Interactions between natural and electrically evoked saccades III. Is the
non-stationarity due to an integrator not instantaneously reset?.
John Schlag, Alexandre Pouget, Safa Sadeghpour, and Madeleine Schlag-Rey.
Brain Research Institute and Department of Neurobiology, UCLA, Los Angeles,
California 90095-1763.
APStracts 4:0278N, 1997.
ABSTRACT
In monkey, fixed-vector saccades evoked by superior colliculus (SC)
stimulation when the animal fixates can be dramatically modified if the
stimulation is applied during or immediately after an initial natural saccade
(Schlag-Rey et al. 1989). The vector is then deviated in the direction
opposite to the displacement just accomplished as if it were compensating for
part of the preceding trajectory. Recently, it was suggested that the
amplitude of the compensatory deviation is related to the amplitude of the
initial saccade linearly, and that the ratio between the two decreases
exponentially as stimulation is applied later. These two findings (spatial
linearity and temporal non-stationarity) were invoked as evidence for the non-
instantaneous resetting of a feedback integrator (Nichols and Sparks 1995;
Kustov and Robinson 1995). Such an integrator is included in most models of
saccade generation for the specific purpose of terminating a saccade when it
has reached its intended goal. However, the hypothesis of a feedback
integrator in the process of being reset implies that the exponential decay of
the compensatory deviation is temporally linked to the end of the initial
saccade. We analyzed the time course of this decay in stimulation experiments
performed at 24 SC sites in 2 monkeys. The results show that, if the start of
the exponential decay of compensation is assumed to be linked to the end of
the initial saccade, then the relation between the amount of compensatory
deviation and the amplitude of the initial saccade is not linear. On the other
hand, it is possible to show a linear relation if the measurements of
compensatory deviation are made in terms of delay of stimulation from the
saccade beginning. We conclude that stimulating the superior colliculus just
after a visually guided saccade does not seem to test the properties of a
feedback integrator. Whether such an integrator is or is not resettable is not
likely to be decided by this approach. Conversely, as the non-stationarity of
compensation is linked to the beginning of the saccade, the non-stationarity
seems to represent a property of an event occurring at saccade onset. We
suggest that this event, close to the input of the oculomotor apparatus, is
the summation of the visual signal with a damped signal of eye position or
displacement.
Received 3 January 1997; accepted in final form 26 September 1997.
APS Manuscript Number J3-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 7 October 1997