Oliver Sacks Hallucinations (2012), Picador, London

Oliver Sacks Hallucinations (2012), Picador, London

All of these effects [various forms of hallucination] seem to show, by default, what a colossal and complicated achievement normal vision is, as the brain constructs a visual world in which color and movement and size and form and stability are all seamlessly meshed and integrated. I came to regard my own migraine experiences as a sort of spontaneous (and fortunately reversible) experiment of nature, a window into the nervous system – and I think this was one reason I decided to become a neurologist.

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Heinrich Klüver, writing about mescal, remarked that the simple geometric hallucinations one might get with hallucinogenic drugs were identical to those found in migraine and many other conditions. Such geometrical forms, he felt, were not dependent on memory or personal experience or desire or imagination; they were built into the very architecture of the brain’s visual system.

Links to Lewis-Williams’ ‘stages of consciousness’ in some cave art

One does not see with the eyes; one sees with the brain, which has dozens of different systems for analyzing the input from the eyes. In the primary visual cortex, located in the occipital lobes, at the back of the brain, there are point-to-point mappings of the retina onto the cortex, and it is here that light, shape, orientation, and location in the visual field are represented. Impulses from the eyes take a circuitous route to the cerebral cortex, some of them crossing to the opposite side of the brain as they do so, so that the left half of the visual field of each eye goes to the right occipital cortex, and vice versa. It, therefore, one occipital lobe is damaged (as by a stroke, for example), there will be blindness or impaired vision in the opposite half of the visual field – a hemianopia. 

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