I shall describe one alternative model of consciousness at some length, to show that this experimental method is adaptable. You may care to devise other models of the mind. For any model that counts as a 'mind-brain identity theory', though, I am confident that one can adapt the hypothetical experiment so that the identity theory will yield unacceptable predictions.
The alternative model assumes that the mind occupies only a small volume of brain matter at any one time, and moves around the brain to whichever area is active like a spotlight. In this model, a mind can be conscious of sensations in only one sensory faculty at a time: when it is in the visual cortex it can experience only visual sensations; when it is in the auditory cortex, it can experience only sound sensations; and so on. The mind may appear to have simultaneous sensations in different faculties (e.g. hearing someone speak and seeing her lips move at the same time). But, on this view, the simultaneity is an illusion produced by the rapid motion of the mind between different sensory faculties in the brain.
Consider again our experiment in which the pricked-finger corpus is kept alive in a test tube. According to this 'spotlight' model, when the corpus is stimulated in vitro, it cannot feel anything because the mind cannot flow out of the head into the test tube.
Or can it? We are assuming, for the sake of argument, that the mind is purely physical. So the presence of the mind in any region of the brain must consist in the occurrence of some special neurological event. Let us call this event the 'consciousness signifier'. So, the mind is said to be in a particular patch of the brain if and only if the signifier is occurring there. (For instance, there might be a special sort of cell, the firing of which constitutes the presence of consciousness.) The brain surgeon could therefore electrically stimulate the signifying cells in the corpus so that the signifier occurs in the test tube. Then, by definition, the mind - or, at least, a mind would be present in the test tube. Hence the finger-prick pain could indeed be made to occur in vitro.
According to the 'spotlight model', to whose mind does this in vitro pain belong? To answer this, we must make some plausible assumption about the model's criterion for a sensation to belong to a mind.
The criterion must surely be the continuity of the chain of cause-and-effect linking the successive occurrences of the consciousness signifier. If the signifier were suddenly to pop up somewhere in the brain as a result of random electrical noise, then a 'mind' would occur there. But it would not be the same mind as the one that usually resides in that brain. It would be a different stream of consciousness. For a brief period, both the regular mind and a spurious mind would co-exist in one head.
In our experiment, the in vitro pain would not be in my usual mind because the occurrence of the signifier in the test tube would not flow causally from earlier occurrences of the signifier in my head. Rather, the signifier in the severed tissue would be caused by a decision taken by the surgeon.
That obstacle can be removed. The surgeon can augment her apparatus so that whenever the consciousness signifier would occur in the region of my brain that deals with finger perceptions, the device will automatically trigger the signifier in the excised tissue. Then it would follow that my mind would indeed be in the test tube.
The modified arrangement that I have just described would not involve sending any signal back from the test tube to my head. For this 'spotlight model' says only that causation flows from the previous presence of the mind to the new presence; and not that causation must flow back to the continuing presence of the mind in the cranium. Consequently: I would have no recollection of the in vitro pain; I would unable to report it; and I would make no grimace or gesture. Both I and any observer would be convinced I had felt no pain. That is to say, in the normal meaning of the terms, I had not in fact felt the pain. Therefore the theory's prediction that I would feel the pain disagrees with what would actually be observed.
Well, let us add another postulate to this 'spotlight' model. In order for the mind to be present in a piece of brain tissue (call it X), not only must the consciousness signifier be caused there by earlier signifiers of that mind; but the signifiers in X must in turn cause further consciousness signifiers in the persisting mind. This is reasonable. But it makes the theory vulnerable to precisely the attack that I proposed in the previous section. For, the experimenter can now use her apparatus to delay or redirect the outgoing flow of causal signals.
© Peter B Lloyd, 1993