FM signals produce a robust paradoxical latency shift that is important for coding of target range in the bat inferior colliculus

Abstract

Echolocating bats utilize the time delay between an outgoing ultrasonic pulse and its echo to determine target range. Many neurons at the inferior colliculus (IC) and above are tuned to time delays between these sound pulses of unequal amplitudes. Sullivan previously proposed that paradoxical latency shift (PLS), characterized by a quantal increase in firing latency to loud sounds, is important for this attribute because PLS permits coincidence detection that is important for the creation of delay-tuned responses. In the IC of little brown bats, Galazyuk and Feng recently reported that, in response to tone pulses, the proportion of neurons showing PLS was low (<20%). This study was undertaken to determine whether PLS is a function of the acoustic stimulus. For this, the temporal discharge patterns of single IC neurons were investigated over a broad range of sound levels, using tone pulses at CF as well as FM sound pulses that mimicked bats’ ultrasonic cry as stimuli. For many IC neurons, tone pulses did not elicit PLS but FM sound pulses produced robust PLS. This result showed that PLS is stimulus dependent and that the bat’s auditory system is optimal for processing FM sounds employed during echolocation. [Work supported by NIH R01DC04998.]