Laminar Organization of FM Direction Selectivity in the Primary Auditory Cortex of the Free-Tailed Bat

Abstract

We studied the columnar and layer-specific response properties of neurons in the primary auditory cortex (A1) of six (four females, two males) anesthetized free-tailed bats, Tadarida brasiliensis, in response to pure tones and down and upward frequency modulated (FM; 50 kHz bandwidth) sweeps. In addition, we calculated current source density (CSD) to test whether lateral intracortical projections facilitate neuronal activation in response to FM echoes containing spectrally distant frequencies from the excitatory frequency response area (FRA). Auditory responses to a set of stimuli changing in frequency and level were recorded along 64 penetrations in the left A1 of six free-tailed bats. FRA shapes were consistent across the cortical depth within a column and there were no obvious differences in tuning properties. Generally, response latencies were shorter (<10 ms) for cortical depths between 500 and 600 μm, which might correspond to thalamocortical input layers IIIb–IV. Most units showed a stronger response to downward FM sweeps, and direction selectivity did not vary across cortical depth. CSD profiles calculated in response to the CF showed a current sink located at depths between 500 and 600 μm. Frequencies lower than the frequency range eliciting a spike response failed to evoke any visible current sink. Frequencies higher than the frequency range producing a spike response evoked layer IV sinks at longer latencies that increased with spectral distance. These data support the hypothesis that a progressive downward relay of spectral information spreads along the tonotopic axis of A1 via lateral connections, contributing to the neural processing of FM down sweeps used in biosonar.

Faster Repetition Rate Sharpens the Cortical Representation of Echo Streams in Echolocating Bats

“…C , Relation between CF and mean FSL. Details about the topographical and functional organization of the A1 in the Mexican free-tailed bat can be found in Macias et al (2019) .…”

“…Since we did not find differences in the frequency tuning, bandwidth of frequency response areas or directional selectivity to the FM sweep across cortical depth ( Macias et al, 2019 ), in this study, we only included data recorded at depths between 400 and 600 μm, corresponding to input Layer IV. The raw signal was digitally bandpass-filtered offline (elliptic, second order) between 500 and 3000 Hz to obtain the multiunit activity.…”

Temporal coding of echo spectral shape in the bat auditory cortex

“…However, information about the frequency response areas of the sampled neurons and the sensitivity to the partial overlap between the two echoes was lacking. In a survey of the response properties of neurons in the A1 of the Mexican free-tailed bat, the majority of neurons were more sensitive to dFM regardless of whether or not they contained spectral notches [ 40 , 64 ]. The dynamic range in spiking rates was similarly low in the Mexican free-tailed bat (roughly 0–6 spikes per stimulus) to that reported for the big-brown bat (0–3 spikes per stimulus) even at high amplitudes [ 8 ].…”

“…Since we did not find differences in the frequency tuning, bandwidth of frequency response areas, or directional selectivity to the FM sweep across cortical depth [ 64 ], in this study we only included data recorded at depths between 400 and 600 μm, corresponding to input layer IV. The raw signal was digitally bandpass-filtered offline (elliptic, second order) between 500 and 3,000 Hz to obtain the multiunit activity.…”