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The Journal of Neurophysiology Vol. 88 No. 1 July 2002, pp. 354-369
Copyright ©2002 by the American Physiological Society
Howard Hughes Medical Institute, Department of Physiology, W. M. Keck Foundation, Center for Integrative Neuroscience, and the Neuroscience Graduate Program, University of California, San Francisco, California 94143
Priebe, Nicholas J.,
Mark M. Churchland, and
Stephen G. Lisberger.
Constraints on the Source of Short-Term Motion Adaptation in
Macaque Area MT. I. The Role of Input and Intrinsic Mechanisms. J. Neurophysiol. 88: 354-369, 2002. Neurons in area MT, a motion-sensitive area of extrastriate
cortex, respond to a step of target velocity with a transient-sustained firing pattern. The transition from a high initial firing rate to a
lower sustained rate occurs over a time course of 20-80 ms and is
considered a form of short-term adaptation. The present paper asks
whether adaptation is due to input-specific mechanisms such as
short-term synaptic depression or if it results from intrinsic cellular
mechanisms such as spike-rate adaptation. We assessed the contribution
of input-specific mechanisms by using a condition/test paradigm to
measure the spatial scale of adaptation. Conditioning and test stimuli
were placed within MT receptive fields but were spatially segregated so
that the two stimuli would activate different populations of inputs
from the primary visual cortex (V1). Conditioning motion at one visual
location caused a reduction of the transient firing to subsequent test
motion at a second location. The adaptation field, estimated as the
region of visual space where conditioning motion caused adaptation, was
always larger than the MT receptive field. Use of the same stimulus
configuration while recording from direction-selective neurons in V1
failed to demonstrate either adaptation or the transient-sustained
response pattern that is the signature of short-term adaptation in MT.
We conclude that the shift from transient to sustained firing in MT
cells does not result from an input-specific mechanism applied to
inputs from V1 because it operates over a wider range of the visual
field than is covered by receptive fields of V1 neurons. We used a
direct analysis of MT neuron spike trains for many repetitions of the same motion stimulus to assess the contribution to adaptation of
intrinsic cellular mechanisms related to spiking. On a trial-by-trial basis, there was no correlation between number of spikes in the transient interval and the interval immediately after the transient period. This is opposite the prediction that there should be a correlation if spikes cause adaptation directly. Further, the transient
was suppressed or extinguished, not delayed, in trials in which the
neuron emitted zero spikes during the interval that showed a transient
in average firing rate. We conclude that the transition from transient
to sustained firing in neurons in area MT is caused by mechanisms that
are neither input-specific nor controlled by the spiking of the
adapting neuron. We propose that the short-term adaptation observed in
area MT emerges from the intracortical circuit within MT.
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