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.


  1. 1. SelfAwarePatterns says:

    The original split-brain results did not completely fail to be reproduced. The actual paper details that there remain significant impairments for one hemisphere to identify what the other hemisphere is seeing. The new development was that the two patients could identify the existence of an object and its location, but not its properties.

    One of the hypotheses the authors put out is that the difference may be that these two patients are being tested decades after their callosotomy, where the original tests took place much sooner. Maybe the full-on split-brain phenomenon is transitory. It’s possible brain plasticity has strengthened these patients’ abilities over the decades. Unfortunately, since the procedure is rarely done anymore, testing this may not be feasible.

    It also seems like these results are compatible with the blindsight phenomenon, where patients with damage to visual areas in the cerebrum can’t consciously see something, but often when pressed can still identify it, likely because the sub-cortical areas can still process the visual information, albeit below the level of consciousness. And plasticity over decades may have strengthened the blindsight mechanism in these patients.

    Interestingly, the authors also see their results as challenging the IIT (Integrated Information Theory) and Global Workspace theories of consciousness, but not “local recurrent processing theory”, whatever that is.

  2. 2. Brian B. says:

    “Now, though, performing similar experiments, Pinto and colleagues unexpectedly failed to get the same results.”

    Similar may not be good enough. Replication requires the exact reproduction of experimental conditions. In this case, the difference in results may come from differences in the surgeries.

    Plasticity and accommodation (where brain functions are moved to other ares when their primary support has been damaged or removed) can also account for a lot.

  3. 3. Life Continues Within the Body After Death – Seeker | We Seek the Truth! says:

    […] inquiry into the Split Brain Fails To Yield Split Consciousness. Pinto’s conclusions on the Split Brain Not Being Reproducible, notes Peter Hankins, aren’t so cut-and-dried. What’s the elephant in this room? How […]

  4. 4. Callan S. says:

    Going a bit further than SAP suggested, have they scanned to check the corpus callosum has not simply healed/reconnected somehow? Or there may be more rerouting that can occur in some other way.

    Also, did they run the tests with patients who had already gone through the tests before (given there are so few split brain patients, I’m guessing yes). Really that’s bad science – there should be a control who has gone through no such experiments. It’s possible that the patients have learnt the test and are skewing the results because of that.

  5. 5. Peter says:

    They did confirm that the corpus callosum was severed. There were in fact only two subjects – I think they were ‘fresh’.

  6. 6. Callan S. says:

    Well, that sets up quite an anomaly. I don’t really dig animal testing, in addition to it being a test on a different species – and there will probably be no more split brain patients soon enough. Thankfully – I think it’s barbaric, at the level cavemen drilling holes in skulls to let demons out.

  7. 7. lornezo sleakes says:

    The cerebral hemispheres may be responsible for a higher level cognitive processing or “access consciousness” and so can understand and interpret the meaning of what is perceived in its respective fields.

    The brainstem may implement a more basic core “Phenomenal consciousness” which enables simple spatial interactions in the present moment. This may explain why some basic interactive capabilities persist while cognitive functions are split.

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