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The Journal of Neuroscience, September 24, 2008, 28(39):9806-9816; doi:10.1523/JNEUROSCI.1293-08.2008

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Behavioral/Systems/Cognitive
Facilitatory Mechanisms Underlying Selectivity for the Direction and Rate of Frequency Modulated Sweeps in the Auditory Cortex

Khaleel A. Razak and Zoltan M. Fuzessery

Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming 82071

Correspondence should be addressed to Dr. Zoltan M. Fuzessery, Department of Zoology and Physiology, University of Wyoming, Department 3166, 1000 East University Avenue, Laramie, WY 82071. Email: zmf{at}uwyo.edu

Neurons selective for frequency modulated (FM) sweeps are common in auditory systems across different vertebrate groups and may underlie representation of species-specific vocalizations. Studies on mechanisms of FM sweep selectivity have primarily focused on sideband inhibition. Here, we present the first evidence for facilitatory mechanisms of FM sweep selectivity. Facilitatory interactions were found in 46 of 264 (17%) neurons tuned in the echolocation range (25–60 kHz) in the auditory cortex of the pallid bat. These neurons respond poorly to individual tones but are facilitated by combinations of tones with specific spectral and temporal intervals. Facilitation neurons show remarkable sensitivity to sub-millisecond differences in time delays between the two tones. Interestingly, the range of delays eliciting facilitation is not fixed but varies systematically with frequency difference between the two tones. Properties of facilitation strongly predict selectivity for the direction and rate of FM sweeps. Together with previous studies, there appear to be at least three mechanisms underlying FM rate and direction selectivity: sideband inhibition, duration tuning, and facilitation. Interestingly, similar mechanisms underlie direction and velocity tuning in the visual system, suggesting the evolution of similar computations across sensory systems to process dynamic sensory stimuli.

Key words: auditory cortex; combination sensitivity; facilitation; sideband inhibition; FM sweeps; rebound from inhibition

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Received March 26, 2008; revised July 23, 2008; accepted Aug. 25, 2008.

Correspondence should be addressed to Dr. Zoltan M. Fuzessery, Department of Zoology and Physiology, University of Wyoming, Department 3166, 1000 East University Avenue, Laramie, WY 82071. Email: zmf{at}uwyo.edu

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