The effects of contrast and length of gratings on the visual evoked potential

Visual evoked potentials (VEP's) were obtained to grating stimuli modulated in phase at 8 Hz. The amplitude of the VEP was linearly related to the length of a centrally-fixated grating up to a critical length value equivalent to 9-16 grating cycles, for a range of spatial frequencies; thereafter amplitude first decreased then increased with increasing length. VEP amplitude was proportional to log. contrast as well as to length, and the intercept of the regression line with the contrast axis coincided with the psychophysical threshold for gratings of different lengths. More sensitive measurements with low-contrast stimuli around threshold indicated that in this range VEP amplitude is linearly related to contrast, and there is no threshold nonlinearity. The effects of visual field location and adaptation on the VEP to gratings of different lengths and spatial frequencies were also studied. The relationship of these findings to psychophysical, single-cell and transient VEP data is discussed.