The Journal of Neuroscience, March 15, 2000, 20(6):2400-2408
Graduate Group in Neuroscience and Department of Psychology,
University of Pennsylvania, Philadelphia, Pennsylvania 19104
The fish auditory system provides important insights into the
evolution and mechanisms of vertebrate hearing. Fish have relatively simple auditory systems, without a cochlea for mechanical frequency analysis. However, as in all vertebrates, the primary auditory afferents of fish represent sounds as stimulus-entrained spike trains.
Thus, fish provide important models for studying how temporal spiking
patterns are used in higher level neural computations. In this paper we
demonstrate that one of the fundamental transformations of information
in the auditory system of a sound-producing fish, Pollimyrus, takes place in the auditory medulla. We
discovered a class of neurons in which evoked spiking patterns were
relatively independent of the stimulus fine structure and appeared to
reflect intrinsic properties of the neurons. These neurons generated
sustained responses but were poorly phase-locked to tones compared with the primary afferents. The interval histograms showed that spike timing
was regular. However, in contrast to primary afferents, the mode of the
interspike interval distribution was independent of the period of tonal
stimuli. The tuning of the neurons was broad, with best sensitivity in
the same spectral region where these animals concentrate energy in
their communication sounds. The physiology of these neurons was similar
to that of the chopper neurons known in the auditory brainstem of
mammals. Our findings suggest that this medullary transformation, from
phase-locked afferent input to chopper-like physiology, is basic to
vertebrate auditory processing, even within lineages that have not
evolved a cochlea.
Key words: auditory communication; chopper; computation; electric fish; hearing; temporal processing; neural transformation