It’s crazy that you can cut all 300 million axons in a person’s corpus callosum and still have a high-functioning person with just a couple of specific quirks. All that white matter is expensive to build, maintain, and operate. This is weird for a species whose evolutionary trajectory has been from higher nutrition needs to lower nutrition needs (I think scientists are spinning this as “more efficient”) and bigger brains to smaller brains.
The purpose of this giant patchboard seems to be maintaining the correspondence between reality and concepts. White matter connections between gray matter nuclei can be conceptualized as wires connecting electronic components where the real processing takes place in specialized functions (like visual processing).
The absence of this structure produces the sort of behavior we expect from homo sapiens in general.
For example, a patient with split brain is shown a picture of a chicken and a snowy field in separate visual fields and asked to choose from a list of words the best association with the pictures. The patient would choose a chicken foot to associate with the chicken and a shovel to associate with the snow; however, when asked to reason why the patient chose the shovel, the response would relate to the chicken (e.g. “the shovel is for cleaning out the chicken coop”).
Isn’t it fascinating that this behavior is not just effortless post-facto rationalizing (like we’d expect from Harvard graduates or Talmudic scholars trained in tactical nihilism) but actually the only possible behavior for split-brained patients?
I have always needed to overcome my disgust reflex when observing this behavior because my instinctive understanding has been that people who do this are pretending to be stupid in order to score debate points, rather than properly examining reality. And sometimes I think it is on purpose. But we’ve also seen corpus callosum breakdown in thals who overextend themselves during a hypomanic phase and then exhaust their reserve mental energy until there isn’t enough chemical energy to power the white matter, which manifests in some bizarre symptoms (like one side of the body becoming much warmer).
So maybe this correspondence breakdown can be triggered voluntarily or by habit, but sometimes it’s just dysfunction in white matter maintenance.
Another curious thing, which I only learned today, is that people who suffer strokes in the left hemisphere often become depressed, and people who suffer strokes in the right hemisphere often become manic. This makes me think that, in general terms, the purpose of having two hemispheres is to maintain homeostasis in counterbalanced systems.