Changes in cortical activity during suppression in stereoblindness

Patients with strabismus or anisometropic amblyopia fixate and attend with one eye and suppress the image from the other eye. Here we use a visual evo ked potential technique to show that patients who lack normal stereopsis re tain suppressive binocular interactions but lack a characteristic form of n onlinear binocular interaction that is present in normal observers. Oscilla ting grating targets presented at different temporal frequencies in the two eyes evoke a strong response in normal observers at a frequency equal to t he sum of the two input frequencies for fusable targets but not for rivalro us ones. However increasing contrast in one eye reduces the response amplit ude from the other eye under either fusable (dichoptic masking) or rivalrou s conditions. Stereo-deficient observers lack the sum-frequency response. b ut retain dichoptic masking interactions. Dichoptic masking is stronger whe n the masker is presented to the patients' dominant rather than non-dominan t eyes, suggesting that a subset of preserved binocular inhibitory interact ions form the basis of clinical suppression. NeuroReport 11:1007-1012 (C) 2 000 Lippincott Williams & Wilkins.