Echolocating bats can use sonar to discriminate among targets which reflect echoes differing in spectral distribution of energy but not in overall intensity. They can detect differences smaller than 1 millimeter in fine target structure. Bats may be capable of classifying targets from echo spectral signatures and might thus be able to distinguish among flying insect prey by sonar.
Echolocating bats can discriminate the range (echo arrival time) of simulated targets with accuracy predictable directly from the sonar waveforms down to the region where echo and noise powers are about equal. Detection and ranging performance are closely linked. Equivalence of performance in simultaneous- and successive-presentation procedures indicates that bats make absolute range judgments and store range images of targets for subsequent comparisons. Targets separated in range and/or angular direction are perceived as distinct events. Although target range is perceived noncoherently, target fine structure may be perceived coherently. Information about the occurrence and timing of transmitted sounds and echoes is conveyed in the time domain over frequency-specific primary auditory neurons and appears, still in the time domain, ascending the lateral lemniscus and in the inferior colliculus. Behavioral data suggest ultimate spatial “receptor-field” representation of range in the brain, so echo-timing information may be receded into range-specific neurons at or above the inferior colliculus. The data generally suggest that target range is processed by channels used for periodicity-pitch perception in nonecholocating animals. [Work supported by NSF.]
In 1948 Pumphrey and Gold investigated the ability to distinguish between a periodic sequence of identical sinusoidal pulses and a second sequence in which alternate pulses were inverted. The ability to detect differences between these sequences led them to calculate the Q of an assumed resonance system representing the auditory filter or critical band. They concluded that the Q was of the order of 100 for the conditions of these experiments. A detailed analysis of the spectrum of the two pulse trains indicates the critical cue for the discrimination is a pitch cue as described by Hiesey and Schubert. This pitch cue is the result of a ripple in the power spectrum of the stimulus and can be related to the pitch of delayed and added noise, as studied by Bilsen and others. [This research was supported by the National Institutes of Health. ]
Jamming experiments were conducted to assess the effectiveness of several different types of signals for interfering with the ability of echolocating bats (Eptesicus fuscus) to discriminate target range. Random interfering noise covering the bat's sonar bandwidth of 25–50 kHz must be sufficiently intense for the echo signal-to-noise ratios to approach 0 dB before marked breakdown of echolocation occurs. This is true for continuous noise, intermittent noise, and for noise incorporating internal correlation at the same period as the travel time of sounds going to and from the targets. FM interference is 30 to 40 dB more effective in jamming the bat than random noise of the same spectrum. The results suggest that the bat's sonar receiver applies frequency-sweep criteria to incoming sounds to decide whether potential echoes are present. Neurons with properties appropriate to this filtering function, analogous to motion detection in vision, are known to exist in the bat's auditory nervous system.
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