The activation of the visual cortex in response to a bright image is not based on the actual size of the image, but the perceived size of the “afterimage”. These findings, reported online this week in Nature Neuroscience, may help explain why objects are perceived to be the same size in real life, even when the image cast on the retina gets smaller as the distance becomes greater. After staring fixedly at a bright image for a little while, people see an ‘after-image’, the size of which varies. The degree of the corresponding visual cortex activation depends on the perceived size of this after-image, not its actual size projected on the retina. If you stare at a dim light-bulb without moving your eyes for about ten seconds, and then look at a blank wall or ceiling, you will likely see an afterimage of the light. The size of this afterimage will depend on how far away the wall or ceiling is: the further away the wall, the larger the afterimage. This happens even though the size of the light-bulb itself does not change, and correspondingly, neither does the size of the bulb’s image projected on the retina. Mel Goodale and colleagues used functional magnetic resonance imaging to track observers’ brain activation as they saw afterimages of varying sizes on a screen positioned at varying distances. They found that the size of the primary visual cortex activation (V1) corresponding to the afterimage was larger when the afterimages were perceived as larger, even though the size of image cast on the retina did not change.
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