There are three types of experiments one can perform in neuroscience: lesions, stimulations, and recording. Obviously, a particular study can use more than one of them.
The most basic natural experiment that one can harness in neuroscience is to study lesions, due to problems in development, disease, and/or trauma.
Of these, perhaps the most striking lesions come from patients with severe hydrocephalus. Hydrocephalus is the accumulation of cerebrospinal fluid in the brain that causes ventricles to enlarge and compress the surrounding brain tissue.
A 2007 case study by Feuillet et al. of a 44-year old man with an IQ of 75 and a civil-servant career is probably the most famous, since they provide a nice brain set of brain scans of the person:
A 1980 paper is also famous for its report of a person with an IQ of 126 and an impressive educational record who also had extensive hydrocephalus. But no image, so not quite as famous.
The 2007 case has been cited as evidence to a) question dogma about the role of the brain in consciousness, b) speculate on how two minds might coalesce following mind uploading, and c) — of course — postulate the existence of extracorporeal information storage. There are also some great comments about this topic at Neuroskeptic.
As far as I can tell, volume loss in moderate hydrocephalus is initially and primarily due to compression of white matter just adjacent to ventricles. On the other hand, in severe hydrocephalus such as the above, the grey matter and associated neuropil also must be compressed.
Most of the cases with normal cognition appear to be due to congenital or developmental hydrocephalus, causing a slow change in brain structure. On the other hand, rapid changes in brain structure due to acute hydrocephalus, such as following trauma, are more likely to lead to more dramatic changes in cognition.
What can we take away from this? A couple of things:
- This is yet another example of the remarkable long-term plasticity of both the white matter and the grey matter of the brain. Note that this plasticity is not always a good thing, but yes, it exists and can be profound.
- It is evidence for hypotheses that the relative positions and locations of neurons and other brain cell types in the brain is the critical component of maintaining cognition and continuity of consciousness, as opposed to their absolute positions in space within the brain. An example of a theory in the supported class is Seung’s “you are your connectome” theory.
- Might it not make the extracellular space theories of memory a little less plausible?