5.4 Disunified Access Consciousness

17. The fourth hypothesis I listed earlier, (d), has it that split-brain subjects have a unified phenomenal conscious­ness and a disunified access consciousness.³ This needs some explanation. Consider the case of blindsight. People with blindsight have large blind areas or scotoma in their visual fields due to brain damage in the postgeniculate region (typically the occipital cortex), and yet, under certain circumstances, they can issue accurate statements with respect to the contents of those areas (see Weiskrantz 1986). For example, blindsight subjects can make accurate guesses with respect to such things as presence, position, orientation, and movement of visual stimuli. They can also guess cor­rectly as to whether an X is present rather than an O, or vice versa. And some blindsight patients can even make accurate color judgments about the blind field (Weiskrantz 1990).

It appears, then, that, given appropriate instructions, blindsight subjects can respond in a way that is significantly like normally sighted subjects with regard to a limited range of stimuli in the blind areas in their visual fields, without there being anything experiential or phenomenally con­scious going on. Blindsight subjects, however, do not spon­taneously issue any public reports about the contents of their blind fields. In each case, they respond only when they are forced to choose between certain alternative possibili- ties.⁴ Moreover, they cannot identify objects in their blind fields as falling into everyday categories; they cannot tell what is present except in rare cases (of the sort mentioned above). So, for example, a thirsty blindsight subject will fail to recognize a glass of water placed before him, and hence will fail to reach out for it. The result is that blindsight sub­jects are invariably "deeply disabled" (Weiskrantz 1990, p. 8).

17. Why should there be such failures in identification or recognition? The natural answer is that blindsight subjects lack phenomenal consciousness (see, e.g., Marcel 1986; Van Gulick 1989; Flanagan 1992). The function (or a function) of phenomenal consciousness is simply to enable creatures to use information represented in their brains in the guidance of rational action (as contrasted with guessing behavior). Blindsight subjects cannot use the information from their senses rationally in the way that the rest of us can.

18. The claim just made about the function (or a function) of phenomenal consciousness is sometimes rejected on the grounds that it improperly transfers a function of "access consciousness" (Block 1995) to phenomenal consciousness. Access consciousness, according to Block, is a matter of a state's having a conceptualized representational content, one that is available for verbal report and plays an appropriate role in reasoning (see Block 1995, p. 232). The blindsight patient who fails to reach for a glass of water in his blind field lacks both sorts of consciousness. Block comments:

There is an obvious explanation of why the patient doesn't reach for the glass in terms of information about it not reaching the mechanisms of reasoning and rational control of speech and action, the machinery of A-consciousness (access consciousness). . . . A function of the mechanisms underlying A-consciousness is com­pletely obvious. If information from the senses did not get to mechanisms of control and reasoning and of rational control of action and reporting, we would not be able to use our senses to guide our action and reporting. But it is a mistake to slide from a function of the machinery of A-consciousness to any function at all of P-consciousness. (p. 242)

So, it could well be that the absence of phenomenal con­sciousness is not a factor in the patient's inaction. For all we know, it is empirically possible for access consciousness of the glass to be missing, while phenomenal consciousness is present; and, in this case, a thirsty subject will still not reach for the glass. This possibility is easy to ignore because access consciousness and phenomenal consciousness usually go hand in hand (and hence it is very easy to mistakenly identify them, according to Block). But it is a real possibility, certainly not one that can be ruled out a priori.⁵

(21) Consider now the earlier split-brain patient, S, who has the words 'pen' and 'knife' flashed up before him. The pro­posal with respect to S is that S has an experience just like that you or I would undergo in the same experimental conditions: S has an experience of 'pen' to the left of 'knife'. But, given the commissurotomy, S does not undergo an access-conscious state, the content of which includes both 'pen' and 'knife'. Using his right hemisphere, S undergoes an access-conscious state of 'pen' but not of 'knife'. Using his left hemisphere, S undergoes an access-conscious state of 'knife' but not of 'pen'.

22. This seems to me unconvincing. In the case of the thirsty blindsight subject, we can agree that there is no access consciousness with respect to the glass. The subject has no state that conceptually represents to him that a glass of water is present on the basis of the information in the light striking his eyes, even though he certainly has the perti­nent concepts at his disposal (he can recognize glasses of water placed in his normal field, for example). But why is this? Why doesn't information about the glass get to the cog­nitive centers, the loci of belief and reasoning? Why is access consciousness missing? That too calls for explanation.

The obvious, commonsense answer surely is that phenom­enal consciousness is absent. The patient has no visual experi­ences with respect to the blind field. There is nothing it is like for him as he stares at the glass. That's why he doesn't con­ceptually represent (or believe) that there is a glass of water present, and that, in turn, is why he doesn't reach for it.

22. Intuitively, basic visual experiences are nonconceptual states. They do not demand that their subjects have concepts for what they represent. The visual sensations involved in seeing a rotten egg, for example, do not require that one believe it to be an egg, think of it as rotten, or even have any idea of what one is seeing at all. Visual states like these form the basis for possible conceptual responses, but they are not themselves conceptual.⁶ When they are absent—when there is no visual phenomenology—there is nothing for the belief- forming processes to go on about what is in the field of view. So, no visual recognition occurs.⁷

23. Perhaps Block would deny that it really is obvious that the blindsight patient does not conceptually represent that the glass of water is present because he has no pertinent visual experience. But it certainly seems to be the obvious response. This is not to say that it is an a priori truth or that it could not possibly be mistaken. The point is simply that to deny it would be very counterintuitive. My suggestion, then, is that access consciousness is missing in the patient because phenomenal consciousness is missing too.⁸

24. Likewise in the case of split-brain subjects. Surely, the most straightforward explanation as to why the split-brain patient S has no access consciousness of 'pen' next to 'knife' is that he has no visual experience of the one design to the left of the other in his visual field. He has an experience of 'pen' and, on this basis, he undergoes a cognitive state rep­resenting 'pen'. He also has an experience of 'knife' and, on this basis, he undergoes a cognitive state representing 'knife'. But he does not experience the two words together. That's why access consciousness of both is missing.

25. Those who take the opposing view seem motivated largely by the belief that disunity in phenomenal conscious- nesss is impossible and perhaps even incoherent (Bayne and Chalmers 2003). However, as I have already argued in response to Nagel, there is no difficulty in imagining the mental life of a split-brain patient, under the phenomenal disunity assumption. So, the hypothesis of a unified phe­nomenal consciousness and a disunified access conscious­ness is neither intuitively plausible nor, so far as I can see, well motivated.