Kodierung von Raum- und Zeitfrequenz in der Echoabbildung von Fledermäusen
Biologie des Verhaltens und der Sinne
Zusammenfassung der Projektergebnisse
Bats emit ultrasonic cries and listen to the reflected sounds to orient and forage in their environment. The rich ecological niche of nocturnal air space became accessible through bats’ capability of sustained flight and echolocation. Their “sixth sense” gained them autonomy from sunlight, but to what extent can hearing replace vision? This thesis addresses the question how echolocation encodes certain spatial and temporal parameters of the environment. Echolocation poses a challenge to the perception of spatial layouts because the auditory sensory epithelium, the cochlea, does not explicitly encode space like the eye’s retina does; space must be computed by comparing echo cues at both ears. In the first set of experiments within this DFG grant, we tested the hypothesis that despite this challenge, bat echolocation utilizes the concept of spatial frequency to form perceptual representations of bats’ habitat. Spatial frequency has been crucial to understand visual perception. In the first set of experiments, we show that both sensory systems, echolocation and vision, have access to spatial frequency information despite their fundamental mechanistic differences. We trained six bats (Phyllostomus discolor) to discriminate ripples of different spatial frequencies from a smooth surface and measured echo-acoustic depth-contrast-sensitivity functions. We show that bats are much more sensitive to high spatial frequencies, exemplifying a spatial high-pass filter. Additionally, we evaluated the perceptual cues available to the bats to assess spatial frequency and found them fundamentally different from those in vision. While spatial frequency perception in vision is a result of spatial tuning, starting already in the retina, spatial frequency perception in echolocation is achieved by object-specific reflection properties that determine the perceived echo-acoustic object signature. The demonstration of a high-pass filter in bat echolocation reveals a functional similarity between vision and echolocation, which underlies figure-groundseparation and allows both systems access to the spatial contours in the environment. The functional similarities, yet mechanistic differences, highlight the need for spatial environmental information, independent of sensory system. The auditory system excels in measuring minute differences in echo arrival times. But when it comes to the tracking of changes of echo properties over time, the echolocation system of a typical bat seems to be at a disadvantage. The echolocation call of frequency-modulating bats is too short to track an entire movement cycle. In order to track movement, bats have to compare memorised sequences of call-echo pairs. In the second and third sets of experiments, we quantified the sensitivity of bat echolocation to the temporal modulation of echo parameters. In nature, fluttering insect wings cause echo modulations; the echoes carry modulations in echo delay and in echo amplitude simultaneously. In the second set of experiments, we introduce an auditory virtual reality where we can manipulate delay independently from amplitude and tease apart the effects of both parameters on perception. We demonstrate that in the frequency-modulating bat Phyllostomus discolor the sensitivity for modulations in echo delay depends on the rate of the modulation, with bats being most sensitive at modulation rates below 20 Hz and above 50 Hz. We show that echolocation is susceptible to interference between call repetition rate and modulation rate. We propose that this phenomenon constitutes an echo-acoustic wagon-wheel effect. We further demonstrate how at high modulation rates sensitivity could be rescued by using spectral and temporal cues introduced by Doppler-distortions. Thus, we present evidence that Doppler distortions may play a crucial role in flutter sensitivity in the hundreds of frequencymodulating bat species worldwide. In the third set of experiments, we use the virtual reality approach to generate modulations in echo amplitude independent from echo delay. We show that Phyllostomus discolor successfully detected these modulations in echo amplitude and that their performance increased with the rate of the modulation. We suggest that amplitude-modulation detection with echolocation differs fundamentally from delay-modulation detection and speculate that the mechanism to detect fast amplitude modulations relies on spectral cues. In summary, within this DFG funding, we provide experimental evidence on important perceptual processes in echolocation of frequency-modulating bats. Our findings highlight the diversity of selective pressures working on the echolocation system of bats.
Projektbezogene Publikationen (Auswahl)
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(2018) Flutter sensitivity in FM bats. Part I: delay modulation. J Comp Physiol A Neuroethol Sens Neural Behav Physiol. 2018 Nov;204(11):929-939
A. Leonie Baier and Lutz Wiegrebe
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(2018) Flutter sensitivity in FM bats. Part II: amplitude modulation. J Comp Physiol A Neuroethol Sens Neural Behav Physiol. 2018 Nov;204(11):941-951
A. Leonie Baier, Kristin-Jasmin Stelzer, and Lutz Wiegrebe
