In his book Hallucinations Oliver Sacks writes of a man Gordon H. who had a stroke and lost left peripheral vision in both eyes – left hemianopia. But he had
“…little awareness of his vision loss, as his brain appears to fill in the missing part. Interestingly, though, his visual hallucinations 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 and distant buildings in his left visual field, which when [he looks at them with his right side] are not really there.”
Sack’s correspondent, Gordon H.’s son, goes on to describe how in the kitchen his father would see the whole of a work surface and even illusory objects on it, which of course vanished when he tried to inspect them more carefully with his good side.
My first reaction on reading this is that my experience is quite different. True, when walking along a lane I see, or imagine I do, vegetation, houses, walls etc. in what is actual my blind region. My brain is filling in these features. But they correspond to real objects, ones I noticed earlier from further off, when they were still in my visual field. So what I am imagining is not the existence of the objects but the fact that I can see them at that moment. My brain is evidently using recent memory to help with the completion process.
I certainly don’t see imaginary objects on a table top, even when I’ve seen their real-life counterparts just a moment ago. I frequently spend minutes trying to locate an object I have put down somewhere. I don’t know if there is a special term for this condition (other than “visual search problems”) but it is recognized problem for those with visual field loss and there is at least one online therapy, Eye-Search, which can help some people.
But wait a minute! My experience is not so different the man Sacks writes about after all. Thinking more carefully I realize that I do sometimes see objects that aren’t there, in both an outdoor and domestic setting, and especially at night. But they are always things that might be a danger. For example in the street I imagine street lights that I might collide with, or branches that might scratch me. Indoors I imagine that my anglepoise lamp is in my way and about to jab into me. These momentary visions are always on my right, in the area where I have vision loss, and they cause me to start and flinch away, until I have checked they are not really there.
So it looks as though my brain is somehow selective about what it fills in and only works hard to invent objects that might be a threat. Perhaps this arises from some kind of instinctive over-caution, a sort defense mechanism in case something dangerous is really there. But I don’t imagine bloodthirsty robbers or snarling wolves, only inanimate objects like rubbish bins. So I don’t know. It’s intriguing.
Grimshaw painting http://commons.wikimedia.org/wiki/File:Grimshaw_-_November,_1879.jpg