Visual illusions and prefrontal cortex

Abstract

A new study demonstrates that prefrontal cortex activity can reflect the perception of a visual illusion [Lebedev, M.A. et al. (2001) J. Neurophysiol. 85, 1395–1411]. The researchers trained a monkey to fixate a spot of light surrounded by a frame of other lights. The lights disappeared and then reappeared, sometimes in a new location. The monkey had to decide whether the spot had moved and to report this by glancing left or right. Like humans performing the same task, the money fell foul of the ‘displacement illusion’ if the frame moved more than the spot. For example, if the spot were to shift 1 degree to the left but the frame shifts 2 degrees left, subjects respond ‘right’. However, they can still make accurate leftward eye movements to the new location – suggesting that the system for shifting fixation to the displaced target is separate from the system for perceiving and reporting its displacement. Recording in prefrontal cortex, the researchers found a population of ‘report-related’ cells that reflected the monkey's report of the displacement (even when wrong). A different population of directionally-selective cells was active when the monkey was simply required to glance at the target in its new location. This is the first study to demonstrate illusion-related activity in prefrontal cortex and to contrast it with movement-related activity. HJB