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Functional organization of the primary auditory cortex of the free-tailed bat Tadarida brasiliensis

Abstract

The Mexican free-tailed bat, Tadarida brasiliensis, is a fast-flying bat that hunts by biosonar at high altitudes in open space. The auditory periphery and ascending auditory pathways have been described in great detail for this species, but nothing is yet known about its auditory cortex. Here we describe the topographical organization of response properties in the primary auditory cortex (AC) of the Mexican free-tailed bat with emphasis on the sensitivity for FM sweeps and echo-delay tuning. Responses of 716 units to pure tones and of 373 units to FM sweeps and FM–FM pairs were recorded extracellularly using multielectrode arrays in anesthetized bats. A general tonotopy was confirmed with low frequencies represented caudally and high frequencies represented rostrally. Characteristic frequencies (CF) ranged from 15 to 70 kHz, and fifty percent of CFs fell between 20 and 30 kHz, reflecting a hyper-representation of a bandwidth corresponding to search-phase echolocation pulses. Most units showed a stronger response to downward rather than upward FM sweeps and forty percent of the neurons interspersed throughout AC (150/371) showed echo-delay sensitivity to FM–FM pairs. Overall, the results illustrate that the free-tailed bat auditory cortex is organized similarly to that of other FM-type insectivorous bats.

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Abbreviations

AC:

Auditory cortex

CD:

Characteristic delay

CF:

Characteristic frequency

CF-FM:

Constant-frequency–frequency-modulated pulses

DRA:

Delay response area

DSI:

Direction selectivity index

FM:

Frequency modulated

FRA:

Frequency response area

MT:

Minimum threshold

PSTH:

Post-stimulus time histogram

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Acknowledgements

All experiments were carried out according to the National Institutes of Health guidelines and were approved by the TAMU Institutional Animal Care and Use Committee. This work was supported by the U.S. Office of Naval Research Grant No. ONRN00014-17-1-2736. KB received the support from the Texas A&M Institute for Neuroscience.

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Correspondence to Silvio Macias.

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Macias, S., Bakshi, K. & Smotherman, M. Functional organization of the primary auditory cortex of the free-tailed bat Tadarida brasiliensis. J Comp Physiol A (2020). https://doi.org/10.1007/s00359-020-01406-w

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Keywords

  • Auditory cortex
  • Tonotopy
  • FM sweep direction
  • Echo-delay tuning
  • Tadarida