A particular type of epilepsy has been treated by cutting the corpus callosum, the tracks of nerves connecting the two hemispheres of the cerebrum. The result had very little side effects on the patients. However, with closer experimental studies, the nature of the split brain was examined. Only the left hemisphere spoke and so only stimuli presented to the left visual field resulted in spoken replies and responses of the right hand. The right hemisphere could understand written language presented to the right visual field and made responses with the left hand but never spoke. Based on this and similar evidence, it was assumed that there were two minds (that is two consciousnesses) in a split brain.
A recent paper has upset this hypothesis: Pinto, Neville, Otten, Corballis, Lamme, de Haan, Foschi, & Fabri; Split brain: divided perception but undivided consciousness; Brain Jan 2017. Here is the abstract:
“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.”
When they showed an object in both visual fields and if the objects were the same or different, the split brain subject could not answer that question with either hand or by speech. They could not examine the objects together – so it was correct that the perception in the two hemispheres was separate and isolated. But if an object was placed in either or both visual fields, the subjects could say how many objects there were in total and there was no different in the answer coming from the left or right hand, or the voice. So although they could not examine the objects together, their consciousness covered the entire visual field – there was only one consciousness.
What can explain this if the results hold up? Perhaps the two hemispheres have learned unusual ways of communicating outside of the normal connections. Perhaps it is some dualistic magic. Or, to me more likely, consciousness is not a product of the cerebrum. It is created in some other part of the brain that can receive information from both hemispheres and can store its creation in immediate memory where it is available to the hemispheres. There is an obvious candidate, the thalamus. It is not cut in half by the cutting of the corpus callosum. It is connected to almost all areas of the brain and almost all information passes through it at some stage of its processing. It is the one part of the brain that must be functioning for consciousness to occur.
There has been for years an assumption that the cerebrum is the engine of thought and a number of things are puzzles because they cannot be understood looking at the cerebral cortex alone. It is time to thing about the possibility that the thalamus drives the cerebrum: it feeds information to the cortex, it creates the rhythms and synchronization in the cortex, and it controls the communication networks in the cortex. The thalamus may have the cortex as an on-line computer, to use that metaphor. But then the thalamus is in the center of the brain and the cortex is laid out on the surface. It is easier the examine the cortex and so the rest of the brain gets neglected. Like the man looking for his keys under the street lamp because the light is better there even though he lost them elsewhere.