A Comparison of Electrophysiological and Psychophysical Temporal Modulation Transfer Functions of Human Vision

Abstract

Although the visual evoked cortical potential (VECP) is a measure of electrical activity at a relatively high level in the visual system, a close correlation is not always found between this response and perceptual phenomena. Part of this difference between electrical and perceptual responses must originate in the inherent variability of the VECP among different subjects (Emrich, 1970), and to the fact that physically similar stimuli can elicit different electrical responses depending upon their significance to the subject (Chapman, 1964). However, even when psychological variables are presumably controlled and reproducible responses are obtained, the VECP may still not be closely related to the subject’s perceptual experience. For example, Regan (1970) found that when flashes of two different colours are alternated, stimulus conditions that result in minimum perceptual flicker are usually not the same as those that elicit the minimum evoked potential. Similarly, Riggs and Whittle (1967) found that the subjective fading of visual stimuli that is caused by retinal image stabilization or binocular rivalry was not accompanied by a reduction in the VECP, although with a different electrode placement MacKay (1968) found that perceptual blanking of one brief stimulus by another that followed immediately was accompanied by suppression of the VECP that was normally elicited by the first stimulus. Kohn and Salisbury (1967) reported no increase in the amplitude of electrical activity elicited by stimuli that were presented at flash rates at which brightness enhancement occurred, although their technique of sharply filtering the VECP at the stimulus frequency and measuring only the amplitude of this fundamental may overlook changes in the amplitude or latency of other components.