Orientational anisotropy in the human visual system: an evoked potential and psychophysical study

Orientational anisotropy in the human visual system was investigated using electrophysiological and psychophysical techniques. Grating patterns of various orientations were presented and contrast detection thresholds were measured using a two alternative forced choice procedure. Using the same observers, visual evoked cortical potentials (VECPs) were elicited by identical gratings set at various contrasts ranging from subthreshold to high levels. Plots of contrast sensitivity versus grating orientation were derived using the amplitude of a negative wave of the VECP for low contrast data and the amplitude of a positive wave for moderate to high contrast data. These plots were compared with plots obtained during the psychophysical experiment. The results indicate that the low contrast VECP functions exhibit an orientational anisotropy that is qualitatively and quantitatively similar to that observed in the psychophysical data. The high contrast VECP functions, however, exhibit an orientational effect that is qualitatively similar but greater in magnitude than that found in the low contrast VECP or psychophysical experiments. These findings suggest that the amplitudes of VECPs elicited by low contrast stimuli are highly correlated with psychophysical detection performance, and that an additional physiological mechanism is reflected in the VECPs elicited by moderate to high contrast stimuli.