Posts tagged ‘split brain’

Classic ‘Split Brain’ experiments by Sperry and Gazzaniga could not be reproduced, according to Yair Pinto of the University of Amsterdam. Has the crisis of reproducibility claimed an extraordinary new victim?

The original experiments studied patients who had undergone surgery to disconnect the two halves of their cerebrum by cutting the corpus callosum and associated commisures. This was a last-ditch method of controlling epilepsy, and it worked. The patients’ symptoms were improved and they did not generally suffer any cognitive deficits. They felt normal and were able to continue their ordinary lives.

However, under conditions that fed different information to the two halves of the brain, something strange showed up. Each half reported only its own data, unaware of what the other half was reporting. The effect is clearly demonstrated in this video with Gazzaniga himself…

On the basis of these remarkable results, it has been widely assumed that these patients have two consciousnesses, or two streams of consciousness, operating separately in each hemisphere but generally in such harmony they don’t notice each other. Now, though, performing similar experiments, Pinto and colleagues unexpectedly failed to get the same results. Is it conceivable that Sperry and Gazzaniga’s patients were to some degree playing along with the Doc, giving him the results he evidently wanted? It seems more likely that the effects are just different in different subjects for some reason. In fact, I believe the details of the surgery as well as the pre-existing condition of the patients varies. Recently the tendency has been towards less radical surgery and new drugs; in fact there may not be any more split-brain patients in future.

I don’t think that accounts for Pinto’s results, though, and they certainly raise interesting problems. In fact the interpretation of split-brain experiments has always been disputed. Although in experimental conditions the subjects seem divided, they don’t feel like two people and generally – not invariably -behave normally outside the lab. We do not have cases where the left hand and right hand write separate personal accounts, nor do these patients

Various people have offered interpretations which preserve the essential unity of consciousness, including Michael Tye and Charles E. Marks. One way of looking at it is to point out how unusual the experimental conditions are. Perhaps these conditions (and occasionally others that arise by chance) temporarily induce a bifurcation in an essentially united consciousness. Incidentally, so far as I know no-one has ever tried to repeat the experiments and get the same effects in people with an intact corpus callosum; maybe now and then it would work?

More recently Tim Bayne has proposed a switching model, in which a single consciousness is supported by different physical bits of brain, moving from one physical basis to another without sacrificing its essential unity.

There is, I think, a degree of neurological snobbery about the whole issue, inasmuch as it takes from granted that consciousness is situated in the cerebrum and that therefore dividing the cerebrum can be expected to divide consciousness. Descartes thought the essential unity of consciousness meant it could not reside in parts of the brain which even in normal people are extensively bifurcated into two hemispheres (actually into a number of quite distinct lobes). We need not follow him in plumping for the pineal gland as the seat of the soul – and we know that the cerebellum, for example, can be removed without destroying consciousness). But what about the lowly brain stem? Lizards make do with a brain that in evolutionary terms is little more than our brain stem, yet I’m inclined to believe they have some sense of self, and though they don’t have human cogitation, they certainly have consciousness in at least the basic sense. Perhaps we should see consciousness as situated as much down there as up in the cortex. Then we might interpret split-brain patients as having a single consciousness which has some particular lateralised difficulties in pulling together the contributions of its fancy cortical operations.

It might be that different people have different wiring patterns between lower and higher brain that either facilitate or suppress the split-brain effects; but that wouldn’t altogether explain why Pinto’s results are so different.