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The Journal of Neurophysiology Vol. 86 No. 3 September 2001, pp. 1333-1350
Copyright ©2001 by the American Physiological Society
Kresge Hearing Research Institute, University of Michigan, Ann Arbor, Michigan 48109-0506
Mickey, Brian J. and
John C. Middlebrooks.
Responses of Auditory Cortical Neurons to Pairs of Sounds:
Correlates of Fusion and Localization. J. Neurophysiol. 86: 1333-1350, 2001. When two brief sounds arrive at a
listener's ears nearly simultaneously from different directions,
localization of the sounds is described by "the precedence effect."
At inter-stimulus delays (ISDs) <5 ms, listeners typically report
hearing not two sounds but a single fused sound. The reported location
of the fused image depends on the ISD. At ISDs of 1-4 ms, listeners
point near the leading source (localization dominance). As the ISD is
decreased from 0.8 to 0 ms, the fused image shifts toward a location
midway between the two sources (summing localization). When an
inter-stimulus level difference (ISLD) is imposed, judgements shift
toward the more intense source. Spatial hearing, including the
precedence effect, is thought to depend on the auditory cortex.
Therefore we tested the hypothesis that the activity of cortical
neurons signals the perceived location of fused pairs of sounds. We
recorded the unit responses of cortical neurons in areas A1 and A2 of
anesthetized cats. Single broadband clicks were presented from various
frontal locations. Paired clicks were presented with various ISDs and ISLDs from two loudspeakers located 50° to the left and right of
midline. Units typically responded to single clicks or paired clicks
with a single burst of spikes. Artificial neural networks were trained
to recognize the spike patterns elicited by single clicks from various
locations. The trained networks were then used to identify the
locations signaled by unit responses to paired clicks. At ISDs of 1-4
ms, unit responses typically signaled locations near that of the
leading source in agreement with localization dominance. Nonetheless
the responses generally exhibited a substantial undershoot; this
finding, too, accorded with psychophysical measurements. As the ISD was
decreased from ~0.4 to 0 ms, network estimates typically shifted from
the leading location toward the midline in agreement with summing
localization. Furthermore a superposed ISLD shifted network estimates
toward the more intense source, reaching an asymptote at an ISLD of
15-20 dB. To allow quantitative comparison of our physiological
findings to psychophysical results, we performed human psychophysical
experiments and made acoustical measurements from the ears of cats and
humans. After accounting for the difference in head size between cats
and humans, the responses of cortical units usually agreed with the
responses of human listeners, although a sizable minority of units
defied psychophysical expectations.
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