VISUAL-EVOKED POTENTIALS IN 3-DIMENSIONAL COLOR SPACE - CORRELATES OFSPATIOCHROMATIC PROCESSING

Visual evoked potentials (VEPs) were measured for sinusoidal gratings with spatio-chromatic modulation defined in a three-dimensional color space. The spatio-chromatic modulation of the gratings can be decompos ed into contributions from an achromatic luminance varying component, an isoluminant component which modulates only the activities of L cone s and M cones, and an isoluminant component corresponding to modulatio n of only S-cone activity. The emphasis of this report is the nature o f VEPs resulting from isoluminant spatio-chromatic modulation. The VEP response was characterized along a number of spatial, temporal, and c hromatic stimulus dimensions: contrast, spatial frequency, chromaticit y in the isoluminant plane, chrominance/luminance ratio, orientation, and temporal frequency. Isoluminant VEPs resulting from stimuli modula ting L and M cones are compared with those from S-cone modulation. Whe n appropriate spatiotemporal conditions are employed, both types produ ce robust VEPs; however, the S-pathway VEPs show considerably longer l atencies than do those from LM-pathwap activation. The VEP results are compared to psychophysical and single unit electrophysiological obser vations. VEP latencies exhibit the lowpass character of psychophysical chromatic contrast sensitivity functions but VEP amplitudes show band pass tuning along both the S and LM axes. An oblique effect, i.e. shor ter latencies for horizontal and vertical gratings than for diagonal, is observed in the isoluminant VEP. S-pathway VE:Ps are used to demons trate an electrophysiological correlate of transient tritanopia. Norma tive amplitude and latency data for VEPs from selectivity stimulated c hromatic mechanisms provide a baseline for clinical electrodiagnostic applications.