Coding and Processing in the Nervous Systems of FM-Signal-Producing Bats

Abstract

For echolocation, certain insectivorous bats emit frequency-modulated (FM) sounds in which frequency always sweeps from high to low. Neurons in the cochlear nerve and nucleus respond to various types of sounds, such as pure tones, FM sounds, and noise bursts, and are therefore not specialized for the analysis of the bat's orientation sounds. In the inferior colliculus and auditory cortex, however, there exist neurons which selectively respond to FM sounds. For the excitation of these FM-specialized neurons, the direction, range, and speed of frequency sweep are important factors. The FM-specialized neurons always have a large inhibitory area and respond to FM sounds sweeping across this area. Although seemingly paradoxical, such properties are easily explained by neural network models. Bats emit short FM sounds when certain parts of the brain are stimulated with electric pulses. The temporal pattern of emission differs according to the location of the stimulating electrodes. Many neurons in the lateral lemniscus and inferior colliculus are excited by the bat's own outgoing sounds, in spite of the contraction of the middle ear muscles, synchronized with sound emission. The lateral lemniscal neurons appear to be specialized for echo detection because of the facilitation of their responses to a second tone pulse (artificial echo), while the inferior collicular neurons appear to be specialized for echo ranging because they exhibit various types of recovery cycles, including temporary recovery. [Supported by grant GB-13904 and PIB-2103 from the N.S.F. and grant HSAA 53837 from the P.H.S.]