Visual detection deficits following inactivation of the superior colliculus in the cat

Lesion or inactivation of the superior colliculus (SC) of the cat results in an animal that fails to orient toward peripheral visual stimuli which normally evoke a brisk, reflexive orienting response. A failure to orient toward a visual stimulus could be the result of a sensory impairment (a failure to detect the visual stimulus) or a motor impairment (an inability to generate the orienting response). Either mechanism could explain the deficit observed during SC inactivation since neurons in the SC can carry visual sensory signals as well as motor commands involved in the generation of head and eye movements. We investigated the effects of SC inactivation in the cat in two ways. First, we tested cats in a visual detection task that required the animals to press a central, stationary foot pedal to indicate detection of a peripheral visual stimulus. Such a motor response does not involve any components of the orienting response and is unlikely to depend on SC motor commands. A deficit in this task would indicate that the SC plays an important role in the detection of visual targets even in a task that does not require visual orienting. Second, to further investigate the visual orienting deficit observed during SC inactivation and to make direct comparisons between detection and orienting performance, we tested cats in a standard perimetry paradigm. Performance in both tasks was tested following focal inactivation of the SC with microinjections of muscimol at various depths and rostral/caudal locations throughout the SC. Our results reveal a dramatic deficit in both the visual detection task and the visual orienting task following inactivation of the SC with muscimol.