Projections from areas 17 and 18 of the visual cortex to the nucleus of the optic tract.

The corticofugal pathway to the nucleus of the optic tract (NOT) in the cat was studied with visual and electrical stimulation in two experimental series. In 10 experiments cells in the NOT were identified, and orthodromic responses evoked by single electric shocks applied through microelectrodes situated at different loci in areas 17 and 18. All but 2 units gave clear responses to shocks applied to either cortical electrode. The mean of response latencies was calculated to be 3.29 ms for area 17 stimulation and 3.04 ms for area 18 stimulation. In 8 further experiments the stimulation and recording sites were reversed; i.e. single shocks through microelectrodes in the NOT were used to elicit antidromic discharges in areas 17 and 18. A third microelectrode was placed in the superior colliculus (SC) at a position in retinotopic register with the units recorded at the NOT stimulation site. Out of 231 cortical units tested, 42 (17%) gave an antidromic response to NOT and/or SC shock. Seventeen units responded both to NOT and SC stimulation, 18 to NOT alone, and 7 to SC alone. The response latencies after NOT shock (mean 2.8 ms +/- 1.5 S.D.) did not differ significantly from those to SC shock (2.9 ms +/- 1.5 S.D.). All cortical cells activated antidromically from NOT and/or SC were located in layer V of areas 17 and 18. These units showed the following response characteristics: they responded well to conventional light bars as well as to large area random dot patterns; they were binocular and showed direction tuning; as compared to NOT cells they were more sharply tuned for stimulus velocity preferring faster movements. The present findings suggest a convergent projection to the NOT from the same type of cortical cells that project to the superior colliculus.