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The Journal of Neurophysiology Vol. 81 No. 5 May 1999, pp. 2279-2296
Copyright ©1999 by the American Physiological Society
Department of Medical Physics and Biophysics, University of Nijmegen, 6500 HB Nijmegen, The Netherlands
Chaturvedi, Vivek and
Jan A. M. van Gisbergen.
Perturbation of Combined Saccade-Vergence Movements by
Microstimulation in Monkey Superior Colliculus. J. Neurophysiol. 81: 2279-2296, 1999.
Perturbation of combined saccade-vergence movements by microstimulation
in monkey superior colliculus. This study investigated the role
of the monkey superior colliculus (SC) in the control of visually
(V)-guided combined saccade-vergence movements by assessing the
perturbing effects of microstimulation. We elicited an electrical
saccade (E) by stimulation (in 20% of trials) in the SC while the
monkey was preparing a V-guided movement to a near target. The target
was aligned such that E- and V-induced saccades had similar amplitudes
but different directions and such that V-induced saccades had a
significant vergence component (saccades to a near target). The onset
of the E-stimulus was varied from immediately after V-target onset to
after V-saccade onset. E-control trials, where stimulation was applied
during fixation of a V-target, yielded the expected saccade but no
vergence. By contrast, early perturbation trials, where the E-stimulus
was applied soon after the onset of the V-target, caused an E-triggered
response with a clear vergence component toward the V-target. Midflight
perturbation, timed to occur just after the monkey initiated the
movement toward the target, markedly curtailed the ongoing vergence
component during the saccade. Examination of pooled responses from both types of perturbation trials showed weighted-averaging effects between
E- and V-stimuli in both saccade and fast vergence components. Both
components exhibited a progression from E- to V-dominance as the
E-stimulus was delayed further. This study shows that artificial intervention in the SC, while a three-dimensional (3D) refixation is
being prepared or is ongoing, can affect the timing (WHEN) and the metric specification (WHERE) of both saccades and
vergence. To explain this we interpret the absence of overt vergence in the E-controls as being caused by a zero-vergence change command rather
than reflecting the mere absence of a collicular vergence signal. In
the perturbation trials, the E-evoked zero-vergence signal competes
with the V-initiated saccade-vergence signal, thereby giving rise to a
compromised 3D response. This effect would be expected if the
population of movement cells at each SC site is tuned in 3D, combining
the well-known topographical code for direction and amplitude with a
nontopographical depth representation. On E-stimulation, the local
population would yield a net saccade signal caused by the topography,
but the cells coding for different depths would be excited equally,
causing the vergence change to be zero.
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