Similarities of Human and Bat Echolocation

Blind and blindfolded sighted human subjects were in fact able to learn to use echolocation to detect objects in their environment (5). Efforts have been made to devise an effective mechanical aid for the blind to improve their auditory perception of obstacles (4).

From 1944 to 1947 the Committee on Sensory Devices of the National Academy of Scien ces developed eighteen different portable devices to aid the blind in avoiding obstacles. Only two performed sufficiently well, but these devices performed analysis on a point-by-point basis. This point-by-point analysis can be compared to a flashlight which "reveals only the small portion of the environment that falls within its scope (18)." Much remains to be learned about the physical properties and cues which affect object perception by the blind.

These issues were addressed by many scientists , including Diderot who, in 1749, recorded his observation that a blind person has the ability to perceive the presence and distance of objects. Since then different theories have been postulated to account for this phenomenon. One theory described obje ct detection through the use of skin sensitivity to temperature or pressure. Another theory offered pressure on the tympanic membrane (which vibrates with sound waves in the inner ear) as the mechanism of detection. Occult theories postulated object det ection through utilization of such phenomena as magnetism, electricity, or a sixth sense. The blind themselves had different interpretations of the mechanism of their perception.

"Hearing" Objects

In 1893, Dresslar conducted expe riments on the detection of echoes reflected from obstacle surfaces by eliminating, one by one, different sensory capabilities of blindfolded sighted subjects and then analyzing the subjects' performance. In one condition subjects' vision alone was elimi nated; in another, vision, thermal, and facial pressure were eliminated by covering exposed skin but not the auditory meatus. In the final condition hearing was eliminated by plugging the ears but leaving the face exposed and the eyes covered. Dresslar concluded that the ability to detect objects was due to auditory mechanisms. However, further experiments by different researchers led to conflicting results and theories on the mechanism of object perception by the blind, the most popular of which suppo rted hearing, skin pressure, and/or thermal sensations (17).

In 1944, Supa, Cotzin, and Dallenbach discovered that stimulation of the auditory system, rather than stimulation of the skin by air and sound waves, was necessary and sufficient for detecti ng obstacles. When air-waves were prevented from impinging on the exposed skin of blind and blindfolded sighted human subjects, the subjects, by listening to footsteps, could still detect obstacles through their uncovered ears. When audition was prevent ed and the skin left exposed, subjects could not detect the obstacles. Worchel and Dallenbach (1947) found that partially deaf and blind subjects could not detect obstacles if audition was prevented. If, however, the skin of the external ears was covere d but the auditory meatus was exposed, subjects could still detect obstacles. Thus, the aural mechanism the blind employed was found to be auditory stimulation and not stimulation of the skin of the external ears. These discoveries corroborated work done on bats by Griffin and Galambos in 1941, who found that gagging bats to prevent their emission of supersonic cries, or plugging their ears to prevent reception of echoes, increased the bats' frequency of collisions with obstacles. Thus, in both bats and humans, eliminating sound production or sound reception decreased or eliminated the ability to detect objects when sight was not available (17).

Important Properties of Sound

After determining the importance of hearing, researche rs focused on specific properties of sound were important. Cotzin and Dallenbach (2) found that in order for blind subjects to avoid colliding with a wall, they must hear changes in the pitch of a sound, with a frequency above 10 kilohertz. The nature o f sound is such that higher frequencies allow better resolution of echoes reflected from small targets. Thus bats, who are able to emit and perceive sounds above the upper limit of human hearing, can derive finer information from targets than humans (9). For example, the big brown bat, Eptesicus Fuscus, uses broad-band FM sound waveforms (approximately 100 to 20 kHz) which, according to Simmons (13), provides good resolution of target distance, size and shape.

Natural Conditions

< b> The range of human echolocation capabilities continued to be explored in further studies. In 1953 Ammons, Worchel, and Dallen-bach found that blindfolded sighted subjects were all able to acquire the ability to detect obstacles outdoors. The expe riment replicated a procedure described by Supa (17), and the results revealed that the subjects relied on and sought out non-auditory cues such as odors and shadows. Thus under natural conditions, detection of obstacles may be enhanced by a number of di fferent senses such as touch and smell, both of which can be controlled in the laboratory.

Size and Distance Discrimination

Distance discrimination was the next step of inquiry in the evolving study of human echolocation. In 1962 Kellogg presented blind subjects with two stimuli, two flat disks of the same diameter. Every trial consisted of a pair of targets, a standard stimulus at a constant distance and a comparison stimulus at a variable distance. Targets were present ed one after another in a pair in rapid succession. Asked to generate any sound they chose, subjects issued a variety of acoustic signals, including tongue clicks, hisses, whistles, and the human voice (which was the subjects' preferred source of audito ry signals). Target size was kept constant while distance between the two targets was changed, and the subject's task was to report the larger of the two targets by listening to the echoes reflected off of the disks. Kellogg found that objects closer to the observer were observed to be larger in size than the standard of same diameter, whereas objects farther away were determined to be smaller.

Kellogg also found that the blind had the ability to discriminate objects of different size. In the sec ond part of Kellogg's experiment, subjects were able to discriminate, with up to a 100 percent accuracy, the smaller object of two different sized objects placed at the same distance. Performance was found to decrease as the task was set at further dis tances.

In 1965 Rice showed that percent correct detection of an object was a function of the size and distance of the object from the observer. Thus, at a greater distance, objects needed to be larger in diameter to be detected by subject. As the ob ject was placed farther from a subject, the sound intensity of the echo became lower, and object detection became more difficult. As object size increased, the subjects' ability to detect the object improved. Sound intensity of echoes, manipulated throu gh object size and distance, affected the ability of subjects to detect object presence.

Types of Sounds

The type of sound suitable for human echolocation was also studied in Rice's 1965 experiment. Subjects used a variety of se lf-produced sounds, including tongue clicks, hisses, and lip-smacks, with each subject consistently using his own preferred pattern of sound. Self-generated preferred sounds (hisses or clicks) were found by Rice in 1967 to be as useful and significant as an artificial sound.

All subjects also oscillated their heads from side to side in what Kellogg (5) described as auditory scanning, which accentuates the intensity and arrival time of the returning echoes at both ears. In this way the blind could enhance binaural localization of sound, or the localization of a target from echoes reaching both ears.

Questions

Kellogg's study of distance discrimination illustrated how changes in target distance could be perceived by subjec ts as changes in target size. Although the variable being changed was target distance and not target size, subjects were asked to respond in terms of target size by reporting which target was perceived as smaller. Left unanswered by thisp ast research is how well subjects can discriminate differences in distance between objects of the same size when the subject's task is to perceive changes in target distance.

Echolocation may in fact be a tool for the blind to perceive, not just the presence of objects, but such dimensions of the objects as size and distance. Both the size and the distance of a target from the observer affects the perceived sound amplitude of the echo. For example, a larger target size reflects more echo energy and may contribute to a perceived louder echo by the subject (15). Other factors that might affect sound amplitude include reflective properties of a target's material, spreading loss as a function of target distance range, and atmospheric at tenuation (6).

Due to spherical spreading loss, a target placed farther away reflects to the observer an echo that arrives as a weaker sound that may be perceived as a smaller size target or as a farther target distance. By contrast, a target place d closer to the observer reflects a stronger sound that might be perceived as a larger size target or as a target placed closer in distance. According to Simmons (12), bats detect target range (by measuring time delay between emission of the sound pulse and return of the echo) and target size by amplitude of an echo. It remains to be seen whether humans, like bats, can utilize the cue of time delay during distance discrimination to separate changes in target size from changes in target distance.

In bats, echo sensation level is stable over a range of about 1.5 meters, in spite of the fact that echo strength decreases by about 12 dB for each halving of target distance. Contractions of the bat's middle ear muscles at the time of vocalization produ ce changes in hearing sensitivity that offset distance - dependent spherical spreading losses. This mechanism, called automatic gain control (AGC) by Kick and Simmons (7), allows the bat to perceive a fixed echo intensity level as it approaches a target. Contraction of the muscles gradually diminishes 5 to 8 msec after sound emission, thus attenuating returning echoes.

The function of AGC may include regulation of the amplitude of stimuli reaching the basilar membrane where vibrations are translat ed into neural discharges. The timing of discharges to echoes is used by bats to determine target range (14). According to Kick and Simmons (7) the stabilization of echo amplitude with respect to hearing threshold prevents large changes in neural respon se latency. As a bat flies closer to its target, the amplitude of echoes might increase 30 to 40 dB, which would stimulate the cochlea to shift the timing of nerve impulses even 1 msec, corresponding to a change of 17 cm in distance. Such a huge error i n target range perception is thought to be prevented by the regulation of echo amplitude through the automatic gain control.

The ability of a bat to maintain a constant sensation level of echo amplitude over a short distance to its target is a resul t of the ability of bats to remove changes in echo amplitude caused by changes in target range only. Therefore, other causes of variations in sound amplitude, such as wing flutter and target size might be rendered more salient to the bat. The bat would thus be able to attribute changes in echo amplitude to target structure rather than target distance (14).

The rationale for placing a smaller target (22.5 cm) at a closer comparison distance and a larger target (45 cm) at a farther comparison d istance is based on the assumption that the effects of size and distance cues on the echo amplitude might not be perceived separately by subjects. In fact, subjects found it easier to discriminate differences in distance between two same-size targets whe n the absolute distance of those targets was closer to them. Changing the size of one of the two targets disrupted the ability of blind subjects to utilize echolocation to discriminate differences in distance between the targets.

The inability of blind humans to use echolocation to discriminate differences in distance when target size changed stands in stark contrast to the ability of bats to isolate size changes from distance changes using echolocation. Inspired by bat echolocation research, thi s human echolocation study was undertaken in the hopes of acquiring a more intimate knowledge about human echolocation ability. Findings demonstrate the ability of blind humans to use echolocation to actively seek out objects in their vicinity and thus t o exert more control over perceiving the qualities of objects in their environment.

Christine Uy is a senior in Quincy House concentrating in psychology. Her thesis explored the ability of blind humans and a blindfolded sighted human to utilize information in echoes in order to perceive the difference in distance between targets that varied in size and in distance from each other.

Editor: Bridget Wagner, Tony Chen

References

1. Ammons, C., W orchel, P., Dallenbach, K. (1953). "Facial Vision": The perception of obstacles out of doors by blindfolded and blindfolded-deafened subjects. The American Journal of Psychology, 66: 519-553. (continued on p.. xx)

Echoloc ation (continued from p. xx)

2. Cotzin, M., Dallenbach, K. (1950). "Facial Vision: the role of pitch and loudness in the perception of obstacles by the blind. The American Journal of Psychology, 63: 485-515.< p> 3. Griffin, D. (1959). Echoes of bats and men. Garden City, NY: Anchor Books Doubleday & Co.

4. Kay, L. (1980). Animal Sonar Systems. Busnel, R., Fish, J., eds NY Plenum Press. 789-816.

5. Kellogg, W.N . (1962). Sonar system of the blind. Science, 137: 3528, 399-404.

6. Kick, S. (1982). Target Detection by the echolocating bat, Eptesicus fuscus. The Journal of Comparative Physiology, 145: 431-435.

7. Kick, S., Simmons, J. (1984). Automatic gain control in the bat's sonar receiver and the neuroethology of echolocation. The Journal of Neuroscience, 4: 2725-2737.

8. Rice, C.E., Feinstein, S.H. (1965). Sonar system of the blind: size discrimination. Science, 148: 21, 1107-1108.

9. Rice, C.E., Feinstein, S.H., Schuster-man, R.J. (1965). Echo detection ability of the blind: size and distance factors. Journal of Experimental Psychology, 7 0: 246-251.

10. Roverud, R.C., Grinnell, A.D. (1985). Discrimination performance and echolocation signal integration requirements for target detection and distance determination in the CF/FM bat, Noctilio albiventris. Journal of C omparative Physiology A, 156: 447-456.

11. Schnitzler, H.U., Henson, O.W. (1980). Performance of Airborne animal sonar systems: I. Microchiroptera. In: Animal Sonar Systems. Busnel, R.G., Fish, J.F. (eds). NY: Plenum Press. 109-181.

12. Simmons, J. (1973). The resolution of target range by echolocating bats. The Journal of the Acoustical Society of America, 54 (1): 157-173.

13. Simmons, J., Grinnell, A. (1988). The performance of echoloca tion: acoustic images perceived by echolocating bats. In: Animal Sonar: Processes and Performance. Nachtigall, P.E., Moore, P.W.B. (eds). NY: Plenum Press. 353-385.

14. Simmons, J., Moffat, A., Masters, W. (1992). Sonar gain control a nd echo detection thresholds in the echolocating bat, Eptesicus fuscus. Journal of the Acoustic Society of America, 91: 1150-1162

15. Simmons, J., Vernon, J. (1971). Echolocation: discrimination of targets by the bat Ept esicus fuscus. Journal of Experimental Zoology, 176: 315-328.

16. Stebbins, W. (1983). The Acoustic Sense of Animals. Cambridge, MA: Harvard University Press. 116-119.

17. Supa, M., Cotzin, M., Dallenbach, K. (1944). "Facial vision": the perception of obstacles by the blind. The American Journal of Psychology, 60: 502-553.

18. Worchel, P., Mauney, J., Andrew, J. (1950). The perception of obstacles by the blind. Journal of Expe rimental Psychology, 40: 746-751.