SACCADES TO SOMATOSENSORY TARGETS. II. MOTOR CONVERGENCE IN PRIMATE
SUPERIOR COLLICULUS.
Groh, Jennifer M., David L. Sparks.
Institute of Neurological Sciences, 140 John Morgan Building, Department of
Psychology, 3815 Walnut Street, University of Pennsylvania, Philadelphia, PA
19104.
APStracts 2:0244N, 1995.
SUMMARY AND CONCLUSIONS
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 a) to determine whether signals from these separate sensory
systems have converged onto a common motor pathway by the level of the SC; b)
to determine the frame of reference of somatosensory saccade signals in the
SC; and c) 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. Out 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.
Received 30 November 1995; accepted in final form 8 August 1995.
APS Manuscript Number J750-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 August 1995.