Evidence for perception of fine echo delay and phase by the FM bat,Eptesicus fuscus

The big brown bat, Eptesicus fuscus, can perceive small changes in the delay of FM sonar echoes and shifts in echo phase, which interact with delay. Using spectral cues caused by interference, Eptesicus also can perceive the individual delays of two overlapping FM echoes at small delay separations. These results have been criticized as due to spectral artifacts caused by overlap between stimulus echoes and extraneous sounds (Pollak 1993). However, no amplitude or spectral variations larger than 0.05 dB accompany delay or phase changes produced by the electronic apparatus. No reverberation falls in the narrow span of delays required to produce the bat's performance curve from echo interference cues. Consistent differences in the durations of sonar sounds for 6 bats that perform the same in the experiments demonstrate that overlap between stimulus echoes and extraneous echoes is not necessary, and changes in the amount of echo overlap have no effect on performance. Noise-induced random variations in echo spectra outweigh putative spectral artifacts, and deliberately-introduced spectral artifacts do not improve performance overall but instead yield new time-frequency images. Amplitude-latency trading of perceived delay, proposed as a demonstration that the latency of neural discharges encodes delay (Pollak et al. 1977), confirms that the bat's fine delay and phase perception depends on a temporal neural code. The perceived delays depend on stimulus delays, not the delays of extraneous sounds. The rejected criticisms are based on physiological results with random-phase FM stimuli which are irrelevant to neural coding of fine echo delay and phase.The contents of this paper first appeared in October 1990 in a letter to G.D. Pollak in response to his unpublished criticisms of echo-jitter experiments. These responses also have been presented at the 1991 and 1992 Association for Research in Otolaryngology midwinter meetings and at the 1992 3rd International Congress of Neuroethology. Several of the control experiments also appeared in Simmons et al. (1990b). The now-published criticisms (Pollak 1993, the preceding paper) have not addressed these responses, including the prior published data demonstrating that the stimulus conditions asserted by these criticisms do not in fact occur.