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Oliver Sacks – Hallucinations

I received the following letter from a physician in England, about his eighty-six-year-old father, Gordon H., who had long-standing glaucoma and macular degeneration. He had never had hallucinations before, but recently he had had a small stroke affecting his right occipital lobe. He was "quite sane and largely intellectually undiminished," his son wrote, but ‘he has not recovered vision and retains a left hemianopia. He has,  however, little awareness of his visual loss as his brain appears to fill in the missing parts. Interestingly, though, his visual hallucinations / filling in always seem to be context-sensitive or consistent.

In other words, if he is walking in a rural setting, he can be aware of bushes and trees or distant buildings in his left visual field, which when he turns to engage his right side, he discovers are not really there. The hallucinations do, however, seem to be filled in seamlessly with his ordinary vision.

If he is at his kitchen bench, he "sees" the entire bench, even to the extent of perceiving a certain bowl or plate within the left side of his vision - but which on turning disappear, because they were never really there. Yet he definitely sees a whole bench, with no clear separation between parts composed of hallucination and true perception.