Echolocation has {} been observed in terrestrial mammals, such as rodents, insectivores, Megachiroptera, and in nocturnal cave-dwelling oil birds and cave swiftlets (11). In the past fifty years, research has re vealed that the auditory system is a major tool of perception employed by blind humans.
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.
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