Consciousness is a “subjective” state

Binary vs. continuous experimental designs for the study of unconscious perceptual processing

Binary vs. continuous conceptualizations of consciousness may have an unstated influence on experimental designs in unconscious perception research. The binary approach aims to compare a conscious condition (e.g., supraliminal, no or weak stimulus masking) to an unconscious condition (e.g., subliminal, heavy stimulus masking). In contrast, continuous designs tend to vary stimulus energy along a near-threshold continuum to determine changes in perception as a function of stimulus energy (or duration). The present study compared two experimental designs, binary vs. continuous, for the influence of target-masked prime stimuli on a Stroop task. The display parameters were inspired by emotional Stroop studies reporting unconscious perception. Neither experiment produced strong evidence of unconscious perception, but the experiment with a continuous design was more informative. We thus recommend sampling a range of near-threshold display parameters to yield straight-forward, unambiguous interpretations.

Classic Stroop Color Words Produce No Stroop Effect When the Display Characteristics Are Based Upon Emotional Stroop Studies With Subliminal Presentations

Two forms of the Stroop task have produced contradictory findings regarding unconscious perceptual processing. Emotional Stroop task studies with prime words presented at an objective threshold (i.e., subliminal) produce Stroop-like effects, but comparable studies conducted with classic Stroop stimuli do not produce Stroop effects. We tested the possibility that differences in the display appearance might explain this discrepancy. Color word prime stimuli from the traditional Stroop task were used with display characteristics based upon the emotional Stroop studies. There was a Stroop effect for the relatively long prime stimulus durations (59, 87, or 108 milliseconds) but not for the brief durations (18, 24, or 38 milliseconds). Accordingly, the discrepancy in research findings cannot be attributed to simple differences in display methodology. The failure to find strong evidence of unconscious perceptual processing is consistent with the negative findings from some emotional Stroop studies that use subliminal stimulus presentations.

Qualitatively Different Effects of Undetected and Unidentified Auditory Primes

The suggestion that semantic activation can occur without conscious identification of the priming stimulus is still controversial. Many studies supporting such a contention, especially those where primes were auditorially presented, suffer from methodological shortcomings, frequently with regard to threshold measurement. In the study reported here 24 subjects underwent a considerably more rigorous thresholding procedure than has been usual, prior to engaging in a forced-choice sentence completion task. The results show that semantic priming operates when subjects were unable to detect the presence of primes and that phonological (but not semantic) priming operates when the primes were invariably detected but never correctly identified. The relevance of these qualitatively different effects of primes, as a function of the level at which they are presented, in discussed in the light of recent accounts of unconscious processing.

Partial Awareness Creates the “Illusion” of Subliminal Semantic Priming

We argue that the lack of consensus regarding the existence of subliminal semantic processing arises from not taking into account the fact that linguistic stimuli are represented across several processing levels (features, letters, word form) that can independently reach or not reach awareness. Using masked words, we constructed conditions in which participants were aware of some letters or fragments of a word, while remaining unaware of the whole word. Three experiments using the Stroop priming paradigm show that when the stimulus set is reduced and participants are encouraged to guess the identity of the prime, such partially perceived stimuli can nonetheless give rise to “semantic” processing. We provide evidence that this effect is due to illusory reconstruction of the incompletely perceived stimulus, followed by usual semantic processing of the result. We conclude that previously reported unconscious Stroop priming is in fact a conscious effect, but applied to a perceptual illusion.

Time and the observer: The where and when of consciousness in the brain

We compare the way two models of consciousness treat subjective timing. According to the standard “Cartesian Theater” model, there is a place in the brain where “it all comes together,” and the discriminations in all modalities are somehow put into registration and “presented” for subjective judgment. The timing of the events in this theater determines subjective order. According to the alternative “Multiple Drafts” model, discriminations are distributed in both space and time in the brain. These events do have temporal properties, but those properties do not determine subjective order because there is no single, definitive “stream of consciousness,” only a parallel stream of conflicting and continuously revised contents. Four puzzling phenomena that resist explanation by the Cartesian model are analyzed: (1) a gradual apparent motion phenomenon involving abrupt color change (Kolers & von Grünau 1976), (2) an illusion of an evenly spaced series of “hops” produced by two or more widely spaced series of taps delivered to the skin (Geldard & Sherrick's “cutaneous rabbit” [1972]), (3) backwards referral in time, and (4) subjective delay of consciousness of intention (both reported in this journal by LIbet 1985a; 1987; 1989a). The unexamined assumptions that have always made the Cartesian Theater so attractive are exposed and dismantled. The Multiple Drafts model provides a better account of the puzzling phenomena, avoiding the scientific and metaphysical extravagances of the Cartesian Theater: The temporal order of subjective events is a product of the brain's interpretational processes, not a direct reflection of events making up those processes.

Characteristics of dissociable human learning systems

A number of ways of taxonomizing human learning have been proposed. We examine the evidence for one such proposal, namely, that there exist independent explicit and implicit learning systems. This combines two further distinctions, (1) between learning that takes place with versus without concurrent awareness, and (2) between learning that involves the encoding of instances (or fragments) versus the induction of abstract rules or hypotheses. Implicit learning is assumed to involve unconscious rule learning. We examine the evidence for implicit learning derived from subliminal learning, conditioning, artificial grammar learning, instrumental learning, and reaction times in sequence learning. We conclude that unconscious learning has not been satisfactorily established in any of these areas. The assumption that learning in some of these tasks (e.g., artificial grammar learning) is predominantly based on rule abstraction is questionable. When subjects cannot report the “implicitly learned” rules that govern stimulus selection, this is often because their knowledge consists of instances or fragments of the training stimuli rather than rules. In contrast to the distinction between conscious and unconscious learning, the distinction between instance and rule learning is a sound and meaningful way of taxonomizing human learning. We discuss various computational models of these two forms of learning.

Unconscious perception: the need for a paradigm shift

According to Snodgrass, Bernat, and Shevrin (2004), unconscious perception can be demonstrated convincingly only at the objective detection threshold, provided that the conditions of their objective detection/strategic model are met, whereas both the subjective threshold model of Cheesman and Merikle (1984, 1986) and the objective threshold/rapid decay model of Greenwald, Draine, and Abrams (1996) are inconclusive. We argue on theoretical, metatheoretical, and empirical grounds that all three dual-process models, which are based on both conscious and unconscious perception, should be rejected in favor of the single-process conscious perception model.

Subliminal perception and its cognates: Theory, indeterminacy, and time

Unconscious processes, by whatever name they may be known (e.g., "subliminal," "implicit"), are invariably operationalized by the dissociation paradigm, any situation involving the dissociation between two indicators (or sets of indicators), one of availability (epsilon) and the other, of accessibility (alpha), such that, epsilon>alpha. Subliminal perception has been traditionally defined by a special case of the dissociation paradigm in which availability exceeds accessibility when accessibility is null (epsilon>alpha/alpha=0). Construct validity issues bedevil all dissociation paradigms since it is not clear what might constitute appropriate indicators that, moreover, are pure and exhaustive. Semantic and theoretic drifts in the recent literature--e.g., the confusion of different versions of the dissociation paradigm, the equation of conscious-unconscious with direct-indirect tests, and the foisting of the criterion of qualitative differences--have tended to undermine emerging theoretic parsimony. On the other hand, a crucial factor has been left out of theory development: time. Both epsilon and alpha can rise and fall over time, often asynchronously, and so dissociations may wax and wane and, even, reverse over time. Some laboratory evidence suggests that accessibility measures (e.g., d'), as they approach chance, may actually dip below chance (subchance perception). If so, d'=0 (the chance-limen or objective threshold), could be an averaging artifact of positive and negative d's. Conscious accessibility is not either-or but more or less, and variable over time.

Confidence and accuracy of near-threshold discrimination responses

This article reports four subliminal perception experiments using the relationship between confidence and accuracy to assess awareness. Subjects discriminated among stimuli and indicated their confidence in each discrimination response. Subjects were classified as being aware of the stimuli if their confidence judgments predicted accuracy and as being unaware if they did not. In the first experiment, confidence predicted accuracy even at stimulus durations so brief that subjects claimed to be performing at chance. This finding indicates that subjects's claims that they are "just guessing" should not be accepted as sufficient evidence that they are completely unaware of the stimuli. Experiments 2-4 tested directly for subliminal perception by comparing the minimum exposure duration needed for better than chance discrimination performance against the minimum needed for confidence to predict accuracy. The latter durations were slightly but significantly longer, suggesting that under certain circumstances people can make perceptual discriminations even though the information that was used to make those discriminations is not consciously available.

Congruity effects evoked by subliminally presented primes: automaticity rather than semantic processing

In a size judgment task on words denoting concrete objects, subliminally presented stimuli that preceded the targets influenced response times and were dependent on whether responses to the prime and the target were congruent or incongruent (Experiment 1). These findings, mirroring S. Dehaene et al. (1998), imply that primes are unconsciously categorized and processed to the response stage. However, the effect does not generalize to primes that are not in the response set (Experiment 2), and even exposure to primes not in the response set in an interleaved naming-size judgment task fails to induce it (Experiment 3). However, the effect generalizes from lowercase primes to the same set of uppercase targets (Experiment 4), suggesting an abstract level of operation. The findings suggest that rather than resulting from unconscious prime categorization, the congruity effect results from automatized stimulus-response mappings. Potential differences between the number and the word domain are discussed.

Parts outweigh the whole (word) in unconscious analysis of meaning

In unconscious semantic priming, an unidentifiable visually masked word (the prime) facilitates semantic classification of a following visible related word (the target). Three experiments reported here provide evidence that masked primes are analyzed mainly at the level of word parts, not whole-word meaning. In Experiment 1, masked nonword primes composed of subword fragments of earlier-viewed targets functioned as effective evaluative primes. (For example, after repeated classification of the targets angel and warm, the nonword anrm acted as an evaluatively positive masked prime.) Experiment 2 showed that this part-word processing was potent enough to oppose analysis at the whole-word level. Thus, smile functioned as an evaluatively negative (!) masked prime after repeated classification of smut and bile. Experiment 3 found no priming when masked word primes contained no parts of earlier targets. These results suggest that robust unconscious priming (a) is driven by analysis of part-word information and (b) requires previous classification of visible targets that contain the fragments later serving as primes. Contrary to a widely held view, analysis of subliminal primes appears not to function at the level of analysis of complete words.

Evaluative learning with "subliminally" presented stimuli

Evaluative learning refers to the change in the affective evaluation of a previously neutral stimulus (NS) that occurs after the stimulus has been associated with a second, positive or negative, affective stimulus (AS). Four experiments are reported in which the AS was presented very briefly. Significant evaluative learning was observed in participants who did not notice the presentation of the affective stimuli (ASi) (Experiment 2) or could not discriminate between the briefly presented positive and negative ASi when asked to do so (Experiment 3). In two other experiments (Experiments 1 and 4), no significant learning effect was obtained. A meta-analysis performed on the present and previously reported results (De Houwer, Baeyens, & Eelen, 1994) gave evidence for a small, though statistically reliable evaluative learning effect when ASi are presented "subliminally." This finding supports the hypothesis that evaluative associations can be learned implicitly.