Fall 2016 Intern
A patient left blind by two strokes—referred
to in studies as “TN”—stands at the end of a hallway. Littered before him are a
series of obstacles: a trash can, a paper shredder, a tripod and more. Without using
a cane, he walks down the hallway, moving to avoid all the obstacles
on his first attempt. When told that he succeeded, TN was shocked. Some unconscious
instinct had caused him to avoid the obstacles on the first try.
This instinct has been documented
as occurring when the eyes and brain are healthy, but the primary visual
cortex—the part of the brain that is necessary for sight—is damaged. This type
of brain damage often occurs in stroke victims. Because of this, signals that
travel from the eyes through the optic nerves cannot be processed. Patients
with otherwise healthy eyes are left unable to see. However, for some reason,
patients with this specific form of blindness are sometimes able to respond to
visual stimuli that they are not consciously aware of at all. This phenomenon
is known as blindsight.
In 1974, psychologists Elizabeth
Warrington and Lawrence Weiskrantz investigated one
case of blindsight where the patient had been left blind in one eye after a
surgery that was meant to cure headaches. The eye was still functional, but the
visual cortex had been damaged. Warrington and Weiskrantz tested the patient by
putting a screen in his blind spot and asking him to point to a shape when it
appeared in different places on the screen. They also tested him with vertical and
horizontal lines, asking him to identify which type of line was showing on the
screen. The patient insisted that he couldn’t see anything, but was correct around
80 percent of the time: much more than chance alone would allow.
Despite multiple documented cases,
the actual cause of blindsight remains a mystery. There are several viable theories,
however. Weiskrantz and Warrington suggest the processing that causes
blindsight occurs in parts of the brain other than the visual cortex. They argue
that the bundles of fibers that travel from the optic nerve to the midbrain
still transmit information, and that the midbrain unconsciously interprets
these signals. Michael Gazzaniga, a neuroscientist at the University of
California at Davis who has also studied blindsight,
argues that this phenomenon is a result of portions of healthy tissue in the
visual cortex. These sections of live tissue, he says, are too small to allow a
patient to consciously register visual stimuli, but they do result in
blindsight.
Research on blindsight allows us a
little more understanding of the human brain and how we perceive our
surroundings, both consciously and unconsciously. It also raises more questions
for scientists—the answers to which I’ll be very interested to see.
Did You Know?
About 30 percent of the brain’s
neurons are designated for visual processing. To compare,
touch uses around 8 percent, while hearing uses 3 percent.
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