Cells projecting into the commissure of the cat's superior colliculus were identified during extracellular recording by antidromic activation. Electrical stimulation of the ipsilateral optic tract evoked action potentials in the majority of commissural neurons. Response latencies of 1.4 ± 0.5 ms (mean ± S.D.) in a few cells indicated that some commissural neurons receive direct input from the axons of retinal Y–cells. Most commissural cells responded 5.9 ± 1.9 ms (mean ± S.D.) following optic tract shock, implying that the responsible pathway was composed of more slowly conducting axons or did not proceed directly to the colliculus. Results of previous studies of retinal inputs to cells in the deep tectal layers suggest that the later responses were mediated by an indirect Y pathway through the visual cortex.
