Echolocation versus echo suppression in humans

Abstract

Several studies have shown that blind humans can gather spatial information through echolocation. However, when localizing sound sources, the precedence effect suppresses spatial information of echoes, and thereby conflicts with effective echolocation. This study investigates the interaction of echolocation and echo suppression in terms of discrimination suppression in virtual acoustic space. In the 'Listening' experiment, sighted subjects discriminated between positions of a single sound source, the leading or the lagging of two sources, respectively. In the 'Echolocation' experiment, the sources were replaced by reflectors. Here, the same subjects evaluated echoes generated in real time from self-produced vocalizations and thereby discriminated between positions of a single reflector, the leading or the lagging of two reflectors, respectively. Two key results were observed. First, sighted subjects can learn to discriminate positions of reflective surfaces echo-acoustically with accuracy comparable to sound source discrimination. Second, in the Listening experiment, the presence of the leading source affected discrimination of lagging sources much more than vice versa. In the Echolocation experiment, however, the presence of both the lead and the lag strongly affected discrimination. These data show that the classically described asymmetry in the perception of leading and lagging sounds is strongly diminished in an echolocation task. Additional control experiments showed that the effect is owing to both the direct sound of the vocalization that precedes the echoes and owing to the fact that the subjects actively vocalize in the echolocation task.

Keywords: auditory; binaural hearing; reverberation; temporal processing.

Figure 1.

Illustration of the Echolocation experiment. In a 2AIFC paradigm, subjects were asked to discriminate between azimuthal positions of a single sound reflector (a), the leading of two reflectors (b), and the lagging of two reflectors (c), respectively. The same paradigm with sound sources instead of sound reflectors was used for the Listening experiment. The temporal delay between leading and lagging sound was 2 ms at the subjects’ ears, which corresponds to a simulated physical distance of Δd = 34 cm in the Echolocation experiment and Δd = 68 cm in the Listening experiment.

Figure 2.

Results of the azimuth discrimination experiments for all six subjects. (a,d,g) MAAs for sound source discrimination, (b,e,h) MAAs for sound reflector discrimination, and (c,f,i) MAAs for the control experiment without and with DS. The rows represent the three conditions, single object, lead and lag discrimination, respectively.

Figure 3.

Normalized results in terms of DDFs. (a,b,c) Lead-discrimination performance was significantly better in the Listening experiment and in the control experiment without DS than in the Echolocation experiment and the control experiment with DS, (d,e,f) while lag-discrimination performance was significantly better in the Echolocation experiment than in all other experiments. (g,h,i) The asymmetry between lead and lag discriminations as the defining factor for the precedence effect was significantly weaker in the Echolocation experiment than in all other experiments.