In his target article, Ned Block is dealing with a difficult problem: how to empirically demonstrate that phenomenal consciousness (hereafter P-consciousness) is dissociable from access consciousness (hereafter A-consciousness). An a priori argument in favor of this dissociation is the common intuition that the representational content of phenomenal experience is much richer than the limited content we can access at a given time. In Block's words, “phenomenology overflows cognitive accessibility” (sect. 8, para. 6). This intuition is so strong that it appears very easy, at first glance, to show how much richer P-consciousness is, compared with A-consciousness.
However, providing an empirical demonstration of this dissociation leads to a major methodological difficulty: any measure of consciousness seems inevitably to require the involvement of A-consciousness. From there on, it seems impossible to show evidence for P-consciousness without A-consciousness. This methodological puzzle arises also in dissociating consciousness and top-down attention. Demonstrating consciousness without attention seems impossible for similar reasons: To assess consciousness of the stimulus, one needs to direct the subject's attention on the stimulus! Although there is converging evidence that attention can affect both conscious and unconscious perception, the reverse dissociation involving the possibility of consciousness without attention remains highly debated (see Dehaene et al. Reference Dehaene, Changeux, Naccache, Sackur and Sergant2006; Koch & Tsuchiya Reference Koch and Tsuchiya2007). Block acknowledges this methodological issue and proposes to take the set of evidence at hand as a whole and see whether it points towards the researched dissociation. Using, among others, examples from perception of unattended objects (e.g., attentional blink, change blindness), and from partial report Sperling-like experiments, Block assumes that we should adopt the A- versus P-consciousness dissociation and look for its respective neural bases.
Unfortunately, the evidence Block is using cannot unequivocally prove his theory. Furthermore, we think that the empirical data reviewed in his target article do not provide more support for his accounts over alternative and crucially simpler explanations. In the empirical phenomena that Block is using, one can distinguish two types of situations: those involving partial access and those involving undetectable stimuli.
The first type of situation involves stimuli that are visible but unattended – and importantly, not even detected – implying the absence of any conscious access. This is usually the case during attentional blink and inattentional blindness experiments. Block uses the fact that the stimulus is supra-threshold (it can be reported when attention is drawn towards it) to argue for P- without A-consciousness. But there is no evidence for this claim, since subjects do not even detect the unattended stimulus. Moreover, Block's claim that participants forget their phenomenal episode appears impossible to test, because any probe would modify their attention and hence make the stimulus consciously accessible (Kouider et al. Reference Kouider, Dehaene, Jobert and Le Bihan2007).
In the second type of situation, by contrast, subjects are aware of “some” information: stimuli are presented very briefly and/or in a degraded fashion, such that they are not fully visible but not subliminal either. Block assumes that this situation gives rise to P- without A-consciousness. We propose instead that what happens here is rather a form of partial awareness in the absence of full awareness. Partial awareness reflects the situation where subjects have transient access to lower but not higher levels of representation. For instance, visual word recognition implies the processing of several hierarchically organized levels (e.g., fragments, letters, whole word). With degraded presentation conditions, lower levels can be accessed (e.g., fragments/letters) while higher levels cannot (whole word). Still, subjects can use such partial information in conjunction with context/expectations to make hypotheses about the representational content at higher levels of processing (Kouider & Dupoux Reference Kouider and Dupoux2004). Under this perspective, Block's richness of phenomenal experience can be reinterpreted as the transient activation of a large quantity of degraded low-level information. In such partial awareness conditions, the available information is quantitatively rich but qualitatively poor.
This hypothesis allows us to construe the Sperling phenomenon (see Sperling Reference Sperling1960) as resulting from partial awareness: subjects have a transient and degraded access to fragments of all the letters in the grid. As subjects are not expecting anything other than letters, fragments are used to reconstruct as many letters as possible. Due to mnemonic decay and attentional overflow, subjects are able to reconstruct at most about four letters. Crucially, the unreported items are never identified as letters per se and remain coded as unidentified letter fragments. A similar situation of partial without full access is the McConkie experiment (McConkie & Rayner Reference McConkie and Rayner1975) in which subjects see “letter-like” fragments in the periphery and infer that these are real letters. In our previous work (Kouider & Dupoux Reference Kouider and Dupoux2004) we have extended this phenomenon to a dissociation between the letter and the word level. We have induced subjects to access some but not all letters of a real or false color word (GREEN or GENER). We found that both the real and false color words are identified and treated as real words, as assessed by both subjective reports and the magnitude of the Stroop effect.
An important question for future research will be to characterize whether such reconstruction processes imply metacognitive/inferential interpretations or rather more direct perceptual illusions. Block acknowledges that McConkie's experiments involve a reconstruction process (what Block labels “cognitive illusions”). However, Block assumes that subjects in Sperling-like experiments are not reconstructing the visual scene but genuinely experiencing the whole set of items. To justify this special treatment, Block argues that the Sperling phenomenon is somehow mandatory and does not require subjects to explicitly report the stimuli, implying that it is a perceptual rather than a metacognitive reinterpretation. Yet, these statements remain highly speculative, as none of them has been empirically demonstrated. For us, it is highly probable that Sperling-like paradigms also lead to the “experience” of letters even when the uncued items consist of false letters. Of course, disentangling this issue requires further empirical research.
All these remarks point towards the same direction: Including a typology in terms of levels of representation during conscious access, along with the associated notion of partial awareness, provides a unified description of the empirical evidence at hands. In particular, this account offers more explicit specifications of the functional mechanisms leading to conscious perception.
