Topographic organization and convergence in corticotectal projections from areas 17, 18, and 19 in the cat

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Abstract
1. Electrical stimulation in areas 17, 18 and 19 of the cat's visual cortex activated neurons in the superficial gray layer of the superior colliculus. The threshold for such excitation was lowest when the receptive fields of cells at the stimulus site lay inside the receptive field of the collicular cell under observation. 2. A substantial percentage of the recorded collicular cells received convergent excitatory input from more than one of the stimulated cortical areas. When stimuli could be applied in parts of areas 17, 18, and 19 related retinotopically to a collicular cell, over half of the units were activated from all three cortical areas by low-intensity stimulation. 3. Cells located near one another in the colliculus received excitatory cortical input from regions of areas 17, 18, and 19 which were not identical in size. This may be attributable to the disparate dimensions of collicular dendritic fields, which permit neighboring cells to sample dissimilar regions of the topographically organized cortical input. It is argued that this projection system preserves the functional unity of cortical and tectal cell groups processing information from a given retinal point.