snodgrass & shevrin 2006 unconscious inhibition and facilitation at the objective detection...

7/28/2019 Snodgrass & Shevrin 2006 Unconscious inhibition and facilitation at the objective detection threshold - Replicable

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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

M. Snodgrass, H. Shevrin / Cognition 101 (2006) 437944


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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.

M. Snodgrass, H. Shevrin / Cognition 101 (2006) 4379 45


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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)


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)


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

snodgrass & shevrin 2006 unconscious inhibition and facilitation at the objective detection...

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