I support Puccetti's arguments and conclusions by providing some additional evidence that Dennett's position with respect to split-brains is untenable. I also suggest that these considerations highlight the need to pay more attention to brain activity as the criterion for consciousness.
1. I would like register my agreement with Puccetti's (1993) argument that Dennett (1991) plays fast and loose with the facts regarding split-brain patients, that Dennett does not adequately support his contention that a split brain does not entail two consciousness, and that by Dennett's own criteria for selfhood, persons in irreversible coma and functioning left hemispherectomies have the same inner life. In each instance, Puccetti is exactly right. Here I would like to amplify my agreement by providing additional evidence for Puccetti's points and suggest that these sorts of considerations should lead us to approach the study of consciousness in a very different way.
2. One question that preoccupies many philosophers and psychologists who worry about consciousness (including Dennett) is whether one can be conscious in the absence linguistic ability. The most commonly cited examples of such people are split-brain patients. In general, the considerations run as follows: since our capacity for language is thought to reside in the left hemisphere (for normal right-handed individuals), if the left hemisphere can be separated from the right, the hemisphere nondominant for language might lose its connection to conscious thought (if it had any to begin with); if we then compare and contrast the behavior elicited by each hemisphere, perhaps we can decide in which hemisphere consciousness is located. If language is in fact necessary for qualitative experience, then we should conclude that the left hemisphere is the seat of consciousness.
3. Not surprisingly, however, it is far from clear how we are to understand the verbal reports and nonlinguistic behavior of split-brain patients. There is an ongoing debate in the cognitive sciences -- Dennett's interpretation notwithstanding -- about whether patients with severed corpus callosa have two separate streams of consciousness as a result, or whether consciousness is localized in only one hemisphere (Gazzaniga 1985; Gazzaniga and LeDoux 1978; Gazzaniga et al. 1979, 1987; Sperry 1965, 1977, 1985; Marks 1981; Natsoulas 1987). Of course, which side of the debate you are on depends a great deal upon what you think consciousness is. Unfortunately, however, most of the controversy centers around whether language is a necessary condition for consciousness and consequently whether language is truly localized in one hemisphere, so it begs the question of where consciousness in fact resides.
4. It is not my purpose, however, to locate myself on the map of this debate. As controversial and exciting as it is, the debate is a poor way to conceptualize and discuss the issues involved. If we look at the corpus callosum developmentally, we have little reason to assume it is in the business of transferring semantic information from one hemisphere to the other (Cyander et al. 1981, Innocenti 1986, Nahm 1989) -- as those who tie consciousness to language and argue that both hemispheres are aware must assume. Hence whether the callosum alone is severed may have little bearing on qualitative experience.
5. If the question is really about whether consciousness is localized in one hemisphere, then a better way to investigate this would be to study hemispherectomies, especially because subcortical or other extraneous influences between the hemispheres would not confound the data in those cases. However, developmental plasticity and individual differences remove any statistical significance of differential processing between hemispheres when one is considering people who only have one hemisphere (Bishop 1983, 1988; Hecaen 1976; St. James-Roberts 1981). Furthermore, there do exist patients with only a right hemisphere who, for all intents and purposes, have command of a language and appear conscious.
6. I conclude that the questions themselves that philosophers and others pose about consciousness with respect to split-brain patients are ill-formed. They do not give enough credence to what is known about individual developmental differences. What we really need first, before we can meaningfully discuss whether split brain patients have two streams of consciousness and whether patients with only one hemisphere are truly aware, are better tests for when people are conscious and a better idea of what "consciousness" includes.
7. The bottom line is that we do not know what consciousness is, nor where or how it appears in the brain. Consequently, postulating that certain organisms are or are not conscious is hopelessly premature. To illustrate this point, let us return to Puccetti's contention that an unintended implication of Dennett's position is that individuals whose neocortex has "died" and right-handers with only a right hemisphere are equally conscious: neocortical death entails a profound and irreversible coma, while the loss of the left hemisphere seems to entail (only) partial paralysis and mutism. Surely, concludes Puccetti, these two sets of individuals do not have the same inner life.
8. At first blush, this seems an entirely reasonable conclusion. Dennett's criterion for conscious selfhood as a robust "center of narrative gravity" (Dennett 1991, p. 418) would also rule out prelinguistic children, certain stroke victims, deaf-mutes, and feral humans as conscious. Certainly, we would not want to do this. Or would we? If a "center of narrative gravity" is in fact a necessary and sufficient requirement for (human-like) consciousness, then, intuitions to the contrary notwithstanding, the examples Puccetti and I have cited are not conscious. Just as certainly, we don't want to claim that just because something acts intelligently, it follows that it is conscious.
9. What we need, in order to decide these sorts of hard cases, is not more intuition-bashing. We need to recognize that being conscious is a property of our brains and as such, there is something in our brains that makes them conscious. We should focus on localizing consciousness in the brain to an area, a network, a circuit type, an oscillation, whatever, BEFORE we start waxing philosophical on who or what lacks consciousness. If we could do this, then we would have a better way to tell whether young or damaged individuals are conscious, rather than relying on our notoriously unreliable intuitions. Criteria like "center of narrative gravity" are only helpful if they can be tied to something more concrete, like mnemonic activations in parietal cortex supported by the hippocampus; and if they can be divorced from our desire for things that act like us to be like us.
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