The Journal of Neurophysiology Vol. 84 No. 1 July 2000, pp. 344-357
Copyright ©2000 by the American Physiological Society
Multielectrode Evidence for Spreading Activity Across the
Superior Colliculus Movement Map
Nicholas L.
Port,
Marc A.
Sommer, and
Robert H.
Wurtz
Laboratory of Sensorimotor Research, National Eye Institute,
National Institutes of Health, Bethesda, Maryland 20982-4435
Port, Nicholas L.,
Marc A. Sommer, and
Robert H. Wurtz.
Multielectrode Evidence for Spreading Activity Across the
Superior Colliculus Movement Map. J. Neurophysiol. 84: 344-357, 2000. The monkey superior colliculus (SC)
has maps for both visual input and movement output in the superficial
and intermediate layers, respectively, and activity on these maps is
generally related to visual stimuli only in one part of the visual
field and/or to a restricted range of saccadic eye movements to those stimuli. For some neurons within these maps, however, activity has been
reported to spread from the caudal SC to the rostral SC during the
course of a saccade. This spread of activity was inferred from averages
of recordings at different sites on the SC movement map during saccades
of different amplitudes and even in different monkeys. In the present
experiments, SC activity was recorded simultaneously in pairs of
neurons to observe the spread of activity during individual saccades.
Two electrodes were positioned along the rostral-caudal axis of the SC
with one being more caudal than the other, and 60 neuron pairs whose
movement fields were large enough to see a spread of activity were
studied. During individual saccades, the relative time of discharge of the two neurons was compared using 1) the time
difference between peak discharge of the two neurons, 2)
the difference between the "median activation time" of the two
neurons, and 3) the shift required to align the two
discharge patterns using cross-correlation. All three analysis methods
gave comparable results. Many pairs of neurons were activated in
sequence during saccade generation, and the order of activation was
most frequently caudal to rostral. Such a sequence of activation was
not observed in every neuron pair, but over the sample of neuron pairs
studied, the spread was statistically significant. When we compared the
time of neuronal activity to the time of saccade onset, we found that
the caudal neuronal activity was more likely to be before the saccade,
whereas the rostral neuronal activity was more likely to be during the saccade. These results demonstrate that when individual pairs of
neurons are examined during single saccades there is evidence of a
caudal to rostral spread of activity within the monkey SC, and they
confirm the previous inferences of a spread of activity drawn from
observations on averaged neuronal activity during multiple saccades.
The functional contribution of this spread of activity remains to be determined.
