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J Neurophysiol (June 14, 2006). doi:10.1152/jn.00020.2006
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Submitted on January 9, 2006
Accepted on June 13, 2006

Neural mechanisms underlying selectivity for the rate and direction of frequency modulated sweeps in the auditory cortex of the pallid bat

Khaleel A Razak1 and Zoltan M Fuzessery1*

1 Zoology and Physiology, Univ of Wyoming, Laramie, Wyoming, United States

* To whom correspondence should be addressed. E-mail: zmf{at}uwyo.edu.

Frequency modulated (FM) sweeps are common in vocalizations, including human speech. Selectivity for FM sweep rate and direction is present in the auditory cortex of many species. The present study sought to determine the mechanisms underlying FM sweep selectivity in the auditory cortex of pallid bats. In the pallid bat inferior colliculus (IC), two mechanisms shape FM sweep rate selectivity. The first mechanism depends on duration tuning for tones. The second mechanism depends on a narrow band of delayed high-frequency inhibition. Direction selectivity depends on a broad band of early low-frequency inhibition. Here, the contributions of these mechanisms to cortical FM sweep selectivity were determined. We show that the majority of cortical neurons tuned to echolocation frequencies are selective for the downward direction and rate of FM sweeps. Unlike in IC neurons tuned in the echolocation range, duration tuning is rare in cortical neurons with similar tuning. As in the IC, consistent spectrotemporal differences exist between low- and high-frequency sidebands. A narrow band of delayed high-frequency inhibition is necessary for FM rate selectivity. Low-frequency inhibition has a broad bandwidth, early arrival time, and creates direction selectivity. Cortical neurons respond better to slower FM rates and exhibit broader rate tuning than IC neurons. Relative arrival time of high-frequency inhibition is slower in the cortex than in the IC. Thus, while similar mechanisms shape direction selectivity in IC and cortical FM sweep-selective neurons, only one of the two mechanisms underlying rate selectivity in the IC is present in the cortex.




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