SACCADES TO SOMATOSENSORY TARGETS .2. MOTOR CONVERGENCE IN PRIMATE SUPERIOR COLLICULUS

1. We examined cells with saccade-related activity in the superior col liculus (SC) of monkeys performing saccades to both somatosensory and visual targets. Our goals were 1) to determine whether signals from th ese separate sensory systems have converged onto a common motor pathwa y by the level of the SC; 2) to determine the frame of reference of so matosensory saccade signals in the SC; and 3) to relate collicular mot or activity to the behavioral characteristics of somatosensory saccade s. 2. Somatosensory targets consisted of vibrotactile stimuli delivere d to the hands, which were held in fixed spatial positions. Saccades o f different directions and amplitudes were elicited from different ini tial eye positions. Of 86 cells with motor-related activity, 85 (99%) discharged for saccades to both visual and somatosensory targets. The remaining cell was active only for visual saccades. 3. Cells with sacc ade-related activity had movement fields representing the direction an d amplitude of saccades to both visual and somatosensory targets. We f ound no cells that discharged for saccades to a particular somatosenso ry target regardless of the vector of the saccade. 4. Small modality-d ependent differences in the spatial tuning of the movement fields were observed, but these variations formed no clear pattern. Given the lar ge population of cells active in conjunction with each saccade, these small tuning differences may have no net effect. Because the visual an d somatosensory movement fields of individual cells were similar to ea ch other, the inaccuracy of somatosensory saccades is likely to be the result of inaccurate signals reaching the SC, rather than an error si gnal added downstream. 5. The peak discharge frequency of collicular m otor cells was lower for somatosensory saccades than for visual saccad es, although the number of spikes in the discharge was about the same. 6. The latency of the onset of the prelude of motor activity followin g the cue to initiate a saccade was about the same for somatosensory a nd visual trials, even though somatosensory saccades have longer react ion times than visual saccades, However, the peak of the motor activit y was delayed on somatosensory trials such that the timing of the peak was the same with respect to the movement on somatosensory and visual trials. 7. We conclude that the same population of saccade-related ne urons in the SC that represents saccades to visual targets also repres ents saccades to somatosensory targets. Somatosensory saccades are enc oded by these cells as the change in eye position necessary to bring t he target onto the fovea, rather than the location of the stimulus on the body surface. Modality-dependent differences in the frequency and timing of collicular motor activity may contribute to velocity and rea ction time differences.