Split Brain Research Fails to Replicate Different Hemisphere Awareness After Corpus Callosotomy

According to the Nobel-prize-winning work in the 1960's by Sperry and Gazzaniga, after the two cerebral hemisphere are cut by callosotomy (a surgery sometimes done for intractable epilepsy), a person seeing an object in one visual field can signal seeing it with the hand on that side, but not with the hand on the other side, which will respond as if no object was seen.

I'd heard several tales in more recent years from neuropsychologists just out of university training of how researchers at their institutions were failing to replicate those classic results. This was usually attributed to variations in the surgery performed, as well as the fact the posterior commissure was usually left intact.

However, in a recent issue of Brain, Pinto and colleagues below report they have systematically re-studied many of the patients from these surgeries and are showing that a single awareness persists in such subjects-- exactly as the persons who had the surgery (I've seen three since the 1990's) themselves will tell you, if you ask them :).

What improved over the decades? Perhaps they got better at re-routing the interrupted communications. Did the subjects ever have more than one center of consciousness? Probably no more than we do when we split our attention between simultaneous tasks.



Split brain: divided perception but undivided consciousness

Yair Pinto David A. Neville Marte Otten Paul M. Corballis Victor A. F. Lamme Edward H. F de Haan Nicoletta Foschi Mara Fabri

Brain (2017) aww358. DOI: https://doi.org/10.1093/brain/aww358

Published: 24 January 2017

In extensive studies with two split-brain patients we replicate the standard finding that stimuli cannot be compared across visual half-fields, indicating that each hemisphere processes information independently of the other. Yet, crucially, we show that the canonical textbook findings that a split-brain patient can only respond to stimuli in the left visual half-field with the left hand, and to stimuli in the right visual half-field with the right hand and verbally, are not universally true. Across a wide variety of tasks, split-brain patients with a complete and radiologically confirmed transection of the corpus callosum showed full awareness of presence, and well above chance-level recognition of location, orientation and identity of stimuli throughout the entire visual field, irrespective of response type (left hand, right hand, or verbally). Crucially, we used confidence ratings to assess conscious awareness. This revealed that also on high confidence trials, indicative of conscious perception, response type did not affect performance. These findings suggest that severing the cortical connections between hemispheres splits visual perception, but does not create two independent conscious perceivers within one brain.

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