Saccades to somatosensory targets .2. Motor convergence in primate superior colliculus

1. We examined cells with saccade-related activity in the superior colliculus (SC) of monkeys performing saccades to both somatosensory and visual targets. Our goals were 1) to determine whether signals from these separate sensory systems have converged onto a common motor pathway by the level of the SC; 2) to determine the frame of reference of somatosensory saccade signals in the SC; and 3) to relate collicular motor activity to the behavioral characteristics of somatosensory saccades. 2. Somatosensory targets consisted of vibrotactile stimuli delivered to the hands, which were held in fixed spatial positions. Saccades of different directions and amplitudes were elicited from different initial 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 saccade-related activity had movement fields representing the direction and amplitude of saccades to both visual and somatosensory targets. We found no cells that discharged for saccades to a particular somatosensory target regardless of the vector of the saccade. 4. Small modality-dependent differences in the spatial tuning of the movement fields were observed, but these variations formed no clear pattern. Given the large population of cells active in conjunction with each saccade, these small tuning differences may have no net effect. Because the visual and somatosensory movement fields of individual cells were similar to each other, the inaccuracy of somatosensory saccades is likely to be the result of inaccurate signals reaching the SC, rather than an error signal added downstream. 5. The peak discharge frequency of collicular motor cells was lower for somatosensory saccades than for visual saccades, although the number of spikes in the discharge was about the same. 6. The latency of the onset of the prelude of motor activity following the cue to initiate a saccade was about the same for somatosensory and visual trials, even though somatosensory saccades have longer reaction times than visual saccades, However, the peak of the motor activity 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 neurons in the SC that represents saccades to visual targets also represents saccades to somatosensory targets. Somatosensory saccades are encoded by these cells as the change in eye position necessary to bring the 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 reaction time differences.