Recognition of aspect-dependent geometric solids by an echolocating Atlantic bottlenosed dolphin

Abstract

This investigation examined the ability of a bottlenosed dolphin (Tursiops truncatus) to recognize aspect-dependent geometric solids using echolocation. Aspect-dependent objects generate different echoes depending on their orientation. Previous studies suggested that dolphins cannot automatically infer object structure from observations of only one orientation of the object, which suggests that dolphins might recognize objects based on the acoustic structure of their echoes rather than the spatial structure of the objects. Such representations are inconsistent, however, with object constancy, because each aspect generates a different echo structure. In contrast, the formation of an aspect-independent cognitive representation implies the ability to recognize an object from multiple viewpoints. The present data support the hypothesis that dolphins form aspect-independent representations. The dolphin recognized the targets even though orientation was free to vary. Variability in the time separations between intra-echo amplitude peaks (inter-highlight intervals) provided evidence that target orientation varied within and between trials. Although no significant differences in the center frequency and RMS bandwidth were observed among the target echoes, differences in the distributions of inter-highlight intervals may have acted as a cue for discrimination. The dolphin's performance was compared with the performance of an Integrator Gateway Network (IGN) neural network and a Linear Discriminant Analysis (LOA). The IGN correctly classified all three targets using the echo spectra, and the LOA correctly classified all three targets using only a combination of amplitude, center frequency and RMS bandwidth of echo spectra. Thus, the dolphin could have used information contained in the echo spectra as the basis of his ability to recognize the geometric targets. The dolphin could not rely on any single acoustic dimension as the basis of his ability to identify the targets because the acoustic properties of the echoes varied as target orientation changed. The ability to match sample and comparison stimuli that may not have been at identical orientations suggests that the dolphin formed object-centered representations of the stimuli. The results suggest that dolphins can form stable representations of objects regardless of orientation, and suggests that dolphins can use varying sensory properties to recognize constant objects.