Putting aside the sneering and philosophy to nerd for a minute, before getting back to it.

For a long time people were very into the split-consciousness notion of what happened to split-brain people, but a couple things have come around and now some people really think that the better way of thinking of it is still-unitary consciousness with a very difficult time moving around information between different sensory/expression modalities.

First, you get people who are born without a corpus callosum who are behaviorally normal (https://www.tandfonline.com/doi/full/10.1080/13554794.2013.826690). They get a bit of extra connectivity sidways through their deep brain structure as some kind of homeostatic compensation, but the total amount is definitely low. What this says is there’s a difference between a brain that grew under a very unusual set of structural constraints, and one that grew normally that gets shredded. Similar with those people you find now and then with a brain that’s 90% fluid (though with the actual cortex pushed up against the skull around a big bubble of CSF) and the only neurological findings are things like weakness in one leg and an IQ of 80 (worth noting that this is still very very different from hydranencephaly) (https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(07)61127-1/fulltext).

Second, when you do a wider range of experiments with the split brain people you find that while they cannot verbally say what is in their left visual field (which goes to the right side, while language is usually a left-side phenomenon) they can reliably state that something is there with speech, or either hand, and approximately where in the visual field it is. The low bandwidth awareness of presence is there, but they cannot get their speech capacity to access the details. It’s like their sight is now multiple separate sensory modalities, some of which is very difficult to talk about and some of which are very difficult to draw with particular hands.

https://www.uva.nl/shared-content/uva/en/news/press-releases/2017/01/split-brain-does-not-lead-to-split-consciousness.html

https://academic.oup.com/brain/article/140/5/1231/2951052

People argue a lot about what this means

https://www.sciencedirect.com/science/article/pii/S0028393221002402

You can also apparently reorganize around very small amounts of remaining fibers to have no deficits like that, with no issues talking about anything in either part of the visual field

https://news.ucsb.edu/2025/022246/new-findings-split-brain-science-even-minimal-fiber-connections-can-unify-consciousness

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Now, getting out of the nerd mode, there’s a LOT of weird literature from the 60s to 80s about people with very strange brain anatomy who nonetheless developed normally or better than expected

https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8749.1965.tb07839.x

“Two cases of hydranencephaly are described in infants. In both these there was evidence of excessive intracranial pressure-as is often the case-and both were operated on to relieve this. The progress of the older child, now 21 months of age, was throughout excellent physically and mentally, and he is considered to be normal. The progress of the second infant was remarkably good for three months, but thereafter mental retardation and spasticity followed; he was also blind. There is no good explanation for the unexpectedly good progress of the first patient.”

https://www.science.org/doi/10.1126/science.7434023

"…the most severe group, in which ventricle expansion fills 95 percent of the cranium. Many of the individuals in this last group, which forms just less than 10 percent of the total sample, are severely disabled, but half of them have IQ’s greater than 100. This group provides some of the most dramatic examples of apparent ly normal function against all odds. Commenting on Lorber’s work, Kenneth Till, a former neurosurgeon at the Great Ormond Street Hospital for Sick Children, London, has this to say: “Interpreting brain scans can be very tricky. There can be a great deal more brain tissue in the cranium than is immediately apparent.” Till echoes the cautions of many practitioners when he says, "Lorber may be being rather overdramatic when he says that someone has ‘virtually no brain.’ "

https://onlinelibrary.wiley.com/doi/10.1111/j.1469-8749.1999.tb00621.x

“Consciousness in congenitally decorticate children: developmental vegetative state as self-fulfilling prophecy”

“According to traditional neurophysiological theory, consciousness requires neocortical functioning, and children born without cerebral hemispheres necessarily remain indefinitely in a developmental vegetative state. Four children between 5 and 17 years old are reported with congenital brain malformations involving total or near-total absence of cerebral cortex but who, nevertheless, possessed discriminative awareness: for example, distinguishing familiar from unfamiliar people and environments, social interaction, functional vision, orienting, musical preferences, appropriate affective responses, and associative learning. These abilities may reflect ‘vertical’ plasticity of brainstem and diencephalic structures. The relative rarity of manifest consciousness in congenitally decorticate children could be due largely to an inherent tendency of the label ‘developmental vegetative state’ to become a self-fulfilling prophecy”

Worth noting that I looked at that paper and these case studies do have noticeable brian mass around the base of the skull, just not much.

Edit I am also very mad at how people so reductionistically talk about different behaviors being restricted to different parts of brain anatomy. It’s different in different creatures. You strip the cortex out of an adult cat and itll still walk around and look at things, though not be all there (yes this was done in the sixties), you strip it from an adult human you get a vegetable. Lots of brain parts are capable of lots of things, its just that as brains get bigger the more peripheral parts are easier to expand faster and grow in importance, their fibers exerting more control over the rest, and I would not be surprised at all at other brain bits being capable of quite a lot when they grow without the influence of the bigger bits.

Anybody ever read the short story “Cutie” by Greg Egan? Very apropos…>