SOME COMMENTS ON THE PROPOSED PERCEPTION OF PHASE AND NANOSECOND TIMEDISPARITIES BY ECHOLOCATING BATS

In a series of recent reports, Simmons and his colleagues propose that bats are able to accurately encode the spectral, temporal and phase i nformation of their emitted calls and echoes. The information so encod ed is then extracted by the networks of the auditory system with speci alized processing. They propose that bats use this information to dete rmine the distance to their target by crosscorrelating the entire stru cture of the emitted call with the structure of the echo. The idea is that slight deviations in the correlation function can be detected by the bat and the degree of mismatch provides an accurate measure of tem poral disparity and hence range. The data in the reports purport to sh ow that bats perceive the phase of ultrasonic signals and that they ca n resolve temporal disparities of about 10 ns, and thus can distinguis h range differences as small as 2 mum. The hypothesis also attempts to explain how a variety of acoustic cues are processed and represented in the auditory system and how they are combined to form a unitary per cept of space and fine structure. The theory incorporates some time ho nored processes of extracting information, such as crosscorrelations. The implications of. the hypothesis, however, go far beyond a theory o f neural processing and representation of information by ensembles of cells. The hypothesis requires some remarkable abilities, such as the phase coding of ultrasonic signals and a temporal acuity on the order of 10 ns. These features have never been seen in any neurophysiologica l study of any animal nor has its existence been implied in behavioral studies of other animals. If bats, in fact, detect and process those signals in the manner proposed by Simmons and his colleagues, it would suggest that bats are supermammals whose auditory systems have evolve d new and extraordinary mechanisms not possessed by other animals. In view of the extraordinary implications of the hypothesis, it seems pru dent to critically evaluate the data upon which the hypothesis is base d. The purpose of this review is to point out a number of technical pr oblems and deficiencies in those experiments which undermine the verac ity of the purported demonstration of phase perception and nanosecond time resolution by bats.