Neural Responses to Overlapping FM Sounds in the Inferior Colliculus of Echolocating Bats

Proc. Natl. Acad. Sci. U.S.A.

et al. 2010

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Sonar broadcasts are followed by echoes at different delays from objects at different distances. When broadcasts are emitted rapidly in cluttered surroundings, echo streams from successive broadcasts overlap and cause ambiguity in matching echoes to corresponding broadcasts. To identify reactions to ambiguity in clutter, echolocating bats that emit multiple-harmonic FM sounds were trained to fly into a dense, extended array of obstacles (multiple rows of vertically hanging chains) while the sonar sounds the bat emitted were recorded with a miniature radio microphone carried by the bat. Flight paths were reconstructed from thermal-infrared video recordings. Successive rows of chains extended more than 6 m in depth, so each broadcast was followed by a series of echoes from multiple rows of chains that lasted up to 40 ms. Bats emitted sounds in pairs ("strobe groups") at short (20-40 ms) interpulse intervals (IPIs) alternating with longer IPIs (>50 ms). For many short IPIs, the stream of echoes from the first broadcast was still arriving when the second broadcast was emitted. This overlap caused ambiguity about matching echoes with broadcasts. Bats shifted frequencies of the first sound in each strobe group upward and the second sound downward by 3-6 kHz. When overlap and ambiguity ceased, frequency shifts ceased also. Frequency differences were small compared with the total broadcast band, which was 75-80 kHz wide, but the harmonic structure of echoes enhances the differences in spectrograms. Bats could use time-frequency comparisons of echoes with broadcasts to assign echoes to the corresponding broadcasts and thus avoid ambiguity.bat sonar | clutter interference | echo delay | FM sounds

Section: Discussionmentioning

confidence: 99%

FM echolocating bats shift frequencies to avoid broadcast–echo ambiguity in clutter

Hiryu

Bates²,

Proc. Natl. Acad. Sci. U.S.A.

et al. 2010

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100

“…2.6s) following the nominal position of FM2 and extends to at least 300s later without serious deterioration of the defocusing effect. This pattern of results suggests that, when responses to FM2 are allowed to shift to a later time than normal, they activate neuronal inhibition that initiates a cascade of responses designed to register the shape of targets from echo interference spectra (see Sanderson and Simmons, 2000;Sanderson and Simmons, 2002).…”

Section: Discussionmentioning

confidence: 98%

“…The target's contribution instead consists of local notches in the spectrum due to reinforcement and cancellation caused by interference between overlapping reflections from multiple parts of the target, or glints (the target's acoustic 'shape') (Kober and Schnitzler, 1990;Moss and Zagaeski, 1994;Simmons and Chen, 1989). Behavioral, neurophysiological and computational studies have identified a process, called spectrogram correlation and transformation (SCAT), that has been hypothesized to explain how big brown bats locate the frequencies of these interference notches and reconstruct the corresponding delay differences between different parts of the target (Matsuo et al, 2004;Neretti et al, 2003;Peremans and Hallam, 1998;Saillant et al, 1993;Sanderson and Simmons 2000;Sanderson and Simmons, 2002;Sanderson and Simmons, 2005;Simmons et al, 1995;Simmons et al, 1998). Using a combination of overall echo delay and the echo interference spectrum as cues, these bats recreate for each echo an image depicting the object as a small number of glints on a perceptual axis of distance.…”

Section: Discussionmentioning

confidence: 99%

“…How is this distinction represented in the bat's auditory system? Neurons at successive stages (cochlear nucleus to auditory cortex) in the big brown bat's auditory system are tuned to different frequencies across the echolocation band of roughly 20-100kHz (Dear et al, 1993a;Dear et al, 1993b;Ferragamo et al, 1998;Haplea et al, 1994;Jen et al, 1989;Ma and Suga, 2008;Pollak et al, 1977;Sanderson and Simmons, 2000;Sanderson and Simmons, 2002). Results of these experiments When FM2 is offset in time (t; horizontal green arrows) relative to FM1, the bats' delay acuity deteriorates, but recovers if FM2 is removed entirely.…”

Section: Discussionmentioning

confidence: 99%

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Perception of echo delay is disrupted by small temporal misalignment of echo harmonics in bat sonar

Bates

Journal of Experimental Biology

2011

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SUMMARYEcholocating big brown bats emit ultrasonic frequency-modulated (FM) biosonar sounds containing two prominent downwardsweeping harmonics (FM1 and FM2) and perceive target distance from echo delay. In naturally occurring echoes, FM1 and FM2 are delayed by the same amount. Even though echoes from targets located off-axis or far away are lowpass filtered, which weakens FM2 relative to FM1, their delays remain the same. We show here that misalignment of FM2 with FM1 by only 2.6s is sufficient to significantly disrupt acuity, which then persists for larger misalignments up to 300s. However, when FM2 is eliminated entirely rather than just misaligned, acuity is effectively restored. For naturally occurring, lowpass-filtered echoes, neuronal responses to weakened FM2 are retarded relative to FM1 because of amplitude-latency trading, which misaligns the harmonics in the bat's internal auditory representations. Electronically delaying FM2 relative to FM1 mimics the retarded neuronal responses for FM2 relative to FM1 caused by amplitude-latency trading. Echoes with either electronically or physiologically misaligned harmonics are not perceived as having a clearly defined delay. This virtual collapse of delay acuity may suppress interference from off-axis or distant clutter through degradation of delay images for clutter in contrast to sharp images for nearer, frontal targets.

“…The mechanical action of the bat's cochlea disperses ultrasonic frequencies to sequential locations along the organ of Corti, creating many parallel frequency-tuned channels which segment the wideband FM sweeps of echoes into numerous overlapping time͞frequency ''slices'' (24). Within these channels, neural transduction generates single spikes that register the times-of-occurrence of different frequencies in the sweeps (25)(26)(27). At higher levels of the auditory pathway, it is likely that volleys of spikes with different latencies representing the same time͞frequency slices are used to make multiple estimates of echo delay (28).…”

Section: Methodsmentioning

confidence: 99%

Delay accuracy in bat sonar is related to the reciprocal of normalized echo bandwidth, or Q

Proc. Natl. Acad. Sci. U.S.A.

Neretti

Intrator

et al. 2004

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Big brown bats (Eptesicus fuscus) emit wideband, frequencymodulated biosonar sounds and perceive the distance to objects from the delay of echoes. Bats remember delays and patterns of delay from one broadcast to the next, and they may rely on delays to perceive target scenes. While emitting a series of broadcasts, they can detect very small changes in delay based on their estimates of delay for successive echoes, which are derived from an auditory time͞frequency representation of frequency-modulated sounds. To understand how bats perceive objects, we need to know how information distributed across the time͞frequency surface is brought together to estimate delay. To assess this transformation, we measured how alteration of the frequency content of echoes affects the sharpness of the bat's delay estimates from the distribution of errors in a psychophysical task for detecting changes in delay. For unrestricted echo frequency content and high echo signal-to-noise ratio, bats can detect extremely small changes in delay of about 10 ns. When echo bandwidth is restricted by filtering out low or high frequencies, the bat's delay acuity declines in relation to the reciprocal of relative echo bandwidth, expressed as Q, which also is the relative width of the target impulse response in cycles rather than time. This normalized-time dimension may be efficient for target classification if it leads to target shape being displayed independent of size. This relation may originate from cochlear transduction by parallel frequency channels with active amplification, which creates the auditory time͞frequency representation itself.