STROBOSCOPIC HEARING AS A MECHANISM FOR PREY DISCRIMINATION IN FREQUENCY-MODULATED BATS

A hypothesis was proposed that bats employing frequency-modulated (FM) echolocation pulses could utilize dynamic information of a flying ins ect to discriminate prey on the basis of ''stroboscopic hearing.'' To test this hypothesis, single unit recordings were made from the inferi or colliculus (IC) of the little brown bat, Myotis lucifugus. Response characteristics of IC units to trains of modulated and unmodulated so und pulses were analyzed at various pulse repetition rates that corres ponded to a bat's pulse emission rates during the different stages of its target directed flight. The results show that amplitude modulation (AM) across a train of sound pulses was faithfully encoded in the uni ts' discharge pattern when the pulse repetition rate was different fro m the AM frequency. When the AM frequency was integer multiples of the pulse repetition rate, the stimulus amplitude was reduced drastically under these conditions. Consequently, the discharge of an IC unit dim inished precipitously, or if there was a phase delay between the pulse onset and the modulating sinusoidal waveform the unit fired to each m odulation cycle with more or less the same vigor as if the modulation was absent. These data indicate that the across-pulse amplitude modula tion becomes undetectable when the AM frequency is integer multiples o f the pulse repetition rate. It is interpreted that FM bats can potent ially employ a ''stroboscopic hearing'' strategy for discriminating in sects on the basis of the wing-beat frequency of the prey.