Invited Session I: 30 years of normalization in the visual system: Contrast normalization accounts for binocular interactions in healthy vision and amblyopia.

Abstract

We investigated the interaction between dichoptic stimuli using steady-state visual evoked potentials and frequency-domain analysis. The stimulus in each eye flickered with a unique temporal frequency, which allowed us to "frequency-tag" the responses to each eye's input (self terms) as well as the responses to the combination of inputs from the two eyes (intermodulation terms). We measured two forms of binocular interaction: one associated with the suppressive effect of one eye's stimulus on the other, and the other associated with a direct measure of interocular interaction between the two eyes' inputs. Fits of a contrast gain control model to the data demonstrated that a common gain control mechanism is consistent with both forms of binocular interaction. We then used the contrast normalization framework to investigate the disruptions to binocular interaction in amblyopia. Although anisometropic amblyopes showed a similar pattern of responses to normal-vision observers, strabismic amblyopes exhibited both reduced responses to the amblyopic eye stimulus in the presence of a mask in the other eye, as well as substantially reduced intermodulation responses indicating reduced interocular interactions in visual cortex. A contrast normalization model that simultaneously fit self- and IM-term responses showed that the excitatory contribution to binocular interaction is significantly reduced in strabismic amblyopia.