SACCADES TO SOMATOSENSORY TARGETS. I. BEHAVIORAL CHARACTERISTICS.
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 2:0243N, 1995.
SUMMARY AND CONCLUSIONS
1. We compared the properties of saccades to somatosensory and visual targets.
This comparison provides insight into the translation of sensory signals
coding target location in different sensory coordinate frameworks into motor
commands of a common format. Vibrotactile stimuli were delivered to the hands,
which were fixed in position and concealed beneath a barrier. Saccades of
different directions and amplitudes were elicited by the same somatosensory
target from different initial eye positions. Both monkeys and humans served as
subjects. 2. Somatosensory saccades were less accurate than visual saccades in
both humans and monkeys. When the barrier concealing the hands was removed,
somatosensory saccade accuracy improved. While the hands were concealed, the
visual frame of reference provided by room illumination did not greatly affect
saccade accuracy: accuracy was not degraded in complete darkness for two of
three monkeys. 3. The endpoints of saccades to a single somatosensory target
varied with initial eye position for the monkeys, but not for the human
subjects. 4. We also found evidence of an effect of limb position on
somatosensory saccades: when human subjects performed the task with crossed
hands, the incidence of curved saccades increased. Saccades often began in the
direction of the unstimulated hand and curved markedly toward the stimulated
hand. When one subject was required to delay the saccade by 600-1000 ms after
target onset (the delayed saccade task), the saccades were straight.
Somatosensory saccades were also straight when the hands were not crossed. 5.
The reaction times of somatosensory saccades were longer than the reaction
times of visual saccades, and they decreased as a function of saccade
amplitude. The delayed saccade task reduced the differences between
somatosensory and visual saccade reaction times. The reaction times of
saccades to very dim visual targets increased into the range found for
saccades to somatosensory targets. When the saccade target was the combination
of the somatosensory and visual stimuli at the same location, the reaction
time was slightly lower than for visual targets alone. 6. The peak velocities
of somatosensory saccades were lower than those of visual saccades of the same
amplitude. The velocities of saccades to combined somatosensory and visual
targets were indistinguishable from those of saccades to visual targets alone.
The differences between somatosensory and visual saccade velocity were
maintained in the delayed trial type. These differences suggests that the main
sequence or velocity-amplitude relationship characteristic of saccades depends
on the modality of the target. 7. The implications of these modality-dependent
differences in accuracy, reaction time and saccade velocity are discussed with
regard to models of the saccade generator and the coordinate transformation
necessary for somatosensory saccades.
Received 30 November 1994; accepted in final form 8 August 1995.
APS Manuscript Number J749-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 August 1995.