Not Seeing Is Believing

Can people be distracted by something they can’t consciously see? Apparently, but only if their eyes are moving at the time and they aren’t engaged in more complex tasks.

This oddity, according to associate professor of psychology Tony Ro, is related to a portion of the midbrain known as the superior colliculus. In nonmammals, this structure handles basic sensory information processing, including eye movements related to vision. As mammals evolved, the visual cortex at the back of the brain took over the more complex processing of visual information. But does the older visual system still function in mammals?

To find out, Ro and his co-researchers, psychology graduate student Erik Chang and undergraduate students Dominique Shelton and Olivia Lee, put six volunteers with normal vision through a series of exercises in which they had to look at a target placed at varying locations on a computer screen. For half of the trials, the participants were asked to move their eyes to the location of the target, and their eye movements were measured electronically. For the other half, the participants were asked to press a button that corresponded with the location of the on-screen target. Researchers sometimes tried to distract the volunteers with an item shown on the center of the screen.

The wrinkle in the tests was that, on about 75 percent of the trials, the researchers stimulated the subjects’ visual cortexes with brief and harmless magnetic pulses that induce a temporary blindness lasting only a fraction of a second.

On the trials where the subjects had to perform the more complex task of pressing the button, the distracting item had no effect. The trials where the magnetic pulse was administered and the subjects only had to visually locate the target, however, produced startling different results. Despite the subjects reporting that they had not perceived the distracting item, the evidence showed that they had been influenced by it. These results indicate that the older visual system still functions and processed the visual information unconsciously.

Ro says that a better understanding of how visual information is processed could help researchers find ways to treat vision problems experienced by patients with brain damage caused by stroke or injury.

—B. J. Almond