Facilitation and Delay Sensitivity of Auditory Cortex Neurons in CF‐FM Bats, Rhinolophus rouxi and Pteronotus p.parnellii

Abstract

Responses of auditory neurons to complex stimuli were recorded in the dorsal belt region of the auditory cortex of two taxonomically unrelated bat species, Rhinolophus rouxi and Pteronotus parnellii parnellii, both showing Doppler shift compensation behaviour. As in P.p.parnellii (Suga et al., J. Neurophysiol., 49, 1573 - 1626, 1983), cortical neurons of R.rouxi show facilitated responses to pairs of pure tones or frequency modulations. Best frequencies for the two components lie near the first and second harmonic of the echolocation call but are in most cases not harmonically related. Neurons facilitated by pairs of pure tones show little dependence on the delay between the stimuli, whereas pairs of frequency modulations evoke best facilitated responses at distinct best delays between 1 and 10 ms. Facilitated neurons are found in distinct portions of the dorsal cortical belt region, with a segregation of facilitated neurons responding to pure tones and to frequency modulations. Non-facilitated neurons are found throughout the field. Neurons are topographically aligned with increasing best delays along a rostrocaudal axis. The best delays between 2 and 4 ms are largely overrepresented numerically, and occupy approximately 56% of the cortical area containing facilitated neurons. A functional interpretation of the large overrepresentation of best delays approximately 3 ms is proposed. Facilitated neurons are located almost entirely within layer V of the dorsal field.

Blurry topography for precise target-distance computations in the auditory cortex of echolocating bats

“…A stronger activation of the FM/FM area could be of advantage for fast initiation of motor responses required to resolve demanding situations such as prey catching or avoiding possible collisions. The latter is known as the 'alarm hypothesis' 5 .…”

“…Neurons tuned to short delays (and hence short target distances) are found rostrally, whereas neurons tuned to longer delays are located more caudally 3,[5][6][7]9 . This organizational pattern was termed 'chronotopic organization' because it describes how time information (in the form of echo delay) is represented in the cortex 6,9 .…”

“…In the auditory cortex, echo delay is encoded by delay-tuned neurons that respond to pulse-echo pairs presented at specific delays [1][2][3] . Cortical delay-tuned neurons of the species Pteronotus parnellii, Pteronotus quadridens, Carollia perspicillata and Rhinolophus rouxi are confined to a specialized region in the dorsal auditory cortex defined as the frequencymodulated (FM)/FM processing area [4][5][6][7] . Some empirical evidence suggests that the FM/FM area has a role in the bat's perception of echo delay 8 .…”

A Neural Correlate of Stochastic Echo Imaging

“…Oberbayern). The principle surgical procedure has been described in detail previously (Schuller et al, 1991). In brief, bats were anesthetized using a combination of medetomidin, midazolam, and fentanyl (0.4, 4, and 0.04 g/g body weight, respectively).…”

Auditory cortex of newborn bats is prewired for echolocation

“…As a neuronal correlate for target distance calculation, neurons in the auditory cortex (AC) of echolocating bats have been described that selectively respond to pairs of FM stimuli that represent pulse and echo 3,[6][7][8][9][10] . These combination-sensitive neurons are maximally facilitated at specific delays between pulse and echo.…”