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 evoked potential technique to show that patients who lack normal stereopsis retain suppressive binocular interactions but lack a characteristic form of non-linear binocular interaction that is present in normal observers. Oscillating grating targets presented at different temporal frequencies in the two eyes evoke a strong response in normal observers at a frequency equal to the sum of the two input frequencies for fusable targets but not for rivalrous ones. However increasing contrast in one eye reduces the response amplitude from the other eye under either fusable (dichoptic masking) or rivalrous conditions. Stereo-deficient observers lack the sum-frequency response, but retain dichoptic masking interactions. Dichoptic masking is stronger when the masker is presented to the patients' dominant rather than non-dominant eyes, suggesting that a subset of preserved binocular inhibitory interactions form the basis of clinical suppression.