The Unabashed Academic

07 June 2011

Local or Global Coherence?

In my last post I wrote about Oliver Sacks’ essay, “Stereo Sue” in his book The Minds Eye where he told of a woman who first developed stereoscopic vision in her 40s. In the following essay, “The Persistence of Vision”, he talks about his own loss of stereopsis as a result of the growth of a melanoma behind his right eye. First, as a result of the tumor, and then as a result of the treatment to kill the tumor, he lost vision in the eye – slowly, over a period of four years.
Besides this being scary (Who knew you could get melanoma behind your eyeball?), it turned out to be fantastically informative about the complex way in which vision works. Sacks, ever the scientist, kept a careful journal, drawing what he saw and describing how his brain interpreted the changing data stream. I think that there’s a lot to learn from his experience, but two points struck me as telling us something interesting about how the brain works: filling in, and reinterpreting, both saying something about the way our brain build the virtual world we live in. (See my earlier post, “Axioms”.)
At one point in his treatment, he had lost vision in the foveal part of the eye – the highly accurate cluster of retinal cells that we do most of our direct looking at things with. He retained peripheral vision (and has interesting things to say about that), but developed a scotoma – a large blind spot in the middle of the vision of his right eye. It’s well known that the brain “fills in” the blind spot – up to a point. (You can try this yourself with your own smaller and normal blind spot.) I read about this in a psych book a couple of decades ago. I recall a story of a man with a scotoma looking at a person sitting on a couch. When he turned to put the person’s head in the hole in his vision the person’s head disappeared – but an image of the patterned wallpaper behind the couch filled in the space where the head should have been. Sacks explored what filled in and what didn’t.
“It was easy to fill in a simple repetitive pattern – I started with the carpet in my office – though a pattern took a bit longer than a color, perhaps needing ten or fifteen second to duplicate. It would fill in from the edges, like ice crystallizing on a pond…I found that movement could be also filled in to some extent. If I looked at the Hudson River, slowly swirling or rippling with small waves, these too were reproduced…But there were strict limits. I could not simulate a face, a person, a complex object.”
Interestingly, he found that if he cut off his foot with his blind spot but wiggled it,
“the stump seemed to grow a sort of translucent pink extension with a ghostly protoplasmic halo around it. As I continued wiggling my toes, this took on a more definite form, until, after a minute or so, I had a complete phantom foot…which seemed to move with the movements I was making.”
This, and other examples he cites, seem to indicate that the brain “recruits resources” – hunting for things that will bring together the various bits of information it has to make a coherent sense.
But a second observation leads in a different direction. Once he lost peripheral vision in his right eye he lost the ability to see anything to the right side of his nose and developed what neurologists call “unilateral neglect.”
“…whatever comes into my visual field from that side is unexpected and startling. I cannot overcome the sense of bewilderment, even shock, when people or objects appear suddenly from my right. A massive slice of space no longer exists for me…”
Particularly surprising is that his brain seems to treat objects that move into that space as vanishing – even people.
“Kate [his wife] and I finished our walk and headed back to my office, I walked ahead and got into the elevator – but Kate had vanished. I presumed she was talking to the doorman or checking the mail, and waited for her to catch up. Then a voice to my right – her voice – said, ‘What are we waiting for?’ I was dumbfounded – not just that I had failed to see her to my right, but that I had even failed to imagine here being there, because ‘there’ did not exist for me.”
This (and other such examples) are fascinating because he knows at some level that he cannot see on the right. Yet the changing in accustomed patterns of sensory data seems to undo a component of his sensory coherence. He cites a colleague (M.-M. Mesulam) as saying, “Patients with unilateral neglect behave not only as if nothing were actually happening in the [blind side] hemisphere, but also as if nothing of importance could be expected to occur there.”
This, and other examples seems to suggest that the brain builds a local coherence, but can neglect large blocks of data that it knows perfectly well in responding to an immediate environment.
One of the most controversial theoretical issues in science education today is the appropriate scale at which to expect coherence in student thinking. I remember a conversation at an international science education conference nearly two decades ago with a friend and colleague about this issue. He took the point of view, “People live in the world. They have to have a coherent view of it. If they don’t, they would go crazy. When students give answers that seem bizarre to us, it’s just that they have a different coherence from ours. As science educators, it’s our job to find out how to one might look at the world coherently and see it the way they do.”
This is an extreme view (and he may have moderated it by now), but many education researchers assume – sometimes tacitly – that students are fully coherent in their thinking about something, even if we, from our perspective, don’t see what the coherence is that they are seeing.
If something as fundamental and as evolutionarily vital to survival as our interpretation of the spatial world around us through vision shows only local but limited coherence, I suggest that we can’t take coherence of brain function for granted. Rather, we have to treat it as an empirical issue, to be explored under varying contexts and conditions.


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