In the target article Block aims to identify circumstances in which stimuli might elicit phenomenal experience but not elicit a representation which is cognitively accessible. In the examples Block considers, cognitive accessibility is impeded through deficits in attention (the extinction example) or through limitations in processing time (the partial report examples). Both mechanisms depend upon some constraint in processing the stimulus. There must be a concern that this might affect sensory processing, and hence that phenomenal experience of the stimulus is affected along with cognitive access. We know, for example, that attention affects visual sensitivity (Solomon Reference Solomon2004). Changes in the response gain of neurons in sensory areas of cortex are as likely to be affecting phenomenal experience as cognitive access (Carrasco et al. Reference Carrasco, Ling and Read2004; Treue & Martìnez Trujillo Reference Treue and Martìnez Trujillo1999). Are there circumstances in which the object of sensory processing can be examined at leisure and can be fully attended, and yet cognitive access is lost when phenomenal experience survives?
What does it mean for a representation to be cognitively accessible? As Block notes, “mechanisms of reporting, reasoning, evaluating, deciding, and remembering” (sect. 11, para. 2) should be able to make use of such a representation – a key aspect of cognitive access in models such as Baars' Global Workspace Theory (Baars Reference Baars1997) is that information about a stimulus becomes widely available. The representation must therefore take a form that permits interaction with memories or plans about other stimuli (in Piagetian terms, for a cognitive system to assimilate a representation, the former must be accommodated to the latter). If stimuli no longer engage representations that can be integrated in any sense with the rest of a person's representational framework, then surely they cannot be said to be cognitively accessible.
How can representations become isolated from cognition? One might erroneously assume that sensations are “raw” – that they do not need to be prepared in a fashion to make them accessible to cognition. Sensations do not, however, correspond simply to the activity of sensory receptors. In color vision, for example, color appearance is far more closely related to a distal property of surfaces (things in the world about which we have cognitions), their spectral reflectance, than to the activations of cone photoreceptors in the retina (Hofer et al. Reference Hofer, Singer and Williams2005). Color sensation is the endpoint of a complex process. If this process is prevented from running to its conclusion we may be left with an incomplete signal that gives rise to sensation but cannot be integrated with cognition. Stimuli may therefore potentially elicit phenomenal experiences and these experiences may be discriminable yet they remain cognitively isolated. There is no framework within which to reason about or evaluate their differences, nor can they be remembered (I am not sure I see mere discrimination or decision as an act of cognition, but neither does Block when he discusses “guessing” in blindsight).
Do such circumstances exist? I will argue there are neurological patients who have all the time in the world to process stimuli, who can attend to them, but who have sensations divorced from cognition.
Cerebral achromatopsia is a neurological condition in which color vision is lost as a consequence of damage to ventromedial occipital cortex, usually in the vicinity of the fusiform and lingual gyri (Meadows Reference Meadows1974). Unlike more usual forms of color blindness there are no deficits or abnormalities in the retinal cone photoreceptors which form the starting point of normal color perception. Patients with cerebral achromatopsia do not usually make spontaneous comments about color. Questions about color sensation seem quite alien to them. They are unable to name the colors of stimuli presented to them or to perform nonverbal tests of color perception such as sorting or odd-one-out tasks. Although they may remember some semantic associations of color words (e.g., that bananas are yellow), they appear to have no sensation of color or means of remembering any aspects of the nature of color sensation (see, e.g., Heywood & Kentridge [2003] for a recent review). It is, however, well known, that cerebral achromatopsics do see (quite consciously) the border formed between regions of equiluminant color (Heywood et al. Reference Heywood, Kentridge and Cowey1998). If a red and a green that a patient cannot tell apart are used to construct a red square against a green background, then the patient will effortlessly see the square although they will be unable to explain how the square and its background differ. My colleagues and I (Kentridge et al. Reference Kentridge, Heywood and Cowey2004) have shown that not only can these patients see such color borders but they can discriminate between borders formed from different colors (and that this discrimination cannot be accounted for on the basis of chromatic contrast salience) even though they cannot see the colors of the adjoining surfaces that form the borders. Again, the discrimination is conscious. The borders somehow look different from one another. The patients cannot, however, explain in any sense how or why the borders differ (they have no accessible representations of color) – they just know that the borders look different. The patient is surely having differing phenomenal experience of these borders, yet these experiences are not cognitively accessible. It is true that they know of the existence of these borders, but subjects in a partial report experiment know of the existence of items they cannot describe. I suggest that this situation, in which sensory representations simply cannot be integrated into the global workspace, provides a better example of phenomenology in the absence of cognitive access than cases in which the integration of representations into the workspace is possible but temporarily unachieved.
