The role of the superior colliculus in facilitating visual attention and form perception.

When interhemispheric transfer in cats is studied from an intact hemisphere to a hemisphere with a suprasylvian cortical lesion, excellent transfer of grating discriminations, but no transfer of forms, is present. Stimuli with global, repetitive features covering a large visual field (gratings), which can be discriminated by preattentive vision, are transferred; perception of stimuli with local features (forms), which require serial exploration using focal vision, is defective in the hemisphere with cortical lesion and transfer is lacking. Influence of the midbrain in facilitating focal vision is shown by the restoration of form discriminations after section of the superior collicular commissure. It is hypothesized that the perceptual defect after lesion in the suprasylvian cortex is due to poor spatial attention and its restoration after midbrain lesion is due to improved function of those collicular cells that mediate orienting of attention.