Adaptation to visual motion in the directional neurons of the nucleus of
the optic tract.
MICHAEL R. IBBOTSON, COLIN W. G. CLIFFORD AND RICHARD F. MARK.
Developmental Neurobiology and Centre for Visual Science, Research School
of Biological Sciences, Australian National University, GPO Box 475, ACT 2601,
Australia; and Department of Psychology, University College London, Gower
Street, London, WC1E 6BT, U.K.
APStracts 4:345N, 1997.
ABSTRACT
Extracellular recordings of action potentials were made from directional
neurons in the nucleus of the optic tract of the wallaby, Macropus eugenii,
while stimulating with moving sine-wave gratings. When a grating was moved at
a constant velocity in the preferred direction through a neuron's receptive
field, the firing rate increased rapidly and then declined exponentially until
reaching a steady-state level. The decline in response is called motion
adaptation. The rate of adaptation increased as the temporal frequency of the
drifting grating increased, up to the frequency that elicited the maximum
firing rate. Beyond this frequency, the adaptation rate decreased. When the
adapting grating's spatial frequency was varied, such that response magnitudes
were significantly different, the maximum adaptation rate occurred at similar
temporal frequencies. Hence the temporal frequency of the stimulus is a major
parameter controlling the rate of adaptation. In most neurons the temporal
frequency response functions measured after adaptation were shifted to the
right when compared to those obtained in the unadapted state. Further insight
into the adaptation process was obtained by measuring the responses of the
cells to grating displacements within 1 frame (10.23 ms). Such impulsive
stimulus movements of less than a quarter cycle elicited a response that rose
rapidly to a maximum and then declined exponentially to the spontaneous firing
rate in several seconds. The level of adaptation was demonstrated by observing
how the time constants of the exponentials varied as a function of the
temporal frequency of a previously presented moving grating. When plotted as
functions of adapting frequency, time constants formed a U-shaped curve. The
shortest time constants occurred at similar temporal frequencies, regardless
of changes in spatial frequency, even when the change in spatial frequency
resulted in large differences in response magnitude during the adaptation
period. The strongest adaptation occurred when the adapting stimulus moved in
the neuron's preferred direction. Stimuli that moved in the anti-preferred
direction or flickered had an adapting influence on the responses to
subsequent impulsive movements but the effect was far smaller than that
elicited by preferred direction adaptation. Adaptation in one region of the
receptive field did not affect the responses elicited by subsequent
stimulation in non-overlapping regions of the field. Adaptation is a
significant property of NOT neurons and probably acts to expand their temporal
resolving power.
Received 20 September 1996; accepted in final form 25 November 1997.
APS Manuscript Number J772-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997