshold effects in disguisein
short, that objective and subjective threshold approaches may index qualitatively distinct
phenomena. Before proceeding, however, it is useful to further examine the issuesconcerning unconscious perceptual influences on direct tasks.
4. Direct measures, unconscious perceptual influences, and the current paradigm
In the current paradigm, SDT detection is the conscious perception index, and forced-
choice identificationalso a direct taskis the unconscious perception index. Although
using direct tasks to index unconscious influences is common (indeed, canonical) in
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subjective threshold approaches, objective threshold approaches typically use indirect
tasks for this purpose. Indeed, it may seem particularly paradoxical to seek unconscious
influences on identification given that it is often used to index conscious perception (e.g.Cheesman & Merikle, 1984). Recalling, however, that direct tasks may be sensitive to both
conscious and unconscious perceptual influences, identification can be used to index either
kind of influence, depending on the research interest and design. For example, if
demonstrating unconscious influences on semantic priming tasks (cf. Cheesman &
Merikle, 1984) is of interest, identification is a perfectly good conscious perception index
because any semantic effect driven by consciously perceived information requires at least
partial stimulus identification (i.e. identification is exhaustively sensitive in this context).
Accordingly, one could infer unconscious semantic priming effects if they occurred
despite null identification sensitivity.
Analogously, where, as in the current paradigm, demonstrating unconscious influences
on identification is of interest, such influences can be inferred if nonzero identification
occurs despite null detection sensitivity, provided that SDT detection is exhaustively
sensitive to all conscious perceptual influences on identification. There are strong reasons,
discussed below, to believe that detection is relevantly exhaustively sensitive in this
context; for now, we simply note that detection is widely agreed to be the most sensitive
conscious perception index of all.
4.1. Intentional versus nonintentional influences
But how, exactly, could unconscious perceptual influences manifest on direct
measures, and how could they be distinguished from conscious perceptual influences?
After all, it seems most parsimonious to simply attribute any obtained direct effects to
the latter, especially given that direct instructions straightforwardly request thatparticipants utilize their conscious perceptions to perform the task. Further, as we
have seen, it seems very difficult to definitively rule out alternative weak conscious
perception accounts, even with objective threshold approaches (cf. null sensitivity
concerns).
If direct task performance is indeed based on conscious perception, however, one might
further expect that participants should be able to selectively control their responses to the
relevant stimuli, and further that performance should be maximal when effortful attempts
to utilize available conscious perception are made. With this in mind, unconscious
perception advocates have generally responded by attempting to distinguish between
qualitatively distinct intentional and nonintentional influences on direct task performance.
That is, although direct performance is clearly intentional in the broadest sense (i.e.participants attempt to comply with the experimental instructions), it might be that the
relevant direct effects are not due to intentionally guided choices based on weak but
conscious perceptions, but rather result from intrinsically uncontrollable, priming-like
influences on direct response selection. Along these lines, Merikle and associates have
presented evidence (e.g. Cheesman & Merikle, 1986; Merikle & Joordens, 1997a,b;
Merikle, Joordens, & Stolz, 1995) suggesting that responses to subjective threshold stimuli
cannot be consciously controlled, and infer accordingly that such stimuli are indeed
unconsciously perceived.
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Similarly, on the objective threshold side, Marcel (1983a, Experiments 1 and 2) found
that unconscious influences on direct tasks occurred only when participants adopted a
passive approach, and moreover that such effects could not be consciously controlled (seealso Dixon, 1981, pp. 9394). With this in mind, in the current paradigm we sought to
examine the influence of task strategy on direct task performance. All participants used
two strategies: In the look (intentional) strategy, participants attempted to base their
identification responses on whatever they could see, whereas in the pop (nonintentional)
strategy participants responded with the first word that popped into their heads. In this
way, although direct instructions were used throughout, any mediating influences of
intentional versus nonintentional task strategies could still be examined. Finally, we also
examined strategy preference, an individual difference measure. Reasoning that
participants attitudes toward the strategies might be important, we asked them which
of the two they preferred or liked better.
5. The original experiments
In the original experiments (Snodgrass et al., 1993a, Experiments 1 and 2; replicated by
Van Selst & Merikle, 1993, Experiments 1 and 2), Preference!Strategy interactions were
repeatedly obtained; there were no main effects. The pooled data are presented in Table 1.
Simple effects analyses indicated that the interaction was primarily carried by a Preference
congruity effect in the pop strategy: look preference participants (lookers) repeatedly
performed below chance (the inhibition effect), whereas weaker evidence suggested that
pop preference participants (poppers) performed above chance.
It is useful to briefly discuss key aspects of the original experiments findings to set the
stage for the current experiments. On the one hand, the weak look strategy findings suggest
that direct tasks may indeed be exclusively sensitive to conscious perceptual influences
when intentional response strategies are used. In contrast, unconscious perceptual
influences readily emerged under nonintentional pop instructions. Notably, however, the
form of the pop strategy findings was highly unusual. Rather than producing greater
overall facilitation, pop instructions produced both facilitation and (especially) inhibition,
mediated by Preference. Moreover, overall performance collapsed across both Preference
and Strategy was at chance (XZ24.96; chanceZ25%). This is quite striking; normally,
putatively unconscious perceptual effects, whether direct or indirect, exhibit overall
Table 1
Performance by Preference and Strategy from Snodgrass et al. (1993a; Experiments 1 and 2); Van Selst and
Merikle (1993; Experiments 1 and 2)pooled data
Strategy
Preference Pop Look
Pop (nZ48) 26.55a (4.11) 24.27 (4.34)
Look (nZ40) 23.31a (3.50) 25.52 (3.51)
Standard deviations and ns are in parentheses. Mean performance is percentage correct collapsed across
individual word (chanceZ25).a The 95% confidence intervals of these means do not include 25 (chance).
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deviations from chancein particular, facilitation. With this in mind, the looker inhibition
effect is especially notable; it seems doubly nonintentional, manifesting both only under
pop instructions and counter to participants intentions to facilitate (i.e. uncontrollable),perhaps providing particularly strong evidence against alternative weak conscious
perception accounts.
5.1. Unidirectional versus bidirectional effects
As Katz (2001) explained, psychological research has concerned itself almost
exclusively with unidirectional effectsthat is, effects where a manipulation produces
changes on overall means, whether in a facilitatory or inhibitory direction. However, it is
also possible for a manipulation to affect the variance rather than the meanthat is,
produce bidirectional effects. Such effects imply the presence of underlying interactions
producing offsetting facilitation and inhibition, and will be missed unless the variance or
mediating factors are examined. Finally, a manipulation might produce both unidirectional
and bidirectional effects; the two kinds of effects are statistically independent. Here, for
example, overall identification could have exceeded chance and Preference!Strategy
interactions could have been obtained. Notably, however, the original findings are
exclusively bidirectional, with no hint of a unidirectional component. This contrasts
sharply with the powerful undirectional influences readily evident on direct tasks under
subjective threshold conditions, and could moreover explain why objective threshold
effects have been difficult to replicate on direct tasks (cf. Holender, 1986). Such
nonreplications (e.g. Nolan & Caramazza, 1982) examined only undirectional effects;
moreover, unlike Marcel (1983a), they did not ensure that nonintentional task strategies
were used.
5.2. Evidence for objective detection threshold status
In the original investigations, detectability was assessed in separate experiments. Van
Selst and Merikle (1993, Experiment 3) replicated our procedures exactly, replacing
identification with SDT detection. They concluded (p. 201) that null detection sensitivity
(overall XZ49.3%) was achieved; further, Preference and Strategy had no effect. Their
findings are particularly compelling given the large number (240) of trials, and replicated
our own null detection results (Snodgrass et al., 1993a, Experiment 3). Moreover, by
conducting the detection and identification tasks under identical conditions, they showed
that the experimental effects were not due to some general increase in visibility caused by
pop instructions.
6. The current experimentsrationale and overview of organization and
presentation
Although the original experiments were promising, we thought further replication of
these effects advisable for two reasons: (1) objective threshold effects remain controversial
(cf. Merikle & Reingold, 1998; Perruchet & Vinter, 2002), perhaps especially using direct
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tasks (cf. Holender, 1986); and (2) their exclusively bidirectional nature is highly unusual,
perhaps reflecting important qualitative differences including a particularly powerful
demonstration of uncontrollability, a widely popular criterion for distinguishingunconscious versus conscious influences.
Because the current (as well as the original) investigations used very similar
procedures, we begin with a General Method section. We then present Experiments 1a and
1b. Although these experiments replicated the primary identification task findings, they
suggested important additional modifications as well. To evaluate the reliability of these
new (as well as the primary) findings, we then present a meta-analysis of these six early
experiments (i.e., Snodgrass et al., 1993a, Experiments 1 and 2; Van Selst & Merikle,
1993, Experiments 1 and 2; the current Experiments 1a and 1b). To anticipate, it will
emerge that both the primary and additional findings are reliable.
However, even if their reliability is assumed, problematic methodological issues
remain. We then derive a novel prediction, which addresses these issues empirically. This
key prediction is tested in Experiment 2, and further evidence suggesting that the current
effects are unconscious is obtained. Following a final meta-analysis (adding Experiment
2), we suggest an interpretation for their bidirectional form. Finally, we address broader
implications in the General discussion.
7. General method
7.1. Participants
University students were paid $1015, depending on the particular study. Allparticipants had normal or corrected-to-normal vision and were native English speakers.
7.2. Apparatus and materials
Words were presented for 1 ms in a Gerbrand Model T3-8 Tachistoscope. Luminance
was 10 fL for the stimulus field, fixation field, and ambient room light. Participants
initially fixated a black dot on an otherwise blank stimulus card. The stimulus field
(containing the word) was then flashed for 1 ms, followed immediately by the fixation field
once more. Luminance levels were constant throughout; there were no dark fields, thus
obviating any lighting adaptation confounds (cf. Holender, 1986). Further, pattern
masking was not used. Instead, the stimuli were unmasked; with such procedures,extremely brief stimulus durations are necessary to prevent stimulus detection. The four
stimulus words were balanced for frequency (Kucera & Francis, 1967) and reflected
pleasant and unpleasant emotional connotations. Pleasure and Rose were the pleasant
words; Fighting and Pain the unpleasant ones (from evaluative norms Osgood, May, &
Miron, 1975). The four words also reflected a structural dimension (two long, two short).
The words were printed on white 4!6 cards in Helvetica Light 18 pt. press type. The
viewing distance was 75 cm; the visual angle ranged from 1 to 2.58, well within foveal
resolution.
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7.3. Procedure
Participants were tested in 112 h sessions, and were informed that we were studyinghow well they could identify briefly presented words. We informed them of the words
identities and that each word would be presented an equal number of times in random
order. They were told that the task was difficult, but to respond even if they saw nothing. It
was sometimes necessary to reassure participants that stimuli were actually being
presented, because they usually felt that nothing at all was occurringno flash, no mask,
no apparent change in the visual display at all. Following an experimenter-provided cue,
participants focused on the fixation point, the stimulus was flashed, and they provided their
4AFC identification response. Participants were also told that we wanted to examine the
effects of two task strategies. Look instructions asked them to look very hard where the
word is presented, around the black dot, for anything you can see. Conversely, pop
instructions asked participants to relax and just look where the word is presented and say
whatever word pops into your head (constrained by the forced-choice task).
After a 24-trial practice block, participants completed five blocks of 24 trials each per
strategy; the two strategy conditions were blocked and counterbalanced. Each word
appeared six times per block in randomized order, but no word appeared more than twice
consecutively. There were thus 120 trials per strategy condition (30 per word), 240 in all.
The experimenter was blind to the stimulus cards identities. Computer-generated
performance feedback was given after each block. Following all experimental trials, we
asked participants which of the two strategies they preferred or liked better, thus yielding
the strategy preference variable. Finally, our dependent variable was percentage correct
identification averaged across the four stimuli.
8. Experiments 1a and 1b
In Experiment 1a, we additionally collected event-related potential (ERP) brain wave
measurements from participants while engaged in our paradigm; these ERP results are
presented elsewhere (Shevrin, Snodgrass, Kushwaha, & Bernat, 2002). In Experiment 1b,
we included female participants; our previous experiments used only males. For both
experiments, we predicted that lookers would inhibit under pop instructions, and
secondarily that poppers might facilitate under pop instructionsthat is, Preference!
Strategy interactions similar to the original experiments. All P-values are two-tailed unless
otherwise noted. Given the original experiments evidence that our exposure conditions
met the objective detection threshold, the current Experiments 1a and 1b did not includedetection tasks.
8.1. Participants, apparatus and materials, and procedure
Experiment 1as had 17 male participants; 1b had 40 female participants. Experiment
1a procedure differed slightly in that participants were urged to keep still to avoid
contaminating the ERP measurements with muscle artifacts. Experiment 1bs procedures
were unchanged.
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8.2. Results
The means for Experiments 1a and 1b are presented in Table 2. Most importantly,the inhibition effect replicatedlookers performed below chance (25%) under pop
instructions [Experiment 1a: t(8)Z2.16, P!.04 (one-tail); 1b: t(17)Z2.06, P!.03
(one-tail). Unlike the original experiments, however, poppers did not facilitate under
pop instructions in either Experiment 1a [both Fs!1]. Moreover, lookers
unexpectedly facilitated under look instructions [Experiment 1a: t(8)Z2.13, P!.07;
1b: t(17)Z3.77, P!.001]. Thus, the Preference!Strategy interaction [Experiment 1a:
F(1,15)Z1.34, ns; 1b: F(1,38)Z14.04, P!.001] now seemed mainly due not only to
looker inhibition under pop instructions, but looker facilitation under look instructions
as wellthat is, a looker simple effect. Finally, some evidence for a Strategy main
effect emerged [Experiment 1a: F(1,15)Z7.90, P!.02; 1b: F(1,38)Z2.94, P!.10],
suggesting a general tendency for pop inhibition and look facilitation. This main
effect seems secondary, however, being carried essentially entirely by the looker
simple effect.
8.2.1. Preliminary meta-analyses
Although the key looker inhibition effects replicated prior results, other findings seemed
to differ in certain respects. To clarify these ambiguities, a meta-analysis was conducted
with the available datathe four original experiments (Snodgrass et al., 1993a,
Experiments 1 and 2; Van Selst & Merikle, 1993, Experiments 1 and 2) and the current
Experiments 1a and 1b. Thanks to Van Selst and Merikle making their raw data available,
we combined the data for all six experiments directly, which Hedges and Olkin (1985, p. 9)
noted is ...the best possible case [for meta-analytic cumulation]. Further, experiment was
initially included in the model to test for heterogeneity; it was dropped because it was
nonsignificant throughout. This finding is important because it means that the apparent
differences between the original and current experiments (e.g. the apparent absence versus
presence of the looker facilitation effect) were likely not genuine, but rather due to sampling
error. Finally, meta-analytic P-values are two-tailed unless otherwise stated.
Table 2
Performance by Preference and Strategy from Experiments 1a and 1b
Experiment 1a Experiment 1b
Strategy Strategy
Preference Pop Look Pop Look
Pop 24.79 (4.49) 26.67 (4.56) 25.00 (4.45) 22.92 (5.29)
(nZ8) (nZ22)
Look 22.53a (3.34) 26.94 (2.73) 22.78a (4.58) 28.38b (3.79)
(nZ9) (nZ18)
Standard deviations and ns are in parentheses. Mean performance is percentage correct collapsed across
individual word (chanceZ25).a These means are below chance (P!.05) by a priori contrasts.b The 95% confidence interval of this mean does not include 25 (chance).
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The key looker inhibition effect under pop instructions was reliable, F(1,66)Z17.66,
PZ8.12!10K5. Interestingly, looker facilitation under look instructions was also
reliable, F(1,66)Z10.92, PZ002. The Preference!Strategy interaction was highlyreliable, F(1,143)Z29.27, PZ2.58!10K7, and was carried primarily by the looker
simple effect, as noted above. On the other hand, popper facilitation under pop instructions
was marginal, F(1,77)Z3.70, PZ.06, as was the Strategy main effect, F(1,143)Z2.81,
PZ.09.
9. Discussion
Experiments 1a and 1b and the preliminary meta-analysis indicated that the
Preference!Strategy interaction, including the looker inhibition effect, was quite reliable.
With the benefit of this additional data, however, the form of this interaction was clarifiedimportantly. Whereas it originally appeared that this interaction was carried by a pop
strategy simple effect, inclusion of Experiments 1a and 1b suggest that it is best
characterized as a looker simple effect, with little indication of popper effects in either
strategy. At the same time, other key features of the obtained effects remained constant
specifically, they are exclusively bidirectional (the overall XZ24.84 did not differ from
chance) and congruent with Preference.
However, the emergence of looker facilitation under look instructions raises important
questions. On the one hand, if this facilitation is produced by unconscious perception, it
suggests that even maximally direct measures (i.e. both direct and intentional) are indeed
sensitive to unconscious perceptual influences, contrary to what the original experiments
suggested. On the other hand, one might skeptically wonder whether this facilitation was
simply caused by weak conscious perception, despite the previously established nulldetection sensitivity. If so, one might also wonder whether the key looker inhibition effect
was somehow conscious after all.
10. Methodological problems in unconscious perception research
Questions such as these return our attention to unresolved methodological issues in
unconscious perception researchmost importantly, ruling out alternative weak
conscious perception explanations. There are two components to this difficultythe
exhaustiveness and null sensitivity problems (Reingold & Merikle, 1988, 1990). The
exhaustiveness problem proper concerns whether the conscious perception index taps therelevant kindof conscious perceptionthat is, any and all conscious perception that could
explain putatively unconscious perception index effects. On the other hand, the null
sensitivity problem is a matter of degreethat is, one could have the right conscious
perception index, but still be unable to demonstrate that the relevant conscious perception
has been completely eliminated due to intrinsic measurement error. Conversely, even if it
were possible to convincingly demonstrate null sensitivity, this alone would not suffice
unless it were further demonstrated that the conscious perception index is relevantly
exhaustive. Finally, the exclusiveness problem apparently implies that objective and
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subjective threshold paradigms cannot both be valid (see Theoretical Tension section
above).
With the Reingold and Merikle critique in mind, then, one could question SDTdetections exhaustiveness, despite its intuitive plausibility, simply because detection and
identification are different measures. Indeed, this was Van Selst and Merikles (1993) main
criticism of our paradigm, and is consistent with Reingold and Merikles (1990) view that
any difference at all between the conscious and unconscious perception indexes is
sufficient to raise insuperable exhaustiveness concerns. Further, null sensitivity concerns
suggest that true detection sensitivity could conceivably have actually exceeded chance.
These two concerns amount to the familiar worry that putatively unconscious effects may
actually be weakly conscious. Finally, following the exclusiveness problem logic, one
could assert that our findings are prima facie impossible, or at least seriously
underestimate the true effects. Accordingly, we now briefly present our methodological
analysis (for a full discussion, see Snodgrass, in press; Snodgrass et al., 2004a),
culminating in a design change for Experiment 2, which allows a direct test of these
skeptical concerns.
11. Inferring unconscious perception by falsifying the conscious-perception-only
model
The core of the dissociation paradigm logic is that higher-level effects (e.g. semantic
processing) should not be possible in the absence of lower-level effects (e.g. stimulus
detection) if only conscious perception is involved. This is so because higher-level effects
require more stimulus information (and stronger stimuli; see below) than lower-level
effects; indeed, the former are built upon and presume the latter. For example, semanticanalysis cannot occur unless the stimuli are at least partially identified; in turn, stimulus
identification cannot occur without some degree of stimulus detection. In short, the
dissociation paradigm logic assumes that conscious perception functions on a hierarchical
strength/complexity continuum, such that greater stimulus intensity is required in order for
more complex effects to occur. Marcel (1983b) called this the Identity Assumption, which
holds that conscious percepts reflect the highest level of analysis (as well as the
constitutive lower levels) achieved by the stimuli.
Accordingly, the two core predictions of the conscious-perception-only model are: (1)
lower-level and higher-level effects should correlate positively, and (2) higher-level
effects should disappear when lower-level effects approach zero. Thus, when higher-level
effects are obtained despite chance performance on relevant lower-level measures, theIdentity Assumption is violated, and inferences for an additional, unconscious perceptual
process are supported. Conversely, the reverse pattern (e.g. above-chance detection but no
semantic effects) can be explained positing only a single, conscious perceptual process.
This is why Marcels (1983a) results were controversialabsent the Identity Assumption,
no one would have been surprised in the first place.
These considerations further clarify why unconscious perception indexes can be either
indirect or direct; what is crucial is that they tap higher levels of processing than the
conscious perception index. Indeed, subjective threshold approaches employ analogous
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reasoning; they assume that participants denial of awareness is a basic, less complex
judgment which should rule out conscious perceptual explanations for more complex,
higher-order effects. Accordingly, using different measures to index conscious andunconscious perception can be a virtue, not a liability, provided that the proper
hierarchical strength/complexity relationships are observed.3
11.1. So which measures are exhaustive?
The above logic has implicitly guided the choice of conscious perception indexes in
almost all dissociation paradigm experiments, and implies an exhaustiveness hierarchy
such that identification is exhaustively sensitive to semantically-relevant conscious
perception, and that detection is exhaustively sensitive to identification-relevant conscious
perception. Available empirical evidence supports the proposed hierarchy; it turns out that
objective identification thresholds are below (i.e. require more stringent exposureconditions than) objective semantic classification thresholds, and in turn that objective
detection thresholds are below objective identification thresholds (see Snodgrass et al.,
2004a for a review). Further, these hierarchical relationships are inherent in basic
cognitive theoryfor example, in standard word recognition models (cf. McClelland,
1987; McClelland & Rumelhart, 1981).
11.2. Specific evidence for SDT detections exhaustiveness
SDT models also support the proposed exhaustiveness hierarchy. For example, and
most importantly for the current paradigm, SDT holds that forced-choice identification
simply is multidimensional detection (Green & Birdsall, 1978; Macmillan & Creelman,1991), rather than being some unrelated, incommensurate task as Reingold and Merikle
(1988, 1990) imply (see Fig. 1A). Crucially, because identification is the multidimensional
distance between the detection vectors, at least one stimulus must be detectable for
nonzero identification to occur. Thus, although identification can exceed detection with
sufficiently orthogonal stimuli, this is only possible with nonzero detection. Thus, SDT
holds that detection is exhaustively sensitive to identification-relevant perception. This is
why Macmillan (1986, p. 39) concluded that Above-chance recognition [i.e.
identification] performance.when detection d0Z0 would be, for almost everyone,
persuasive evidence for subliminal perception.
Further, the fact that objective detection thresholds are below objective identification
thresholds (see, e.g. Dagenbach, Carr, & Wilhelmsen, 1989) suggests that typical wordstimuli reflect highly correlated rather than orthogonal stimulus dimensions, and
accordingly that overlapping and thus nondiscriminative lower-level stimulus information
(e.g. darkness) can support detection but not identification. In contrast, identification will
exceed zero only when nonoverlapping information is perceived (cf. Triesman, 1999).
3 Although it is possible to seek evidence for unconscious perception by contrasting performance on the same
task across direct and indirect instructions, the few experiments using this approach have been unsuccessful
(Reingold & Merikle, 1988).
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Fig. 1. (A) SDT two-dimensional representation of identification discriminations, illustrating identifications
dependence on and derivation from detection. The three circles represent the two-dimensional distributions
of the noise only, A, and B stimuli. Each circle is one SD away from the mean of that distribution.
Identification d0
(d0
1;2) is the distance between the detectability of stimulus A (d0
1) and B (d0
2). This distancedepends on both the magnitudes of d01 and d
0
2 and the correlation between the stimulus A and B dimensions.
An idealized case is depicted here, wherein d01 and d0
2 are equal and the stimulus A and B dimensions are
orthogonal. (B) Here, a more realistic depiction of the relationship between detection and identification is
given, reflecting the highly correlated (i.e. nonorthogonal) nature of typical word stimuli. Consequently,
identification d0 is smaller than detection d0. Moreover, at low levels of detection the stimulus A and B
dimensions overlap completely. The internal dotted lines depict this situation, where identification d0
exceeds zero only at substantial levels of detection d0.
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This more realistic nonorthogonal situation is depicted in Fig. 1B. Once again, then,
identification should not be possible when detection d0Z0 if only conscious perceptual
influences are at work.
11.3. Application to the exhaustiveness, null sensitivity, and exclusiveness problems
Notably, the SDT model predicts that detection and identification should be strongly
and positively correlated. With detectable stimuli, this is exactly what occurs; for example,
reanalysis ofHaase, Theios, and Jenisons (1999) data (mean detection d0Z.61) revealed a
correlation ofC.84. On the other hand, if detection is not exhaustively sensitive, either a
weak positive correlation (if imperfectly exhaustive) or no relationship (if detection and
identification indexed completely distinct forms of conscious perception) should occur.
Similarly, the null sensitivity problem posits that putatively unconscious effects are
actually weakly conscious, and thus that they would disappear if true null sensitivity was
actually attained. With this in mind, it also predicts that the conscious and unconscious
perception indexes should be positively correlated. Analogously, the exclusiveness
problem also predicts a positive correlation (cf. Merikle & Reingold, 1998, p. 309).
Otherwise, attaining null sensitivity on the conscious perception index would not reduce
unconscious perception index effects, as feared.
12. The fundamental qualitative difference
Given the above, skeptical concerns all make the same crucial predictionthat the
conscious and unconscious perception indexes should correlate positively, or at least
nonnegatively. More precisely, these alternative conscious-perception-only accountspredict that the size of the ostensibly unconscious effect (i.e. its absolute deviation from
chance, whether positive or negative) should correlate positively, if at all, with the
conscious perception indexand, in particular, that this should occur as stimulus intensity
begins to exceed the relevant objective threshold. Accordingly, the most convincing
evidence for an additional, unconscious perceptual process would accrue if the two
indexes were negatively, rather than positively, correlated (for similar reasoning, see
Eimer & Schleghacken, 2002; Klapp & Hinckley, 2002; Macleod, 1998). Indeed, we
suggest elsewhere (Snodgrass, 2004b; in press; Snodgrass et al., 2004a) that such negative
relationships provide strong qualitative differences because they directly contradict
alternative conscious perception accounts. In contrast, positive relationships (e.g. finding
partial word effects with weak stimuli versus whole-word effects with strong stimulicf.Abrams & Greenwald, 2000) yield only weak qualitative differences because they are
interpretable in terms of weak versus strong conscious perception.
13. Experiment 2
The primary change in Experiment 2 was the addition of an SDT detection task
following the unconscious perception index (identification) trials. By having the same
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participants provide both detection and identification information, we could test the
crucial skeptical prediction just described. If a positive correlation is obtained,
alternative conscious perception accounts become much more plausible. Conversely,finding a negative relationship would contradict such skeptical accounts, suggesting
instead that the Preference!Strategy interaction is indeed due to unconscious perceptual
processes. Further, finding any relationship would also imply that the conscious
perception index was not simply invalid or insensitiveif so, no relationship should
occur.
Additionally, Experiment 2 changed when preference ratings were obtained. In all
previous work, this was done following all identification trials. Because performance
feedback was provided after each trial block, however, it was possible that preference
ratings were influenced by this feedback, rather than reflecting independently existing
attitudes towards the strategies. If so, results involving Preference could be artifactual.
Although previous control experiments using blank stimuli and bogus feedback(Snodgrass et al., 1993a, Experiment 3; Van Selst & Merikle, 1993, Experiment 2)
indicated that this was not the case, we felt the definitive control was to obtain preference
ratings before the identification trials and without performance feedback. If these early
preference ratings produced similar results, using the usual, late preference ratings
would be justified, which were likely more accurate given participants greater experience
with the strategies. Finally, Experiment 2 was also a large-scale replication, enabling
further checks on the reliability of the just-clarified looker facilitation effect, the crucial
inhibition and Preference!Strategy effects, and previously established null detection
sensitivity.
13.1. Participants, apparatus and materials, and procedure
Forty-nine males and 50 females participated. Six additional participants were dropped
because they did not follow instructions (e.g. showing extreme response biases). Three
changes from our standard procedure were introduced. First, to obtain the early
preference rating, participants used the strategies (pop and look, 12 trials each) on the 24-
trial practice block. After the practice trials, the usual preference inquiry was given. No
feedback was provided to either the participants or the experimenter regarding practice
block performance. Following the experimental trials, we additionally obtained late
(standard) strategy preference, as usual.
Second, following the identification trials, participants underwent a 32-trial SDT
detection task with 16 word (the four words presented four times each) and 16 blankcard trials; stimulus order was completely randomized. Participants said whether a
word or blank was presented on each trial; identifications were not requested.
Participants were informed that words and blanks were presented with equal
probability, and were urged to keep this in mind as they responded (i.e. to respond
yes and no roughly equally). Third, identification stimulus order was now
completely randomized within blocks (i.e. stimulus repetition constraints used in the
prior experiments were dropped), eliminating any possible extraneous influence of
such constraints.
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13.2. Results
13.2.1. Identification
Early preference and late (standard) preference results were similar (seeTable 3). The
early Preference!Strategy interaction was reliable, F(1,97)Z4.01, P!.05, as was the
Strategy main effect, F(1,97)Z7.62, P!.01. Early preference lookers inhibited under pop
instructions, F(1,31)Z4.81, P!.04, and facilitated under look instructions, F(1,31)Z
6.33, P!.02. We therefore concluded that previous Preference-related effects were not
artifactual and used late (standard) preference in subsequent analyses. The standard
Preference!Strategy interaction held, F(1,97)Z8.85, P!.004; as did the Strategy main
effect, F(1,97)Z8.27, P!.005. Critically, lookers inhibited under pop instructions,F(1,37)Z12.35, P!.001, and also facilitated under look instructions, F(1,37)Z5.03, P!
.04. However, poppers did not facilitate under pop instructions, F(1,60)Z1.00, ns. These
findings replicate the critical inhibition and novel facilitation effects, and again suggest
that the Preference!Strategy interaction was carried by the looker simple effect. With this
in mind, the Strategy main effect again seems secondary.
13.2.2. Detection
Mean response bias was minimal (cZ.02), indicating that participants indeed
distributed their yes and no responses equally. Hit rates ranged from .19 to .94;
false alarms from .06 to .81. Because no zeros were obtained for the hits and false alarms,
correction formulas were unnecessary, and d0
could be computed in the usual manner.Overall detection performance (d0Z.05, SDZ.53) did not exceed chance, t(99)!1.
Further, considered separately, neither poppers (d0Z.07, SDZ.57) nor lookers (d0Z.03,
SDZ.47) exceeded chance (both ts!1). Thus, the current effects are indeed at the
objective detection threshold, supporting the conclusion that both the looker facilitation
effect and the inhibition effect are unconscious. As described above, however, examining
mean detection d0 alone still leaves room for skeptical doubts (e.g. null sensitivity
concerns). Accordingly, we conducted a regression analysis which showed that d0 was
indeed negatively related to the Preference!Strategy interaction, contradicting skeptical
Table 3
Performance by Early or Late (standard) Preference and Strategy from Experiment 2
Preference rating type
Early Late (standard)
Strategy Strategy
Preference Pop Look Pop Look
Pop 25.04 (3.96) 25.48 (4.38) 25.50 (3.95) 25.45 (4.29)
(nZ67) (nZ61)
Look 23.54a (3.76) 26.35a (3.04) 23.03a (3.46) 26.27a (3.50)
(nZ32) (nZ38)
Standard deviations and ns are in parentheses. Mean performance is percentage correct collapsed across
individual words (chanceZ25).a The 95% confidence intervals of these means do not include 25 (chance).
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conscious perception predictions. For economy of exposition, we present this regression
analysis, along with discussion of its methodological and substantive implications, in the
General discussion below.
13.2.3. Final meta-analyses
As before, experiment was initially included in the model to test for heterogeneity, but
was again dropped because it was nonsignificant throughout, again indicating that any
apparent differences between experiments were due to random rather than systematic
factors. Table 4 presents the grand means collapsed across Experiment. Final meta-
analyses including Experiment 2 (seven experiments in all) showed that looker inhibition
under pop instructions was quite reliable, F(1,104)Z30.04, PZ2.98!10K7. Lookers also
facilitated under look instructions, F(1,104)Z16.07, PZ1.14!10K4. Poppers facilitated
weakly under pop instructions, F(1,138)Z4.53, PZ.035. The Preference!Strategy
interaction was very reliable, F(1,242)Z36.23, PZ6.45!10K9, and the Strategy main
effect was reliable, F(1,242)Z10.15, PZ.002.Further, when all detection findings were cumulated (Snodgrass et al., 1993a,
Experiment 3; Van Selst & Merikle, Experiment 3; the current Experiment 2), overall
detection performance (7, 248 trials in all) was 49.9%, clearly at chance. This average is
presented as percentage correct (PC) rather than d0 because Van Selst and Merikles
detection data were preserved only in the former form. Given the virtual absence of
response bias in their (and our) data, PC is an adequate detection index.
Clearly, then, the current effects are reliable and occur at the objective detection
threshold. Further, they are substantive despite appearing small in raw units. For
example, expressed as Cohens d, the effect size for the overall Preference!Strategy
effect is .77. Examining the main constituents of this interaction, Cohens dZK.54
for looker inhibition under pop instructions, and dZ.39 for looker facilitation underlook instructions. According to Cohens rules of thumb, these are moderate-to-large,
not small, effects. By comparison, many standard cognitive effects also appear small
in raw units (e.g. 3050 ms. semantic priming effects), but also possess moderate
effect sizes. Moreover, it is unlikely that unpublished null findings would substantially
affect the current conclusions. For example, Rosenthals Fail-safe N for the overall
Preference!Strategy effect is 83, meaning that 83 samples (with about 35 participants
each) with null findings would have to exist in order to render this effect
nonsignificant.
Table 4
Performance by Preference and Strategypooled data from the original and current experiments
Strategy
Preference Pop Look
Pop (nZ139) 25.74 (4.12) 24.71 (4.55)
Look (nZ105) 23.06 (3.64) 26.40 (3.59)
Standard deviations and ns are in parentheses. Mean performance is percentage correct (chanceZ25). See text
for significance levels.
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14. Discussion
Here, we propose an explanation for the Preference!Strategy interaction; moregeneral implications are addressed in the General discussion below. As perusal ofTable 4
indicates, the overall pattern suggests a Preference/Strategy congruity effect. Given this
pattern, the obtained effects may reflect an unconscious attribution process. To begin with,
the stimulus presentation on each trial presumably increases that words activation relative
to the other response alternatives. However, whether this activation is attributed correctly
to the words presentation depends on participants attitudes toward the strategies. When
utilizing the strategy congruent with their preference, perhaps participants unconsciously
allow this activation to influence their response, elevating performance above chance. In
contrast, when utilizing the incongruent strategy, such influences are unconsciously
rejected and below-chance performance ensues. This unconscious attribution explanation
is analogous to those postulated in investigations involving perceptual fluency (e.g.Whittlesea, 1993). The essence of such explanations is that the fluency produced by prior
stimulus presentation is unconsciously attributed to plausible, but not implausible,
sources.
There are, however, several important differences between typical fluency attribution
effects and the current effects. For example, typical fluency stimuli are phenomenally
conscious, albeit with some misdirection and/or diminished attention. At most, subjective
threshold conditions are used. Here, the stimuli are objectively undetectable and thus
unambiguously unconscious. Perhaps relatedly, our participants denied any phenomenal
basis whatsoever for response selection. In contrast, in typical fluency experiments,
participants experience previously presented items as possessing distinctive phenomenal
aspects (e.g. more familiar, clearer, etc.). Further, typical fluency effects are usuallyindirect, involving erroneous attributions of fluency to plausible but irrelevant stimulus
dimensions. In contrast, the current attribution effects occur on a direct identification task.
Finally, in typical fluency experiments below chance performance is rarely, if ever,
present; in contrast, such inhibition is prominent in the current experiments.
Keeping these differences in mind, another (quite speculative) possibility is that looker
inhibition might reflect a simple form of unconscious defense (cf. Snodgrass et al., 1993a).
Along these lines, lookers consistently expressed a strong preference for activity and
control, explaining that they disliked doing nothing as the pop instructions required.
Obliging lookers to relinquish conscious control with pop instructions might instantiate a
mildly conflictual situation, producing inhibition, whereas more congenial look
instructions would not, yielding facilitation. In contrast, poppers expressed various, lessstriking motivations. Some true poppers believed that popping made sense; others
deemed looking futile; still others simply said looking was too much work. In short,
poppers appeared more heterogeneous, perhaps producing weak findings.4 Consistent with
this notion, poppers variance consistently exceeded lookers in our experiments. In
4 We also analyzed the most reliable poppers (i.e. those who consistently expressed preference for the pop
strategy across both the early and late preference assessments) in an effort to obtain stronger popper effects;
results were slightly but nonremarkably stronger.
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keeping with these apparent differences in motivational strength and consistency, the
Preference/Strategy congruity effect was clearly strongest for lookers. At the same time,
although much weaker, poppers displayed the opposite performance pattern, consistentwith both the attribution and defense accounts. Importantly, the core structure of these
accounts is similar; only the underlying motives differ (i.e. maintaining consistency versus
maintaining conscious control). In any event, further work is clearly necessary to clarify
the mechanisms for the current effects.
Notably, both the attribution and defense accounts can explain looker inhibition. In
contrast, standard cognitive inhibitory accounts are implausible because they require
stimulus or response competition to produce and explain inhibitory effects (e.g. Stroop
interference). For example, Milliken, Joordens, Merikle, and Seiffert (1998) showed that
negative priming does not occur without response competition. In our paradigm, however,
there is no stimulus or response competition. Further, Dagenbach and Carrs (1994)
center-surround mechanism seems unlikely because it predicts maximal inhibition withincreased effort and attention. We found the reverseinhibition with pop rather than look
instructions. Moreover, center-surround theory predicts inhibition only for semantically
related associates, not for the target words themselves.
14.1. Strategic factors mediate unconscious influences
In recent years, increasing evidence suggests that unconscious effects, long thought
to be immune to strategic influences, are in fact routinely mediated by them.
Dagenbach et al. (1989), for example, showed that participants failed efforts to
extract specifically semantic information from undetectable primes produced inhibitory
priming effects. Since then, many other strategic influences on ostensibly automaticpriming effects have been demonstrated. For example, semantic priming is severely
curtailed if participants focus on the primes nonsemantic properties (e.g. Smith,
Theodor, & Franklin, 1983; see Maxfield, 1997 for a review). More recently,
Naccache, Blandin, and Dehaene (2002) have shown that unconscious masked priming
requires temporal attention; analogously, Kentridge, Heywood, and Weiskrantz (1999)
found that blindsight performance benefited from temporal cueing. In another
example, Abrams and Greenwald (2000; see also Greenwald, Abrams, Naccache, &
Dehaene, 2003) showed that whether or not a masked prime had been previously seen
as a target mediated its effects.
The current effects add to this previous work in two ways. First, analogous to the
above, a strategic factornamely, whether pop or look strategies were adopted (cf.Dulanys, 1997 evocative versus deliberative distinction)crucially mediated uncon-
scious perceptual influences. Second, the current findings make a relatively novel
contribution in demonstrating the importance of individual differenceshere,
participants strategy preference. Unlike strategic effects, individual difference influences
have hitherto received little study. Clearly, the current results suggest that further
investigation of such influences, and their interaction with strategic factors, is in order.
Indeed, if they had not been examined, no results at all would have been obtained in the
current studies.
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14.2. Unconscious influences at the objective detection threshold are not short-lived
Finally, Greenwald and associates (e.g. Greenwald, Draine, & Abrams, 1996) haveargued that unconscious perceptual influences are extremely short-lived, necessitating
very highly speeded (cf. 400500 ms.) responses in order to capture their effects. In
contrast, in the current paradigm responses were not speeded, and at least several seconds
elapsed on each trial between the stimulus presentation and participants verbal response.5
Accordingly, although we did not time participants responses, it is clear that such
influences are not very short-lived, at least at the objective detection threshold.
15. General discussion
In our view, the current, clearly reliable findings exhibit qualitative differences, whichhave two major implications. First, they provide strong evidence against alternative
conscious perception accounts. Second, they also suggest that objective and subjective
threshold phenomena are likely fundamentally distinct, rather than the former simply
being a weaker version of the latter.
15.1. Evidence contradicting the conscious-perception-only model
Although we have already provided evidence that the Preference!Strategy interaction
was not due to conscious perception, perhaps the clearest and most useful way to rule out
such skeptical concerns is by utilizing the regression approach (cf. Draine & Greenwald,
1998). On the one hand, if conscious perception were actually responsible for thePreference!Strategy interaction, a positive relationship should manifest; moreover, the
interaction should approach zero as null detection sensitivity is approached, producing a
nonsignificant y-intercept. In contrast, negative relationships render conscious perceptual
explanations for the Preference!Strategy interaction much less plausible because the
interaction would diverge from rather than converge to zero as null detection sensitivity is
approached, yielding a significant y-intercept.
The regression approach provides just such information (see Fig. 2). Interaction scores,
plotted on the y-axis, were derived by subtracting each participants nonpreferred strategy
performance from their preferred strategy performance. Accordingly, larger positive
interaction scores indicate greater Preference/Strategy congruity effects. These scores
were regressed onto detection d0, plotted on the x-axis. As Fig. 2 shows, detection was
negatively, not positively, related to the Preference!Strategy interaction. This negativerelationship (rZK.22) was significant, t(98)ZK2.23, PZ.028. Moreover, the y-
5 Recall that in our paradigm, there is no apparent change in the visual display throughout the entire trial (i.e. no
flash, mask, brightness change, etc.). Accordingly, in our procedure the experimenter first alerted participants that
a stimulus presentation was immanent, participants signaled their readiness, the stimulus was presented,
participants were informed that the stimulus had just been presented, and finally participants responded. Thus,
participants could not respond until after being told that that the stimulus had just been presented, creating a built-
in delay of several seconds on each trial.
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interceptthat is, the point estimate of the interaction where detection d0Z0was
significant; t(98)Z2.55, PZ.012.
These results are particularly powerful because they are not subject to the usual criticisms
of the regression method. First, unlike many applications (e.g. Draine & Greenwald, 1998),
here there is a relationship between the conscious and unconscious perception indexes, thus
allowing clearly valid use of the regression equation for predictive purposes (e.g. y-intercept
estimation). Further, in situations where this relationship is negative and the means of both
indexes are positive, as they are here, measurement error in the predictor underestimates y-
interceptsexactly the reverse of the problematic overestimation which occurs when the
relationshipis positive. Accordingly,one neednot be concerned about correcting for predictor
measurement error because such error works against, rather than in favor of, obtaining
significant y-intercepts. Consequently, these regression results indicate that the Preference!Strategy interaction was significant (indeed, maximal) when detection d0 was clearly zero
clearly ruling out alternative conscious perception accounts.
16. But what does the negative relationship mean?
Above and beyond its salutatory methodological import, however, one might wonder
about the substantive meaning of the negative relationship. To begin with, obtaining any
Fig. 2. Regression of Preference!Strategy congruity effect (identification percentage correct performance in
preferred minus nonpreferred strategy) on detection d0.
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relationship at all between the Preference!Strategy interaction and detection indicates
that systematic variance is present in the latter as well as the former, despite the obtained
null detection sensitivity. With this in mind, such variation could be due to conscious orunconscious perceptual influences on detection.
16.1. Conscious influences on detection?
First, it could be that some participants indeed possessed very small amounts of
conscious detection, but that the obtained detection d0 did not significantly exceed zero
either because the extant conscious perception was too weak (leading to low power)
and/or obscured by measurement error (cf. the null sensitivity problem). Crucially,
however, the above regression analysis indicates that such weak conscious detection
could not account for the Preference!Strategy effect, given the latters presence even
when detection d0
Z0.Moreover, if very weak conscious detection was indeed present, the observed negative
relationship is predicted by our recently proposed objective threshold/strategic model
(see, e.g. Snodgrass, 2004a; in press; Snodgrass et al., 2004a,b; see also Bernat, Shevrin,
& Snodgrass, 2001). Although space reasons prevent a full discussion, our review of
unconscious perception research suggests that reliable effects occur at the objective
detection threshold (ODT), but that null findings usually obtain at the objective
identification threshold (OIT). For example, Dagenbach et al. (1989) and Klinger and
Greenwald (1995) repeatedly obtained this pattern. Along these lines, is it important to
note that Cheesman and Merikles (1984) influential null findings were actually obtained
under OIT, not ODT, conditions. Notably, prior reviews have conflated ODT and OIT
studies, leading to the erroneous conclusion that objective threshold effects areunreliable.
Crucially, because the ODT occurs at briefer stimulus durations than the OIT, this
pattern amounts to a negative correlation between the conscious and unconscious
perception indexes in the ODT-OIT region. We interpret these negative relationships as
suggesting that conscious and unconscious perceptions exert functionally exclusive
influences on performance, such that the former override the latter when both are present.
When conscious perception is virtually absent, which occurs only at the ODT, reliable
unconscious perceptual effects are readily obtainable, as in the current experiments. At
stimulus intensities between the ODT and OIT, the crucial stimuli are consciously
detectable; however, this overriding conscious perception is not yet sufficient to support
higher-level (i.e. identification-dependent) effects. Consequently, a negative relationship
between stimulus detectability and unconscious perception effects occurs in the ODT-OITregion.6 For example, we (Bernat et al., 2001) recently investigated unconscious P300
6 Importantly, our review suggests that ODT-OIT negative relationships occur only when participants regard
their conscious perceptions as relevant. Although this seems to occur frequently even with nominally indirect
measures, there are exceptions. For example, conscious perception may also become irrelevant when indirect task
parameters do not allow sufficient time to attend to the primes. This could account for the flat regression slopes
characteristic of Greenwald and associates (e.g. Abrams & Greenwald, 2000; Draine & Greenwald, 1998;
Klinger, Burton, & Pitts, 2000) response window experiments, which force very rapid responding (400 ms).
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oddball effects (i.e. greater brain wave amplitude to rare rather than frequent stimuli) using
visual stimuli. With supraliminal stimuli, P300 oddball effects are a classic
psychophysiological effect (Pritchard, 1981). Here, we used ODT stimuli (mean detectiond0Z.10); moreover, detection and the oddball effect were negatively related (rZK.44,
P!.05), and the y-intercept was significant (P!.0005).
Conversely, as stimulus intensity surpasses the OIT (i.e. to the subjective identification
threshold and beyond), higher-level task performance also increases. Beyond the OIT,
then, conscious and unconscious perception index performance is positively related,
consistent with the hypothesis that conscious perception now exclusively drives
performance. For example, P300 oddball effects correlate positively, not negatively,
with stimulus discriminability with supra-OIT stimuli (Pritchard, 1981). Similarly,
Cheesman and Merikle (1984) found a strong positive relationship (rZ.71) between
semantic priming and identification, with null effects at the OIT.
16.2. Unconscious influences on detection?
On the other hand, the current negative relationship may stem from unconscious rather
than conscious perceptual influences on detection. Given that overall detection d0 did not
exceed zero, such unconscious influences would most likely be exclusively bidirectional,
just as they are with identification (cf. the Preference!Strategy interaction)that is,
underlying mediating factors are systematically producing both facilitation and inhibition
on detection performance.7 Unlike identification, however, the mediating factors for
detection are as yet unknown, and do not appear to include Preference and Strategy (cf.
Van Selst & Merikle, 1993, Experiment 3; see above). In any case, whether unconscious
influences on detection are exclusively bidirectional or (less likely) weakly unidirectional,it is not clear why detection would be negatively related to bidirectional identification
effects. In any event, however, these all-unconscious scenarios do not threaten the
unconscious status of the Preference!Strategy interaction.
16.3. Could the current effects be weak subjective threshold effects?
Merikle and Daneman (2000) (see also Merikle et al., 2001) suggest that subjective and
objective threshold approaches index the same underlying constructphenomenal
awarenessand that objective threshold effects are simply conservative subjective
threshold effects. Relatedly, their formulation of the exclusiveness problem suggests that
reducing direct task performance should reduce unconscious influences generally.
7 Some researchers have found the idea of negative detection d0s strange or nonsensical (cf. Greenwald et al.,
1995), but this is likely because it is often implicitly assumed that detection is exclusively sensitive to conscious