When author Howard Engel picked up his newspaper one morning, he found himself unable to read it. He could sense it was the same 26 alphabets of English he'd grown up with, yet the shapes seemed to shift from Cyrillic to Korean — it was as if the paper was written in Serbo-Croatian. Engel reached a conclusion, calmly:
I HAVE SUFFERED A STROKE.
Engel is one of the many we meet in The Mind's Eye, the latest book from celebrated neurologist Oliver Sacks. Like his famous The Man Who Mistook His Wife for a Hat, the book compiles insightful first hand accounts and patient profiles that illuminate the perplexing workings of the human brain. This time, the overarching theme is the brain’s role in our visual abilities.
Agnosia, Alexia, Agraphia
Agnosia is the inability to interpret sensory input. Howard Engel's condition, a form of visual agnosia, is alexia, the inability to interpret text. Agraphia, likewise, is the inability to write; people who suffer from alexia often do not yet suffer from agraphia (“alexia sine agraphia”): in such a condition, they are able to write a letter, let's say, but not read what they have just written. The two functions, it turns out, are less connected than we think.
The Wallace Problem
Examinations show that in the dominant hemisphere of the brain is a neural system that specializes in processing visual notation — letters, numbers, symbols, musical notation, and so on. Alfred Russell Wallace, who came to the conclusion of natural selection independently of Charles Darwin, was plagued with the paradox of the brain’s lexical abilities: how was it that our prehistoric brain was equipped for reading though writing itself is a fairly new phenomenon in human history? The culture of writing and reading is considered too new for our brain to have evolved specifically for that purpose. It was this paradox that led Wallace to conclude that the brain, which seemed to have anticipated the future of reading, was a gift from God.
“Exaptation” and the Plastic Mind
Darwin responded to Wallace, “I hope you have not murdered completely your own and my child”. For Darwin, there was no paradox; a system that evolved for a certain function can be expected to find a new use. This phenomenon has been recently given the name “exaptation”. It’s no coincidence that the types of writing systems that now survive can be grouped into just two categories: alphabetical and ideograms; it was a pre-existing neural system that dictated the nature of our writing systems. As Oliver Sacks explains, the brain’s power of combination must be called upon to make sense of a finite set of shapes that can be arranged in infinite ways, and, it turns out, the brain’s plasticity makes it highly responsive to new experiences and training. “Inferotemporal neurons evolved,” he writes, “for general visual recognition, but they may be recruited for other purposes — most notably reading.”
A Bundle of Wonders
When something works seamlessly — you look at the word “cat” and understand its meaning instantaneously — it’s hard to take note of the complexities that go behind the scenes. It’s only when something goes awry — a stroke, a loss of sight — that the wonders of our nerve tissue bundle come unraveling; one person’s total inability to conjure up visual images after the loss of sight, another’s heightened development of visual imagery after the loss of sight, Sacks’ own hallucinations while undergoing eye cancer treatment, the brain’s own Photoshop Auto Fill function, and so on.
Hi Mom, You Look Different
Oliver Sacks himself suffers from a mild form of another kind of
visual agnosia: face blindness, or prosopagnosia, the inability to
locate a face in one's memory; he sees the features of a face, can even
judge its beauty, but has no memory of having seen it before, let alone assign it to a person he knows. As a
result, Sacks has stared at another bearded man thinking it to be his
own reflection, and started at his own reflection thinking it to be another
man.
Further Reading
