Binocular facilitation in the visual-evoked potential of strabismic amblyopes

Electrophysiological correlates of abnormal binocular function were investigated by studying the monocular and binocular visual-evoked potentials (VEPs) of strabismic amblyopes. Steady-state evoked potentials were elicited by sinusoidal gratings which were temporally modulated in counterphase and logarithmically swept in spatial frequency from 0.2 to 20 c/deg in a 20 sec epoch. The VEP spatial frequency tuning functions of amblyopic eyes are abnormal in that they tend to show a reduced cutoff spatial frequency in relation to the nonamblyopic fellow eye. In contrast, the binocular interactions in strabismic amblyopes are similar in many respects to those of normal observers. These binocular interactions depend upon the spatial frequency and contrast of the stimulus, and may vary from inhibition to marked facilitation. The results suggest that some binocular function may be preserved in the cortex of patients with strabismic amblyopia, and that caution should be exercised in attempting to use the steady-state VEP to differentiate normal from abnormal binocular vision.