A Brain of Two Halves

At a recent talk, Iain McGilchrist was promoting his new book ‘The Master and His Emissary’. In it, he examines the differences between right and left hemispheres of the brain and proposes a metaphor. The left hemisphere is the ’emissary’, attending to details; the right hemisphere is the ‘master’, more interested in an overview.
McGilchrist argues that, whilst supposedly subordinate to the master, the emissary has become too powerful. ‘Left-brain thinking’ has come to dominate modern thought, to the detriment of society.

I like bold conjectures – thinking with a broad brush. It reminded me of Jared Diamond’s ‘Guns, Germs and Steel’ which posits the theory that the current Western hegemony is due to the accident of geography: for example, the temperate East-West (‘landscape’) orientation of Eurasia and the ‘crinkliness’ of European coastline in contrast to the tropical North-South (‘portrait’) alignment of Africa. Both are susceptible to the charge of ‘greedy reductionism’ but I’d want to defend them. Neither are definitively ‘right’ but both shed some light on hugely complex topics – and in doing so advance our understanding.

In his talk, McGilchrist made two particular claims which stuck in my mind:

  1. The Corpus Callosum has decreased in size as the brain has evolved to its current large size. But this claim is misleading. The book clarifies. The number of connections between left and right-hand halves has stayed the same. It is the number of connections as a proportion of the total number of connections within the entire brain that has decreased as a result of the brain getting larger. This correction does not invalidate the point that McGilchrist is making.
  2. During evolution, one eye of a fish focuses on a narrow region (‘telephoto’) for seeing details whereas the other eye sees ‘wide angle’ (to provide an alarm for predator, for example) [haven’t found a reference to this].

More recently, I saw the Building the Brain episode of BBC Michael Mosley’s Inside the Human Body, which highlights the case of a girl with Sturges-Weber Syndrome (approx 28 minutes into the program). Half of her brain is diseased [which half?], so surgeons perform a hemispherectomy – removal or disconnection of the diseased half of the cortex. This is done at a very young age, when there is highest plasticity to allow ‘right-half’ functions to be mapped to parts of the left hemisphere. See the video below for another example: a successful hemispherectomy (of the right hemisphere) to treat Rasmussen’s Encephalitis in a 3-year-old (now 9-year-old):

It is incredible that this can be done at all. Unfortunately McGilchrist does not mention hemispherectomies.

(This blog will be updated when I’ve read the whole book!)

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