Object-centered representations in echolocating dolphins: evidence from acoustic analyses of object echoes and a human listening study

Date
2003-12
Authors
DeLong, Caroline M.
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University of Hawaii at Manoa
Abstract
Although the echolocation capabilities of bottlenose dolphins have been examined, little is known about how echolocating dolphins represent the information they use to recognize objects. When a dolphin echolocates a fish, it may represent only the sound of the fish's echoes. In contrast, the dolphin may represent the characteristics of the fish (e.g., size, shape) as well as the sound elements that convey its characteristics. The purpose of this project was to evaluate these representation theories by measuring a dolphin's ability to discriminate among objects, and then characterizing the acoustic information contained in the object echoes. A dolphin performed a match-to-sample task in which it was presented with a sample object and then had to choose the same object from a group of three objects. Acoustic measurements of the objects were made by ensonifying the objects with dolphin-like clicks and capturing the returning echoes. Acoustic features were extracted from the echoes: target strength, number of highlights, duration, peak frequency, center frequency, and bandwidth. To determine the role played by acoustic features in the dolphin's representations, an analysis of the dolphin's errors on the matching task was combined with information on the variation in acoustic features between objects. The dolphin's errors could not be fully explained by similarities in single acoustic features, linear combinations of these features, or correlations of echo spectra. These results suggest that dolphins use acoustic information in a complex way to determine object characteristics. To further investigate the nature of dolphins' representations, human participants were presented with slowed-down echoes from the same objects used in the dolphin matching task. The participants were asked to discriminate between the objects and report relevant discrimination cues. The participants' reports and an analysis of their errors indicated that the participants represented the sounds of the echoes. The error patterns of the humans did not always consistently match the error patterns of the dolphin, which suggests that the dolphin represented the objects differently than the humans. This project's results support the idea that dolphins directly represent object characteristics, not merely the sound elements that convey those characteristics.
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xi, 167 leaves
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