Perception & Psychophysics 2006, 68 (3), 489-504 Criteria for unconscious cognition: Three types of dissociation THOMAS SCHMIDT Universität Gießen, Gießen, Germany and DIRK VORBERG Technische Universität Braunschweig, Braunschweig, Germany To demonstrate unconscious cognition, researchers commonly compare a direct measure (D) of awareness for a critical stimulus with an indirect measure (I) showing that the stimulus was cognitively processed at all. We discuss and empirically demonstrate three types of dissociation with distinct ap- pearances in D–I plots, in which direct and indirect effects are plotted against each other in a shared effect size metric. Simple dissociations between D and I occur when I has some nonzero value and D is at chance level; the traditional requirement of zero awareness is necessary for this criterion only. Sensitivity dissociations only require that I be larger than D; double dissociations occur when some experimental manipulation has opposite effects on I and D. We show that double dissociations require much weaker measurement assumptions than do other criteria. Several alternative approaches can be considered special cases of our framework. [what do you see?/ level and that the indirect measure has some nonzero nothing, absolutely nothing] value. This so-called zero-awareness criterion may seem —Paul Auster, “Hide and Seek” (in Auster, 1997) like a straightforward research strategy, but historically it The traditional way of establishing unconscious percep- has encountered severe difficulties. From the beginning, tion has been to demonstrate that awareness of some criti- the field was plagued with methodological criticism con- cal stimulus is absent, even though the same stimulus af- cerning how to make sure that a stimulus was completely fects behavior (Reingold & Merikle, 1988). To show this, outside of awareness. This criticism is still at the heart two types of measurement are needed. First, the degree of the most recent debates (e.g., in Erdelyi, 2004) and to which the critical stimulus reaches conscious aware- has overshadowed the most thought-provoking findings ness must be assessed, for example by asking observers in unconscious cognition (e.g., Kunst-Wilson & Zajonc, whether or not they are aware of it or by testing their abil- 1980; Marcel, 1983). It has placed the burden of proof so ity to identify or detect it. This is called a direct measure one-sidedly on the shoulders of the unconscious cognition (D) of processing, because the task explicitly requires hypothesis that the zero-awareness criterion seems to have some type of direct report on the perception of the critical been more effective in hindering scientific progress than stimulus. Second, one must assess the degree to which the in helping it. critical stimulus, even if not consciously perceived, affects The zero-awareness criterion critically depends on some other behavior. This is called an indirect measure (I) showing that the value of the direct measure is at chance because responses are usually made to some stimulus other level, which introduces the statistical problem of corrobo- than the critical one, with the latter exerting an influence rating a null hypothesis. In principle, this is not a substan- on processing the former. This research paradigm of com- tive theoretical problem—it could be dealt with pragmati- paring direct and indirect measures has been called the dis- cally by setting appropriately strict criteria for maximum sociation procedure (Reingold & Merikle, 1988, 1993). effect sizes in the direct measure and for minimum statisti- The traditional criterion for perception without aware- cal power to detect such effects at a fixed level of signifi- ness is to establish that the direct measure is at chance cance (Murphy & Myors, 1998). But binding standards of this sort have never been established in applied statistics or in the field of unconscious cognition, and many semi- nal studies have invited attack by employing somewhat le- Part of this work was supported by Deutsche Forschungsgemeinschaft nient criteria or low statistical power for “proving the null” Grant Schm1671/1-1 to T.S. Correspondence may be sent to T. Schmidt, (e.g., Dehaene et al., 1998; Marcel, 1983). It may thus Abteilung Allgemeine Psychologie, Universität Gießen, Otto-Behaghel- come as little surprise that in 1960, Eriksen concluded in Straße 10F, D-35394 Gießen, Germany, or D. Vorberg, Institut für Psy- chologie, Technische Universität Braunschweig, Spielmannstraße 19, an extensive review of the literature that no unequivocal 38106 Braunschweig, Germany (e-mail: thomas.schmidt@psychol evidence for unconscious perception existed at all; but it is .uni-giessen.de or [email protected]). irritating that a quarter of a century later, a new review by 489 Copyright 2006 Psychonomic Society, Inc. 490 SCHMIDT AND VORBERG Holender (1986) reached the same conclusion with essen- ally, then, direct and indirect measures should be based on tially the same arguments. In a recent article, that author tasks that are identical in all respects except which stimu- has defended the theory that at least semantic processing lus serves as target. In a priming experiment as typically is exclusively conscious (Holender & Duscherer, 2004). employed in unconscious perception research, this would These skeptics felt that the hypothesis of unconscious mean that participants always indicate the same feature cognition should be tested as rigorously as possible. In discrimination on the same stimuli and with the same set their view, proponents of unconscious processing have to of responses; once with respect to the primed target (indi- refute what we will call the null model of unconscious rect task), and once with respect to the prime itself (direct cognition. Under the null model, there is only one type task). of information processing, conscious processing, which Unfortunately, D–I mismatch has been the rule rather accounts for purportedly unconscious effects and could than the exception. For example, in Dehaene et al.’s (1998) be revealed as the sole source of these effects by some study, the indirect task was to indicate as quickly as pos- sufficiently sensitive measure. In this view, the problem sible whether a target number was numerically smaller or of demonstrating unconscious perception simplifies to larger than 5, in the presence of masked number primes that disproving the null model. were also smaller or larger than 5. The indirect measure Our purpose here is to explore several methods for was the difference in response times when the response refuting the null model, by distinguishing three types of evoked by the prime was congruent or incongruent with dissociation between direct and indirect measures. We that evoked by the target (for example, when the prime start by specifying the conditions under which behavioral was smaller but the target was larger than 5, as compared measures can be compared at all. Then we state a set of with when both were larger than 5). To match this indi- minimal measurement-theoretical assumptions that must rect task, the optimal direct task would have asked for the be met for any reasonable kind of comparison between di- same feature discrimination—namely, deciding whether rect and indirect measures, showing that strong additional the prime was larger or smaller than 5—because this was requirements must be met for the traditional zero-awareness the information in the prime driving the indirect effect. criterion, but also that alternative criteria exist that rest on Instead, the authors employed two direct tasks, detection much milder assumptions. Because these other criteria do of the primes against an empty background and discrimi- not require the critical stimulus to remain outside aware- nation of the primes from random letter strings, neither ness, they turn out to be more powerful for detecting un- of which captured the critical distinction of whether the conscious influences. Through a comparison of the pros prime was smaller or larger than 5. and cons of the different approaches, we will argue that As another example, Draine and Greenwald (1998, zero awareness of critical stimuli is neither necessary nor Experiment 1) used semantic priming effects on the clas- desirable for establishing unconscious perception. We will sification of target words (e.g., pleasant vs. unpleasant) then demonstrate each type of dissociation with empirical as the indirect measure. The direct measure, however, in- data from a response priming paradigm (Vorberg, Mat- volved discriminating the prime words from letter strings tler, Heinecke, Schmidt, & Schwarzbach, 2003, 2004) and consisting of Xs and Gs. These tasks differed not only in show how some previously proposed criteria for uncon- the stimuli employed (the XG strings were never presented scious cognition can be interpreted as special cases of our in the indirect task), but also in the stimulus dimension framework. Although we will deal primarily with uncon- judged. Because the XG task was supposedly easier than scious visual perception, our results generalize naturally the semantic classification task, it was argued that fail- to other areas of unconscious cognition and, in fact, to any ing to discriminate words from XG strings would give field of research that involves dissociations between two even more convincing evidence for unconscious percep- or more variables. tion than would failure to classify the words semantically. However, the direct task may have invited a feature search Avoiding D–I Mismatch for Xs rather than a pleasant–unpleasant discrimination of The most important requirement for any direct mea- the primes, so different aspects of conscious processing sure is that it be a valid measure of those conscious inputs might have driven the two tasks. D–I mismatch could have that might explain nonzero performance in the indirect been avoided if both tasks had involved exactly the same task—in other words, D must be a valid measure not of prime stimuli (i.e., no XG strings), as well as the same conscious processing per se, but of those sources of con- type of semantic classification (i.e., categorizing primes scious processing that are potentially relevant for I.