A NEW VEP SYSTEM FOR STUDYING BINOCULAR SINGLE VISION IN HUMAN INFANTS

Visual evoked potentials (VEPs), that provide unequivocal objective ev idence of cortical binocularity have been recorded from adults and you ng infants using a new VEP system developed for this purpose. The syst em uses alternating field stereoscopy (AFS) to present separate visual stimuli to each eye. With this system, the binocular image pairs to t he right and left eyes alternate at a high rate on a single video moni tor. The subject wears spectacles incorporating light-scattering liqui d crystal lenses which alternate electronically between opaque and cle ar modes in synchrony with the video monitor. To detect cortical binoc ularity, the system analyzes VEP activity mathematically and identifie s significant responses at test frequencies reflecting binocular corti cal interactions exclusively. Three types of binocular stimuli were pr esented: (1) dynamic random dot correlograms (correlograms); (2) dynam ic random dot stereograms (stereograms); and (3) dichoptic checkerboar d stimuli. The correlograms are generated when moving random dot patte rns presented to each eye alternate between two phases, correlated and anticorrelated. With the stereograms, portions of random dot patterns presented to each eye are shifted horizontally relative to each other at a fixed rate, alternately producing crossed and uncrossed binocula r disparities. Subjectively, these patterns appear to shift in depth. Dichoptic checkerboard stimuli are regular checkerboard patterns which reverse at different rates (frequencies) for each eye. Binocular VEPs are generated due to cortical interactions at the difference (beat) f requency. Using this VEP system, we have recorded binocular VEPs from 10 normal adults and more than 40 infant subjects. Responses to the co rrelograms, which we believe reflect binocular fusion, have been detec ted as early as 5 weeks of age, while responses to the stereograms, wh ich require sensitivity to disparity changes, have been recorded in ba bies as young as 12 weeks.