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The Journal of Neurophysiology Vol. 85 No. 6 June 2001, pp. 2364-2380
Copyright ©2001 by the American Physiological Society
Laboratory of Auditory Neurophysiology, Department of Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
Lu, Thomas,
Li Liang, and
Xiaoqin Wang.
Neural Representations of Temporally Asymmetric Stimuli in
the Auditory Cortex of Awake Primates. J. Neurophysiol. 85: 2364-2380, 2001. The representation of rapid acoustic
transients by the auditory cortex is a fundamental issue that is still
unresolved. Auditory cortical neurons have been shown to be limited in
their stimulus-synchronized responses, yet the perceptual performances
of humans and animals in discriminating temporal variations in complex
sounds are better than what existing neurophysiological data would
predict. This study investigated the neural representation of
temporally asymmetric stimuli in the primary auditory cortex of awake
marmoset monkeys. The stimuli, ramped and damped sinusoids, were
systematically manipulated (by means of half-life of the exponential
envelope) within a cortical neuron's presumed temporal integration
window. The main findings of this study are as follows: 1)
temporal asymmetry in ramped and damped sinusoids with a short period
(25 ms) was clearly reflected by average discharge rate but not
necessarily by temporal discharge patterns of auditory cortical
neurons. There was considerable response specificity to these stimuli
such that some neurons were strongly responsive to a ramped sinusoid
but almost completely unresponsive to its damped counterpart or vice versa. Of 181 neurons studied, 140 (77%) showed significant response asymmetry in at least one of the tested half-life values of the exponential envelope. Forty-six neurons showed significant
response asymmetry over all half-lives tested. Sustained firing,
commonly observed under awake conditions, contributed to greater
response asymmetry than that of onset responses in many neurons.
2) A greater proportion of the neurons (32/46) that
exhibited significant overall response asymmetry showed stronger
responses to the ramped sinusoids than to the damped
sinusoids, possibly contributing to the difference in the perceived
loudness between these two classes of sounds. 3) The
asymmetry preference of a neuron to ramped or damped sinusoids did not
appear to be correlated with its characteristic frequency or minimum
response latency, suggesting that this is a general phenomenon that
exists across populations of cortical neurons. Moreover, the intensity
of the stimuli did not have significant effects on the measure of the
asymmetry preference based on discharge rate. 4) A
population measure of response preference, based on discharge rate, of
cortical neurons to the temporally asymmetric stimuli was
qualitatively similar to the performance of human listeners in
discriminating ramped versus damped sinusoids at different half-life
values. These findings suggest that rapid acoustic transients embedded
in complex sounds can be represented by discharge rates of cortical
neurons instead of or in the absence of stimulus-synchronized discharges.
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