The effects of pretectal and superior collicular lesions on binocular vision

Monkeys with mid-brain lesions involving the pre-tectum and superior colliculi often have an odd stare, as if the visual axes were parallel and the animal was looking into the distance. Such a visuomotor abnormality could lead to double-vision for objects close to the animal. The experiments reported here were designed to test this hypothesis of diplopia in monkeys with combined bilateral ablations of the colliculi and frontal eye-fields (area 8). These animals performed better on a task of visually-guided reaching, and in a series of visual pattern discriminations, when they viewed stimuli monocularly rather than binocularly. Monkeys with other cortical lesions showed no such monocular superiority. We propose that an abnormality of vergence eye movements provides a simple explanation for some of the so-called perceptual impairments that follow damage to the mid-brain visual pathways in monkeys.