Saccade-related adtivity in monkey superior colliculus; I. characteristics
of burst and buildup cells.
Munoz, Douglas P., Robert H. Wurtz.
Laboratory of Sensorimotor Research, National Eye Institute, Bethesda, MD
20892-4435, USA and MRC Group in Sensory-Motor Physiology, Department of
Physiology, Queen's University, Department of Physiology, Kingston, Ontario,
Canada K7L 3N6.
APStracts 2:0072N, 1995.
SUMMARY AND CONCLUSIONS
1. In the monkey superior colliculus (SC), the activity of most saccade-
related neurons studied so far consists of a burst of activity in a population
of cells at one place on the SC movement map. In contrast, recent experiments
in the cat have described saccade-related activity as a slow increase in
discharge before saccades followed by a hill of activity moving across the SC
map (Munoz et al. 1991). In order to explore this striking difference in the
distribution of activity across the SC, we recorded from all saccade-related
neurons that we encountered in microelectrode penetrations through the monkey
SC, and placed them in categories according to their activity during the
generation of saccades. 2. When we considered the activity preceding the onset
of the saccade, we could divide the cells into two categories. Cells with
burst activity had a high frequency discharge just before saccade onset but
little activity between the signal to make a saccade and saccade onset. About
two thirds of the saccade-related cells had only a burst of activity. Cells
with a build up of activity began to discharge at a low frequency after the
signal to make a saccade and the discharge continued until generation of the
saccade. About one third of the saccade- related cells studied had a build up
of activity, and about three quarters of these cells also gave a burst of
activity with the saccade in addition to the slow buildup of activity. 3. The
build up of activity seemed to be more closely -3- J229-4RR related to
preparation to make a saccade than to the generation of the saccade. The build
up developed even in cases when no saccade occurred. 4. The falling phase of
the discharge of these saccade- related cells stopped with the end of the
saccade (a clipped discharge), shortly after (partially clipped), or long
after (unclipped). 5. Some cells had closed movement fields in which saccades
that were substantially smaller or larger than the optimal amplitude were not
associated with increased activity. Other cells tended to have open-ended
movement fields without any peripheral border; they were active for all
saccades of optimal direction whose amplitudes were equal to or greater than a
given amplitude. We found both types of movement fields at all movement field
eccentricities studied within the SC. 6. The activity of cells with open-ended
movement fields did not result from the smear of the visual target as it swept
across the retina during a saccade because the discharge of the cell was still
present when saccades were made in the dark to remembered rather than visual
targets. The activity of these cells was also not due to the occurrence of
corrective saccades since the activity was visible whether or not there was
one. 7. In penetrations through the intermediate layers of the SC, we usually
found cells with a burst of activity and those with closed movement fields to
lie more dorsally than those with -4- J229-4RR a build up of activity and
open-ended movement fields. 8. We also compared the activity of the saccade-
related cells to the activity of fixation cells located in the rostral pole of
the SC (Munoz and Wurtz 1993). We found a transition between saccade-related
cells with open-ended movement fields and fixation cells. Cells within this
transition zone were tonically active during fixation but also discharged
during small contraversive saccades. These fixation cells were encountered
deeper in the intermediate layers, at the same level as the cells with open-
ended movement fields and buildup of activity. We propose that fixation cells
form a rostral extension of the layer of cells with a buildup of activity. 9.
We conclude that these characteristics of the saccade- related cells overlap
sufficiently to allow us to place the cells into two groups. Burst cells have
a high frequency burst occurring immediately prior to saccades and no build up
of activity, the majority have clipped activity at the end of the saccade, and
usually have closed movement fields. In contrast, buildup cells show activity
beginning with the signal to make a saccade that continues until the
generation of the saccade, the majority have partially clipped activity at the
end of the saccade, and have open-ended movement fields. Since we encountered
the cells with burst activity and closed movement fields more dorsally than we
did cells with buildup activity and open-ended movement fields, we hypothesize
further that the burst and buildup cells can be regarded as separate
functional -5- J229-4RR sublayers with the burst layer on top and the buildup
layer below. The buildup cells are similar to the saccade-related cells in the
cat SC, but the burst cells may be an added feature of the primate SC.
Received 29 April 1994; accepted in final form 13 February 1995.
APS Manuscript Number J229-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 3 April 1995.