SACCADES TO SOMATOSENSORY TARGETS. III. EYE POSITION-DEPENDENT
SOMATOSENSORY ACTIVITY 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 2:0245N, 1995.
SUMMARY AND CONCLUSIONS
1. We recorded from cells with sensory responses to somatosensory stimuli in
the superior colliculus (SC) of awake monkeys. Our goal was to determine the
frame of reference of collicular somatosensory signals by seeing whether the
positions of the eyes influence the responses of cells to a given tactile
stimulus. Somatosensory targets consisted of vibrotactile stimuli delivered to
the hands, which were held in fixed spatial positions. Monkeys performed a
delayed saccade task from different initial fixation positions to the
locations of these tactile stimuli or to visual stimuli at approximately the
same location. 2. The responses of a majority of somatosensory cells (25/34 or
74%) were significantly affected by eye position. Nearly all somatosensory
cells also responded to visual targets (28/30, 93%). Cells whose somatosensory
responses depended on eye position responded to visual and somatosensory
targets located at approximately the same direction in space with respect to
the eyes. 3. The activity of these cells exhibited both sensory and motor
qualities. The discharge was more closely linked in time to stimulus onset
than to the movement. Sensory features of the stimulus were reflected in the
responses: the discharge of a number of cells was phaselocked to the pulses of
vibration. The sensory responses occurred even if the animal's next saccade
was not directed into the response field of the cell. However, two thirds of
the cells also exhibited a burst of motor activity in conjunction with the
saccade to the somatosensory target. Sensory and motor activity were not
always spatially coextensive. When different, the tuning of motor activity was
broader. 4. Cells with somatosensory responses to vibratory stimulation of the
hands were found in a wide region of the SC, spanning a 40 range of movement
amplitudes. 5. These data show that somatosensory signals in the SC are not
purely somatotopic but are dependent on eye position. For stimuli at a fixed
location, this eye position-dependence allows somatosensory and visual signals
to be in register and share a premotor circuitry for guiding saccadic eye
movements. 6. The dependence of the somatosensory responses on eye position
suggests that the somatosensory receptive fields may either shift on the body
surface or they may be restricted to a limited region of the body surface but
be gated by eye (and body) position. Future experiments varying body position
and the location of the stimulus on the body surface are needed to determine
which of these alternatives is correct. Cells with either type of receptive
field could provide an unambiguous signal of the location of somatosensory
saccade targets with respect to the eyes. The transformation of somatosensory
signals from a body-centered frame of reference to a frame of reference that
depends on the position of the stimulus with respect to the eyes is necessary
for the correct activation of collicular neurons with motor activity, since
this activity encodes saccades as desired changes in eye position.
Received 30 November 1995; accepted in final form 8 August 1995.
APS Manuscript Number J751-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 August 1995.