Frequency-modulated up-chirps produce larger evoked responses than down-chirps in the big brown bat auditory brainstem

A novel method for determining ventilatory and gas exchange dynamics during exercise: the "chirp" waveform

Quantitating exercise ventilatory and gas exchange dynamics affords insights into physiological control processes and cardiopulmonary dysfunction. We designed a novel waveform, the chirp waveform, to efficiently extract moderate-intensity exercise response dynamics. In the chirp waveform, work rate fluctuates sinusoidally with constant amplitude as sinusoidal period decreases progressively from ∼8.5 to 1.4 min over 30 min of cycle ergometry. We hypothesized that response dynamics of pulmonary ventilation (V̇e) and gas exchange [oxygen uptake (V̇o2) and carbon dioxide output (V̇co2)] extracted from chirp waveform are similar to those obtained from stepwise transitions. Thirty-one participants [14 young healthy, 7 older healthy, and 10 patients with chronic obstructive pulmonary disease (COPD)] exercised on three occasions. Participants first performed ramp-incremental exercise for gas exchange threshold (GET) determination. In randomized order, the next two visits involved either chirp or stepwise waveforms. Work rate amplitude (20 W to ∼95% GET work rate) and exercise duration (30 min) were the same for both waveforms. A first-order linear transfer function with a single system gain (G) and time constant (τ) characterized response dynamics. Agreement between model parameters extracted from chirp and stepwise waveforms was established using Bland-Altman analysis and Rothery's concordance coefficient (RCC). V̇e, V̇o2, and V̇co2 Gs showed no systematic bias (P > 0.178) and moderate-to-good agreement (RCC > 0.772, P < 0.01) between waveforms. Similarly, no systematic bias (P = 0.815) and good agreement (RCC = 0.837, P < 0.001) was found for τV̇o2. Despite moderate agreement for τV̇co2 (RCC = 0.794, P < 0.001) and τV̇e (RCC = 0.722, P = 0.083), chirp τ was less [-6.9(11.7) s and -12.2(22.5) s, respectively]. We conclude that the chirp waveform is a promising method for measuring exercise response dynamics and investigating physiological control mechanisms.NEW & NOTEWORTHY We investigated the ability of a novel waveform to extract exercise ventilatory and gas exchange dynamics. In the chirp waveform, work rate fluctuates sinusoidally with constant amplitude as sinusoidal period decreases progressively over 30 min of exercise. In a study of 31 healthy individuals and patients with COPD, comparison of exercise dynamics derived from chirp to those from stepwise waveforms suggests that the chirp waveform is a promising method for derivation of exercise response dynamics.

Representation of frequency-modulated sweeps in the cochlear nucleus of the big brown bat

The cochlear nucleus (CN) receives ipsilateral input from the auditory nerve and projects to other auditory brainstem nuclei. Little is known about CN processing of signals used for echolocation. This study recorded multiple unit activity in the CN of anesthetized big brown bats (Eptesicus fuscus) to ultrasonic frequency-modulated (FM) sweeps differing in sweep direction. FM up-sweeps evoke larger peak amplitudes at shorter onset latencies and with smaller amplitude-latency trading ratios than FM down-sweeps. Variability of onset latencies is in the tens of microsecond ranges, indicating sharp temporal precision in the CN for coding of FM signals.

Non-invasive auditory brainstem responses to FM sweeps in awake big brown bats

We introduce two EEG techniques, one based on conventional monopolar electrodes and one based on a novel tripolar electrode, to record for the first time auditory brainstem responses (ABRs) from the scalp of unanesthetized, unrestrained big brown bats. Stimuli were frequency-modulated (FM) sweeps varying in sweep direction, sweep duration, and harmonic structure. As expected from previous invasive ABR recordings, upward-sweeping FM signals evoked larger amplitude responses (peak-to-trough amplitude in the latency range of 3–5 ms post-stimulus onset) than downward-sweeping FM signals. Scalp-recorded responses displayed amplitude-latency trading effects as expected from invasive recordings. These two findings validate the reliability of our noninvasive recording techniques. The feasibility of recording noninvasively in unanesthetized, unrestrained bats will energize future research uncovering electrophysiological signatures of perceptual and cognitive processing of biosonar signals in these animals, and allows for better comparison with ABR data from echolocating cetaceans, where invasive experiments are heavily restricted.

A comprehensive computational model of animal biosonar signal processing

Computational models of animal biosonar seek to identify critical aspects of echo processing responsible for the superior, real-time performance of echolocating bats and dolphins in target tracking and clutter rejection. The Spectrogram Correlation and Transformation (SCAT) model replicates aspects of biosonar imaging in both species by processing wideband biosonar sounds and echoes with auditory mechanisms identified from experiments with bats. The model acquires broadband biosonar broadcasts and echoes, represents them as time-frequency spectrograms using parallel bandpass filters, translates the filtered signals into ten parallel amplitude threshold levels, and then operates on the resulting time-of-occurrence values at each frequency to estimate overall echo range delay. It uses the structure of the echo spectrum by depicting it as a series of local frequency nulls arranged regularly along the frequency axis of the spectrograms after dechirping them relative to the broadcast. Computations take place entirely on the timing of threshold-crossing events for each echo relative to threshold-events for the broadcast. Threshold-crossing times take into account amplitude-latency trading, a physiological feature absent from conventional digital signal processing. Amplitude-latency trading transposes the profile of amplitudes across frequencies into a profile of time-registrations across frequencies. Target shape is extracted from the spacing of the object's individual acoustic reflecting points, or glints, using the mutual interference pattern of peaks and nulls in the echo spectrum. These are merged with the overall range-delay estimate to produce a delay-based reconstruction of the object's distance as well as its glints. Clutter echoes indiscriminately activate multiple parts in the null-detecting system, which then produces the equivalent glint-delay spacings in images, thus blurring the overall echo-delay estimates by adding spurious glint delays to the image. Blurring acts as an anticorrelation process that rejects clutter intrusion into perceptions.