The brightness of a flash of light is reduced when it is followed by a second flash in an adjacent region of the field. This effect, metacontrast, depends upon, among other things, the luminance, duration, exposure asynchrony, spatial separation of the two flashes, and the region of the field in which the flashes appear. These variables have been systematically investigated in the present study. The results indicated that the magnitude of the effect increased as the luminance or duration of the second flash increased, as the luminance or duration of the first flash decreased, and as the angular separation of the two flashes in space decreased. If the two flashes were confined to the two degree center of the visual field, the effect disappeared, but as they were moved gradually out into the peripheral field the effect quickly appeared.
Theoretical considerations imply that the effect is explicable as the result of: (a) the interaction of two neural events, or (b) the interaction of the neural events produced by the first flash and the extraneural (electrical?) events produced by the second flash. Available evidence does not allow a conclusive choice between these two alternatives, but the second hypothesis gives a more satisfactory explanation for the delay between flashes required for maximum metacontrast.
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