Visual evoked potentials: Concomitants of metacontrast in late components

Metacontrast is the masking of a briefly flashed target by a subsequent mask that shares common or nearby contours with the target. It is of interest because the effect seems to work backward in time, with a later mask obscuring an earlier target. The delay in masking suggests that the internal representation of a stimulus (the target) remains vulnerable for a time to interference in the nervous system. Thus, metacontrast provides a psychophysical tool for the dissection of temporal aspects of visual coding. In addition to psychophysical studies, the visual evoked potential (YEP) is a source of information used to differentiate the metacontrast theories currently proposed. The YEP has shown promise in revealing where and when in the brain metacontrast interactions occur. A recent paper on electrophysiological correlates of metacontrast showed no YEP correlates of metacontrast in a "C1" component that reflected activity of striate cortex (Jeffreys & Musselwhite, 1986). However, while C1 is an early potential that reflects afferent activity, extensive evidence, much of it published in Perception & Psychophysics, has shown metacontrast to be a complex phenomenon that is influenced by repetition, practice, binocularity, and other effects to be expected at a higher level (Schiller & Greenfield, 1969; Schiller & Smith, 1968; Ventura, 1980; Weisstein, 1971). Thus, we would expect correlates of metacontrast in laterVEP components, where higher level influences can be seen. This paper will show that although correlates of metacontrast are indeed present in the Jeffreys and Musselwhite data, they occur in later components, where other studies would lead us to expect them. Several studies have found a suppression of later components in the YEP that matchespsychophysical U-shaped metacontrast functions, even while early components are unaffected (Andreassi, DeSimone, & Mellers, 1975; Vaughan & Silverstein, 1968). Jeffreys and Musselwhite (1986) added to this literature by using electrode placements and stimulus locations that isolated responses of either striate or extrastriate visual cortex (Jeffreys, 1971; Jeffreys & Axford, 1972); the components are termed C1 and C2, respectively. The method is based on the differ-