Short-term Adaptation of Electrically-induced Saccades in Monkey Superior
MELIS, BART J.M. and JAN A.M. VAN GISBERGEN.
Department of Medical Physics and Biophysics, University of Nijmegen,
P.O.Box 9101, NL-6500 HB Nijmegen, The Netherlands.
APStracts 3:0089N, 1996.
SUMMARY AND CONCLUSIONS
This study focuses on the neural mechanisms underlying short-term adaptation
of saccadic eye movements in the rhesus monkey. Involuntary saccades of
various amplitudes and directions (E-saccades) were elicited in complete
darkness by electrical stimulation (<= 50 microA) in the deeper layers of the
superior colliculus (SC), at 30 different sites in two monkeys. E-saccades at
a given site could be adapted by presenting a visual target at a small
distance from the expected end point, immediately after their occurrence. The
monkeys were trained to null the ensuing error signal by making the
appropriate correction saccade to the visual target in many successive trials
(E-adap paradigm). By properly adjusting the location of the visual target
relative to the end point of the E-saccade, the latter could be modified in
amplitude as well as in direction. E-saccade modifications were highly
significant, always in the intended direction and occurred only if a post-
saccadic visual error signal was created. These changes were plastic and
required a subsequent E-adap series with an opposite error signal to cancel
them. Their time course, both during the adaptation and the readaptation
period, indicated that the modification was a slow and gradual process, as has
been observed earlier in classical visual-adaptation experiments. Post-
adaptation tests, assessing whether the adaptation of E-saccades was also
noticeable in normal visually-guided saccades (V-saccades), showed incomplete
adaptation transfer which was significant in most cases. A similar result,
significant in all cases, was obtained with an extended version of the E-adap
paradigm in which movement planning on the basis of target selection was
possible. In this case, a pre-saccadic visual target was presented at the
expected end point of the E-saccade, which was evoked just before the monkey
would make a voluntary saccade himself (VE-adap). In another series of
experiments, V-saccades, which were matched to the optimal saccade vector of
the particular collicular site under investigation, were adapted with the
classical intra-saccadic target shift paradigm (V-adap). In agreement with
earlier findings, this V-adaptation showed no transfer to the E-saccades. This
result was obtained even in trials where movement planning on the basis of
target selection was possible (VE-test). Our experiments have shown that
saccades of collicular origin can be adapted and that pre-saccadic target
selection is not crucial for this process. Both results are nicely in line
with an existing model featuring a downstream adaptive corrector with access
to SC inputs. This scheme, however, does not explain why the degree of
saccadic adaptation, achieved by applying any of the three adaptation
paradigms (E-, EV or V-adap), was never equally expressed in V- and E-
saccades. Arguments for extending the model by adding a cortical input from
the frontal eye fields (FEF) to the adaptive corrector are discussed.
Received 31 October 1995; accepted in final form 23 February 1996.
APS Manuscript Number J736-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 19 May 96