My aunt Jeannie died of brain cancer when she was just in her 30s. Though her death was tragic, her illness did allow me to witness firsthand a most curious vision impairment. A few months after her cancer was diagnosed, she suffered a stroke in her right visual cortex. Since the visual cortex in some ways serves as a mirror image of the area we’re looking at, this meant that she had a very large blind spot covering most of the left side of her field of vision.
This cortical blindness is different from other sorts of blindness, because the viewer doesn’t perceive that something is “missing” from the visual field: the brain simply doesn’t process part of the visual information it receives from the eye. Jeannie wasn’t even aware of what she was missing. If you placed a plate of food in front of her, she would eat the food on the right side of the plate, leaving the left side intact. You could rotate the plate 180 degrees so the food on the left was visible, and from her perspective it was as if new food appeared by magic.
But far from being amusing, this phenomenon upset Jeannie terribly: she felt as if she was being tricked, the butt of jokes. My mother, who was providing much of Jeannie’s care, finally took to serving her food only on the right side of the plate.
Recent research has found, however, that people with cortical blindness do sometimes recognize stimuli presented in their blind spots: They can recognize, for example, whether a briefly flashed shape is a triangle or a square, at rates better than chance.
Now Scientific American has an article about a new experiment where people with cortical blindness were trained to recognize such shapes. After completing a home training session, they could recognize shapes in the lab 5 to 10 percent better than previously. Perhaps most interesting at all, if the laboratory tested a different part of the blind spot, where participants hadn’t previously been trained, there was no vision improvement.
The researchers argue this is evidence that the visual cortex is adaptable, and that even long after the damage has occurred, the vision system may be able to self-adjust, perhaps using different portions of the visual cortex to perform the functions that had previously been lost.