Copyright © 1999 Massachusetts Institute of Technology
Echolocation, a term first coined by Donald Griffin in 1944, refers to the use of sound reflections to localize objects and orient in the environment (Griffin 1958). Echolocating animals transmit acoustic signals and process information contained in the reflected signals, permitting the detection, localization and identification of objects. The use of echolocation has been documented in bats (e.g., Griffin 1958), marine mammals (e.g., Norris et al. 1961; Au 1993), some species of nocturnal birds (e.g., Griffin 1953) and to a limited extent in blind or blindfolded humans (e.g., Rice 1967). Only in bats and dolphins have specialized perceptual and neural processes for echolocation been detailed.
Acoustic signals for echolocation in bats and marine mammals are primarily in the ultrasonic range, above 20 kHz and the upper limit of human hearing. The short wavelengths of these ultrasound signals permit reflections from small objects in the environment. All bat species of the suborder Microchiroptera produce echolocation calls, either through the open mouth or through a nose-leaf, depending on the species. The signal types used by different bat species vary widely, but all contain some frequency modulated (FM) components, which are well suited to carry information about the arrival time of target echoes. Constant frequency (CF) signal components are sometimes combined with FM components, and these signals are well suited to carry information about target movement through Doppler shifts in the returning echoes. There is evidence that species using both FM and CF signals show individual variations in signal structure that could facilitate identification of self-produced echoes (see Suga et al. 1987; Masters, Jacobs, and Simmons 1991). One species of echolocating bat of the suborder Megachiropetera, Rosettus aegyptiacus, produces clicklike sounds with the tongue for echolocation (Novick 1958). The most widely studied echolocating marine mammal, the bottlenose dolphin (Tursiops truncatus), emits brief clicks, typically less than 50 µs in duration, with spectral energy from 20 kHz to over 100 kHz, depending on the acoustic environment in which the sounds are produced (Au 1993).
In echolocating animals, detection of a sonar target depends on the strength of the returning echo (Griffin 1958). Large sonar targets reflecting strong echoes are detected at greater distances than small sonar targets (Kick 1982; Au 1993). Psychophysical studies of echo detection in bats and dolphins indicate a strong dependence of performance on the acoustic environment. Forward and backward masking, background noise level, and reverberation can all influence sonar target detection (Au 1993; Moss and Schnitzler 1995).
Once an animal detects a sonar target, it must localize the object in three-dimensional space. In bats, the horizontal location of the target influences the features of the echo at the two ears, and these interaural cues permit calculation of a target's azimuthal position in space (Shimozowa et al. 1974). Laboratory studies of target tracking along the horizontal axis in bats suggest an accuracy of approximately 1 deg (Masters et al. 1985). The vertical location of a target results in a distinctive travel path of the echo into the bat's external ear, producing spectral changes in the returning sound that can be used to code target elevation (Grinnell and Grinnell 1965). Accuracy of vertical localization in bats is approximately 3 deg (Lawrence and Simmons 1982). The third dimension, target distance, depends on the time delay between the outgoing sound and returning echo (Hartridge 1945; Simmons 1973). Psychophysical studies of distance discrimination in FM bats report thresholds of about 1 cm, corresponding to a difference in echo arrival time of approximately 60 microseconds. Experiments that require the bat to detect a change in the distance (echo delay) of a jittering target report thresholds of less than 0.1 mm, corresponding to a temporal jitter in echo arrival time of less than 1 microsecond. Successful interception of insect prey by bats requires accuracy of only 1-2 cm (summarized in Moss and Schnitzler 1995). In marine mammals, psychophysical data show that the dolphin can discriminate a target range difference of approximately 1 cm, performance similar to that of the echolocating bat (Murchison 1980).
Many bats that use CF-FM signals are specialized to detect and process frequency and amplitude modulations in the returning echoes that are produced by fluttering insect prey. The CF components of these signals are relatively long in duration (up to 100 ms), sufficient to encode target movement from a fluttering insect over one or more wingbeat cycles. The CF-FM greater horseshoe bat, Rhinolophus ferrumequinum, can discriminate frequency modulations in the returning echo of approximately 30 Hz (less than 0.5% of the bat's 83 kHz CF signal component), and can discriminate fluttering insect species with different echo signatures (von der Emde and Schnitzler 1990). Several bat species that use CF-FM signals for echolocation exhibit Doppler shift compensation behavior: the bat adjusts the frequency of its sonar transmission to offset a Doppler shift in the returning echo, the magnitude of which depends on the bat's flight velocity (Schnitzler and Henson 1980). Doppler shift compensation allows the bat to isolate small amplitude and frequency modulations in sonar echoes that are produced by fluttering insects.
High-level perception by sonar has been examined in some bat species. Early work by Griffin et al. (1965) demonstrated that FM-bats can discriminate between mealworms and disks tossed into the air. Both mealworms and disks presented changing surface areas as they tumbled through the air, and this study suggested that FM-bats use complex echo features to discriminate target shape. The acoustic basis for target shape discrimination by FM-bats has been considered in detail by Simmons and Chen (1989); however, researchers have not yet determined whether FM bat species develop three-dimensional representations of objects using sonar (see Moss and Schnitzler 1995). Three-dimensional recognition of fluttering insects has been reported in the greater horseshoe bat, a species that uses a CF-FM signal for echolocation (von der Emde and Schnitzler 1990).
Successful echolocation depends on specializations in the auditory receiver to detect and process echoes of the transmitted sonar signals. Central to the conclusive demonstration of echolocation in bats was data on hearing sensitivity in the ultrasonic range of the biological sonar signals (Griffin 1958), and subsequent research has detailed many interesting specializations for the processing of sonar echoes in the auditory receiver of bats. In dolphins, studies of the central auditory system have been limited, but early work clearly documents high frequency hearing in the ultrasonic range of echolocation calls (e.g., Bullock et al. 1968).
In some CF-FM bat species, there are specializations in the peripheral and central auditory systems for processing echoes in the frequency range of the CF sonar component. The greater horseshoe bat, for example, adjusts the frequency of its sonar emissions to receive echoes at a reference frequency of approximately 83 kHz. The auditory system of this species shows a large proportion of neurons devoted to processing this reference frequency, and this expanded representation of 83 kHz can be traced to mechanical specializations of this bat's cochlea (Kössl and Vater 1995).
There are other specializations in the bat central auditory system for echo processing that may play a role in the perception of target distance. In bat species that utilize CF-FM signals and those that utilize FM sonar components alone, there are neurons in the midbrain, THALAMUS and cortex that respond selectively to pairs of FM sounds separated by a delay (e.g., Yan and Suga 1996). The pairs of FM sounds simulate the bat's sonar transmissions and returning echoes, and the time delay separating the two corresponds to a particular target distance. The pulse-echo delay evoking the largest facilitated response, referred to as the best delay (BD), is in some CF-FM bat species topographically organized (Suga and O'Neill 1979). Neural BD's fall into a biologically relevant range of 2-40 ms, corresponding to target distances of approximately 34 to 690 cm. Such topography has not been demonstrated in FM-bat species (e.g., Dear et al. 1993).
Many specializations in behavior and central auditory processing appear in echolocating animals; however, research findings suggest that echolocation builds on the neural and perceptual systems that evolved for hearing in less specialized animals.
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Copyright © 1999 Massachusetts Institute of Technology