Corticopontine cells in area 18 of the cat

Area 18 projects to the rostral pontine nuclei. The visual response properties of rostral pontine cells differ greatly from those that have been reported for area 18 cells. The authors identified and studied corticopontine cells in area 18 and compared their receptive-field properties to those of other area 18 cells and to pontine visual cells. They first located the visual area in the rostral pons by microelectrode recording and placed stimulating electrodes at the same site. Antidromically invaded cells were then recorded in area 18. The antidromic invasion of each cell was verified by orthodromic-antidromic spike collision. Fifty-seven well-isolated corticopontine cells were studied in detail. They also recorded 466 unitary antidromic potentials with a mean invasion latency of 3.5 ms and recorded from 40 additional area 18 units to serve as a comparison group for the corticopontine cells. The comparison group cells were located in the same area in the visual field as the corticopontine cells. The average receptive-field area for corticopontine cells (485 deg²) was much larger than the comparison cells (59 deg²). Forty percent of the corticopontine cells responded preferentially to multiple-spot targets, while no area 18 comparison cell responded preferentially to a multiple-spot target. Properly oriented gratings, slits, or edges were the most effective stimuli for the comparison cells, but rarely were effective for corticopontine cells. Eighty-two percent of the corticopontine cells showed clear directional preferences to moving-spot stimuli, and downward movements were most commonly preferred. Fifty-five percent of the area 18 comparison cells showed some directional preference, but no particular direction was preferred. The optimal stimulus speeds for corticopontine cells were higher than those for the comparison cells. The response properties of the area 18 corticopontine cells are similar to the response properties of rostral pontine visual cells, except for a somewhat higher selectivity for orientation in the corticopontine cells. It is concluded that most response properties of rostral pontine visual cells are already present in a subset of area 18 cortical cells which project to the pons. The corticopontine cells are sensitive to multiple-spot targets moving in particular directions over large portions of the visual field, such properties are consistent with a visuomotor function for the corticopontocerebellar pathway.