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The Journal of Neurophysiology Vol. 88 No. 1 July 2002, pp. 370-382
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. and
Stephen G. Lisberger.
Constraints on the Source of Short-Term Motion Adaptation in
Macaque Area MT. II. Tuning of Neural Circuit Mechanisms. J. Neurophysiol. 88: 370-382, 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. In the present paper, we compared the tuning of
the adaptation to the neuron's tuning to direction and speed. The
tuning of adaptation was measured with a condition/test paradigm in
which a testing motion of the preferred direction and speed of the
neuron under study was preceded by a conditioning motion: the direction
and speed of the conditioning motion were varied systematically. The
response to the test motion depended strongly on the direction of the
conditioning motion. It was suppressed in almost all neurons by
conditioning motion in the same direction and could be either
suppressed or enhanced by conditioning motion in the opposite
direction. Even in neurons that showed suppression for target motion in
the nonpreferred direction, the adaptation and response direction
tuning were the same. The speed tuning of adaptation was linked much
less tightly to the speed tuning of the response of the neuron under
study. For just more than 50% of neurons, the preferred speed of
adaptation was more than 1 log unit different from the preferred
response speed. Many neurons responded best when slow motions were
followed by faster motions (acceleration) or vice versa (deceleration), suggesting that MT neurons may encode information about the change of
target velocity over time. Finally, adaptation by conditioning motions
of different directions, but not different speeds, altered the latency
of the response to the test motion. The adaptation of latency recovered
with shorter intervals between the conditioning and test motions than
did the adaptation of response size, suggesting that latency and
amplitude adaptation are mediated by separate mechanisms. Taken
together with the companion paper, our data suggest that short-term
motion adaptation in MT is a consequence of the neural circuit in MT
and is not mediated by either input-specific mechanisms or intrinsic
mechanisms related to the spiking of individual neurons. The circuit
responsible for adaptation is tuned for both speed and direction and
has the same direction tuning as the circuit responsible for the
initial response of MT neurons.
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