Temporal analysis of the multifocal achromatic VEP yields separate magnocellular and parvocellular components

Purpose. To investigate the contrast response functions of various peaks of the multifocal visual evoked potential (VEP), and in particular of its temporal nonlinearities, as a means of determining the generating pathway. Methods. The multifocal cortical visual evoked potential was studied using pseudo-random (m-sequence) achromatic stimulation (VERIS). Luminance contrasts from 1% to 94% were employed. Results. The waveforms of the first order response varied with luminance contrast. At low to mid contrasts (<60%), a wave doublet (P90-N110) predominates. A second wave complex (N100-P120-N160) dominates at high contrasts. A rapidly adapting non-linearity (second order response) saturating at low contrasts was only observable when comparing the responses from two consecutive flashes. Its latency coincided with the saturating first order response component. By comparison, the nonlinearities derived from the responses to the stimuli with longer interstimulus intervals (interaction times of 30 or 45 msec) yielded a linear contrast response function and latencies which corresponded to the longer latency component of the first order response. A simple two component model with one component saturating at about 40% contrast while the other demonstrated a linear luminance contrast response to the highest contrast recorded (94%) fit the data well for all contrasts recorded. Conclusions. Intrinsic differences in the adaptation time of the generators of these two components with different contrast response functions caused a distinct separation in the slices of the second order response. Thus, the second order responses show a first slice which is predominantly driven by neural elements that have a latency and contrast function that mimic those of the magnocellular neurons of the primate LGN and a second slice which is dominated by a generator whose properties resemble primate parvocellular function. This division into magno and parvocellular contribution to the VEP is far simpler than other currently available analyses, with potential for functional analysis of visual disease.