Frequency-Doubling in the Visual Evoked Potential: Implications for Assessment of Spatiotemporal Vision

Abstract

Several investigators have noted that the suprathreshold visual evoked potential (VEP) characterizes spatiotemporal tuning in primates in different ways than do psychophysical measures. In particular, narrow spatial and temporal tuning (often surrounding multiple peaks) have frequently been observed (1), and in many cases, the peak of the spatial tuning function is higher than predicted behaviorally. One of the most intriguing aspects of the suprathreshold VEP is the frequency-doubling (i.e., predominance of power at twice the reversal frequency) which is observed when sinusoidal luminance-contrast gratings are used to elicit the VEP (2). Since the doubling phenomenon apparently does not occur when gratings are counter-phased using a square waveform, one hypothesis argues that the more gradual sinusoidal reversal dissociates the VEP onset and offset responses which are believed to combine during square-wave counterphasing (2,3). However, the fact that frequency-doubling does not typically occur in transcortical recordings from rhesus monkeys argues against this hypothesis.