Visual evoked cortical potential (VECP) elicited by pseudoisochromatic stimuli

Abstract

Purpose: Pseudoisochromatic stimuli are widely used in psychophysics studies of color vision. There have not been reports about their use in visual electrophysiology. Their use could be advantageous to avoid isoluminance estimative and to investigate the physiology of cortical response to stimuli comprising illusory contours. Our purpose was to study VECPs elicited by pseudoisochromatic gratings.

Methods: Seven healthy trichromatc subjects, 20-28 years old, were studied. A ViSaGe system was used to generate pseudoiscochromatic or sinusoidal red-green gratings at 0.33, 0.66, 1, 1.33, 1.66, and 2 cycles/degree. The stimuli measured 40 and were presented in pattern reversal (1 Hz) and pattern onset (300 ms) - offset (700 ms) modes. A CED system with Spike 2 software was used to record the cortical responses from the scalp using a channel of surface electrodes placed at Oz (active), Fpz (reference), and Fp (ground). The electrophysiological signals were amplified x30,000 and digitized at 1 kHz; 480 sweeps of 1000 ms were averaged. Amplitude and latency of VECP components were studied across the spatial frequency domain.

Results: For patern-reversal sinusoidal gratings, the VECPs were dominated by a negative component peaking between 110-140 ms. For patern-reversal pseudoisochromatic gratings, the VECPs were dominated by a negative component peaking between 130-150 ms. Both responses decreased in amplitude as a function of the spatial frequency in a similar rate. The onset response for sinusoidal and pseudoisochromatic gratings was dominated by a negative component. The latency of the onset response was shorter for sinusoidal gratings (70-100 ms) than for pseudoisochromatic gratings (120-130 ms). The amplitude of the VECP component elicited by sinusoidal gratings decreased as a function of spatial frequency at higher rate than the amplitude of the VECP component elicited by pseudoisochromatic gratings. Positive offset responses for sinusoidal gratings were faster and larger than those elicited by pseudoisochromatic stimuli.

Conclusions: Real and illusory chromatic gratings evoked different cortical responses when showed in onset-offset mode.