Summer 1996 Volume 5, Number 3

Now you see it,
now you don't

Look at a window or at a nearby light for a moment, then close your eyes and count how many seconds it takes the afterimage (the blue splotches that persist) to disappear. If images persist like this, why doesn't every scene we see look like a time-lapse photograph ?

Steve Macknik, in the laboratory of Margaret Livingstone at Harvard Medical School, has found that the answer may be in an illusion called "visual masking." In this illusion, objects that are easily visible under some conditions vanish in others. Of particular interest, it is possible to inhibit perception of an otherwise visible object (called a target) by following it with another object, called a "mask" (see diagram). That means that the image of the second object enters the brain after the first, catches up to it, and somehow destroys it. This must occur in order to erase smeared afterimages that would otherwise follow moving objects in the real-world: The object in its new position destroys the afterimage of itself from its earlier position.

Macknik used human subjects in behavioral experiments to characterize the relationships of targets and masks in space and across time, and has measured the firing of neurons in the visual cortex of monkeys to correlate human perception with neuronal signaling.

Table of Contents

Now you see it, now you don't

Now you see it,
now you don't