The fine-grain movement illusion: A perceptual probe of neuronal connectivity in the human visual system

Abstract

In the visual periphery, brief presentation of two very closely spaced luminous point stimuli, in rapid sequence, causes the illusory impression that a single dot moves over a path of considerable extent. The interactions obtainable between two illusions near one another are described for various configurations of the inducing stimuli. Only when such illusions are codirectional are they found not to interfere with one another. The effective field position and extent of the illusion were measured by pitting two suitably separated illusions against one another. The extent varies from about 2° to 6° as stimulus eccentricity is increased from 10° to 24°. However, when mapped onto visual cortex by means of human cortical magnification factor, the illusion spans a patch of cortex about 3 mm dia, regardless of stimulus eccentricity; such a region in primate visual cortex corresponds approximately- to the locus of cortical cells that “see” a given retinal point. Finally, it is suggested that these fine-grain effects may underlie certain perceptual responses to sequential random-dot displays.