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.