Perception of structured phantom targets in the echolocating bat, M e g a d e r m a l y r a

Bats can discriminate among objects with different texture using echolocation. The mechanisms involved in texture discrimination are unknown. In real echoes reflected from three-dimensional objects, the spatial structure of the target creates a specific temporal and spectral pattern. In the experiments described here, simply structured phantom targets mimicking echoes reflected from an object with two parallel planes were generated by adding two differentially delayed copies of the bat's vocalizations. The discrimination performance of Megaderma lyra was studied in a two-alternative, forced-choice experiment using phantom targets with different internal delays and relative amplitudes of the two copies. When the reference target to which the bats were trained was presented, targets differing in internal delay by about 1 microseconds were discriminated. However, discrimination performance was not a globally monotonic function of the internal delay of the unrewarded target. When both targets presented in a trial differed from the reference, the bats still preferred one of them. Changes in overall level of the phantom target echo had little effect on performance; however, performance dropped considerably if only one of the two copies in an echo was attenuated. A model framework is introduced to account for these results. Models based on frequency processing yield better approximations than does a model based on time differences. The model based on spectral correlation gives a unified description of all the data. Although time domain models for texture discrimination cannot be generally refuted, the results presented here show that the discrimination of target surface structure can be explained as a discrimination of echo spectra.