Does McGilchrist provide good scientific evidence for the thesis in The Master and his Emissary?

(The outline of McGilchrist’s thesis is here: https://jotsandscribbles.blog/2023/11/07/introduction-to-the-master-and-his-emissary/)

McGilchrist has spent several decades reading the neuroscience literature, and he presents lots of studies to back up his thesis that the two hemispheres (half brains) process the world differently. I’m obviously not in a position to fully evaluate his scientific evidence. McGilchrist spends about 60 pages going through studies showing left right differences, and I’ll present a few examples here. Then next post I’ll look at some objections scientists have raised to his claims.

McGilchrist uses animal studies, imaging, and especially studies of patients with lesions (damage) to parts of the brain. For example, showing hemisphere differences in animals:

many types of birds show more alarm behaviour when viewing a predator with the left eye (right hemisphere), are better at detecting approaches with the left eye, and will choose to examine predators with their left eye, to the extent that they have detected a predator with their right eye, they were actually turned their head so as to examine it further with the left.

In general toads attend to their prey with their left hemisphere, but interact with their fellow toads using the right hemisphere.

The two hemispheres respond differently to hormones (the right hemisphere is more sensitive to testosterone) and rely more on different neurotransmitters (left more reliant on dopamine, right more on noradrenaline).

Patients with brain damage, whether individual cases or wider studies, feature regularly. There are also ways to temporarily inactivate parts of the brain.

Split brain patients from earlier treatment for epilepsy can be good subjects for hemisphere experiments. The right brain gets the left field of vision, and the left brain receives the right field of vision. So by putting objects on one side of the field of vision, you can see how that hemisphere (no longer connected to the other hemisphere) functions. Objects shown in the left visual field cannot be named since the right hemisphere in most subjects cannot speak. But the person can indicate the correct choice of object with their left hand, showing they have recognised it.

Other findings from lesions:

“vigilance and sustained attention are grossly impaired in subjects with right-hemisphere lesions…in patients with left hemisphere lesions… vigilance is preserved.”

“Deficits in focused attention are more severe with left-hemisphere injury.”

As evidence that the right hemisphere receives new things before passing them on to the left, “Patients with right hemisphere damagedon’t seem to be able to adjust the breadth of the ‘spotlight’ of their attention.: they suffer ‘an excessive and more or less permanent narrowing of their attentional window’.”

A patient of mine with a right temporoparietal deficit asked me ‘What’s all this with the eyes?’ When I asked what she meant, she explained that she had noticed people apparently communicating coded messages with their eyes, but could not understand what they were, presumably because the part of her brain that would have interpreted it was no longer functioning – further grounds for paranoia and those who have to rely on their left hemisphere to constitute the world.

While individual studies may have their weakness, the cumulative effect of the studies McGilchrist presents provides good evidence that the right and left hemispheres are perceiving the world in very different ways, and that we need both to understand and function well. But we’ll need to hear from some critical neuroscientists to test the claims- next post in this series…