The corpus callosum is a structure that connects the cerebral cortices of the two hemispheres of the brain. In the 1940s, the surgical operation of callosotomy (cutting the corpus callosum) was introduced by William P. van Wagenen as a treatment of last resort for patients with severe epilepsy. Twenty years later, Roger Sperry conducted a series of experiments on these ‘split -brain patients’, which revealed peculiar behavioural features indicating that the two hemispheres were working independently. This gave rise to the popular view that the conscious mind of the patient had been split into two minds, only one of which (in the left hemisphere) possessed language skills.
The notion of two minds in a split-brain patient is entirely consistent with the philosophical theory that the conscious mind is literally situated inside the physical brain tissue. It is harder to reconcile this notion with the rival theory of the conscious mind as non-local entity, which I have been arguing for in my previous posts.
The two-minds notion remained the standard understanding of split-brain patients until 2017, when Yaïr Pinto published new studies that revolutionised the field. In fact, as Pinto pointed out, there had been inconsistencies in Sperry’s original papers from the beginning, but these had been overlooked and the standard view was ‘cast in stone’ in textbooks and review papers.
In short, Pinto showed that there was actually a single conscious mind spanning the two separated hemispheres. There is not, as yet, any plausible theory of how this could happen if the mind is supervenient on brain tissue. But it is what one would expect if the conscious mind were a non-local entity that functions independently of the brain and communicates with the brain via local neurological portals.
The new research
Pinto et al. (2017) first of all observe that split-brain patients don’t notice anything odd, either in their day-to-day lives or in the experimental set-up. They do not report any big holes in their perceptual fields, nor any weird involuntary movements of their hands. But the crucial finding is that the left brain can report (verbally or using the right hand) conscious experiences arising in the right hemisphere, and vice versa. The patients shouldn’t be able to do this if conscious experiences are situated in isolated hemispheres. Moreover, the patients report high subjective confidence in the (correct) judgements about the presence and position of things seen in the other hemisphere. What the split-brain patients can’t do is to make direct comparisons between visual experiences in the visual fields that are associated with the two hemispheres. That is, the left-hemisphere can verbally report the presence and position of a shape in the right visual hemi-field, but cannot say whether it resembles a shape im the other hemi-field.
Further experiments (Pinto et al. 2023) showed that the cross-hemisphere comparisons became doable when the subject was asked to perform some task that involved paying conscious attention to the images. In summary, what the research seems to be showing is that low-level, pre-conscious brain activity is split by the callosotomy, but conscious experiences and actions that are based on separated stimuli to the two hemispheres are co-conscious in one unified mind.
The authors observe that this undermines two popular theories of consciousness—the global workspace theory and integrated information theory. They briefly mention the possibility of a quantum mechanical explanation but this is not tenable as the brain is a large, warm body that would quickly decohere any quantum superpositions over relevant distances across hemispheres.
de Haan et al. (2020) provide a useful review of this evolving experimental field as it was five years ago. Jordan Accardo (2023) has uploaded a very good introductory video, including an interview with Yaïr Pinto. And David Chalmers (2018) has uploaded a video of a debate with Yaïr Pinto, Elizabeth Schechter, and Joseph LeDoux.
The papers referenced below are very readable, and I would definitely urge you to take a look at them, as well as the two online videos.
Relevance
Why this neuropsychological is relevant to consciousness engineering is that it may corroborate the theory that I have been proposing in this series of posts.
My assertion is that the conscious mind is a non-physical, non-local information processing entity that exchanges information with its embodying object (be it a brain or a computer) via portals that are macromolecules uniquely tagged for the individual mind, which can go into conformational superposition that is collapsed by the conscious mind. The secondary hypothesis is that the actual macromolecule in human brains is a microtubule bound to DNA.
In the split-brain experiments, there is prima facie evidence that physical transmission of information between cerebral hemispheres has been blocked. Nevertheless the DNA in both hemispheres is obviously the same, so the molecular tagging of portals is the same. So, by hypothesis, the mind-brain portals in both hemispheres should map to the same mind. Which, apparently, they do.
In summary, what we have is:
Experimental results with split-brain patients that lack a plausible physical explanation.
A theory of non-local consciousness that, if true, might provide an explanatory framework for single consciousness in split-brain patients.
The renewed experimental investigation of mind localisation in split-brain patients is still nascent, and the results do not yet prove any model. Nonetheless, the theory of non-local consciousness is a contender and therefore worth developing further.
In my next post, I will consider anomalous communication between identical twins, who of course also share the same DNA.
References
Pinto et al. (2017): Yaïr Pinto, David A. Neville, Marte Otten, Paul M. Corballis, Victor A.F. Lamme, Edward H.F. de Haan, Nicoletta Foschi, Mara Fabri, “Split brain: divided perception but undivided consciousness”, Brain 140(5):1231–1237, May 2017. https://doi.org/10.1093/brain/aww358 (https://academic.oup.com/brain/article/140/5/1231/2951052)
de Haan et al. (2020): Edward H.F. de Haan, P.M. Corballis, S.A. Hillyard, C.A. Marzi, Anil Seth, V.A.F. Lamme, L. Volz, M. Fabri, Elizabeth Schechter, T. Bayne, M. Corballis, Yaïr Pinto, “Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness”, Neuropsychological Review 30(2):224-233, June 2020. https://doi.org/10.1007/s11065-020-09439-3.
Pinto et al. (2023): Yaïr Pinto, Maria-Chiara Villa, Sabrina Siliquini, Gabriele Polonara, Claudia Passamonti, Simona Lattanzi, Nicoletta Foschi, Mara Fabri, Edward H. F. de Haan (2023), “Visual integration across fixation: automatic processes are split but conscious processes remain unified in the split-brain”, Frontiers in Human Neuroscience 17, 10 November 2023. https://doi.org/10.3389/fnhum.2023.1278025 (https://www.frontiersin.org/journals/human-neuroscience/articles/10.3389/fnhum.2023.1278025/full)
Jordan Accardo (2023): “Split Brain Research: A Cautionary Tale of Sloppy Science”, Tangled Tangent, 10 June 2023, YouTube link below:
Chalmers et al. (2018). “Debate: ‘Do Split-Brain Patients Have Two Minds?’ (LeDoux, Pinto, Schechter)”, NYU Center for Mind, Brain and Consciousness, 25 October 2018, chaired by David Chalmers, with Yaïr Pinto, Elizabeth Schechter, and Joseph LeDoux. YouTube link:
Will dig into this paper, I’m deep down a rabbit hole about quantum consciousness and love finding new examples to test out some things I’m working on.