Your Brain Sees Things That You Don’t

Photo credit: Beawiharta Beawiharta/Reuters

By Michael Graziano

In the 1950s, an 8-year-old boy suffered a head injury in a road accident. The back left part of his brain was damaged, specifically the primary visual cortex. As a consequence, he went blind in a large part of his right visual field. Years after the injury, his neurologists uncovered something strange. He could still see on the right side, even though he didn’t entirely know it.

The neurologists told him to face a screen and look at a small cross at the center to stabilize his eyes. A single dot appeared in his blind area, and he was told to point to the dot. In frustration, he insists there is no dot. But then, he takes a wild guess, and points right at it. Try after try, as the dot is flashed in different locations in the blind area, he points accurately most of the time.

This bizarre phenomenon is called blindsight. It was discovered in the 1970s by British researchers Larry Weiskrantz, Nicholas Humphrey, and others. It’s caused by damage to the primary visual cortex, one part of a vast network of brain areas that process vision. Without that part, some aspects of vision are still possible but the conscious visual experience disappears.

Blindsight offers a tantalizing hint about human consciousness. It demonstrates the difference between merely processing visual information in the brain, like in a computer, versus having a reportable conscious experience of it.

But this hint from blindsight proved hard to interpret. Does the primary visual cortex somehow generate awareness? If so, what exactly is being generated and how does it get from the back of the brain into our speech circuitry so that we can say that we have it? Maybe the primary visual cortex doesn’t create awareness itself, but instead sends visual information to a different system in the brain that is more closely related to consciousness. But if that’s so, what is this brain system that causes consciousness, and how does it work?

For a while, it seemed as though blindsight would remain only a tantalizing hint, but in 1999, Robert Kentridge, Charles Heywood, and Larry Weiskrantz stumbled on a new quirk of blindsight. It’s easy to mistake their discovery for a minor detail, but it turned out to be one of the most important insights into consciousness in decades.

Imagine you’re looking at a screen. A distracting dot flashes on the right side. A fraction of a second later, a number appears at exactly the same location. Your job is to report the number as quickly as possible. Your response time is probably pretty fast because the initial dot automatically drew your attention to that location. On the other hand, suppose the dot flashes on the left side of the screen. A fraction of a second later, the number appears on the right side. Now you’re probably slower to read the number. The dot automatically drew your attention to the wrong side and it takes a moment for your attention to readjust. This simple experiment allows researchers to measure how much attention was snagged by that initial dot.

It turns out that in people with blindsight, the dot can snag attention even when it doesn’t snag conscious experience. Bizarrely, attention and awareness can be separated.

Continue reading by clicking the name of the source below.