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Abstract:
In mammals, there are two pathways, a lemniscal and an
extralemniscal, along which auditory information ascends to the
auditory and frontal cortices. The lemniscal circuitry is
well-suited for sound localization. In the mustached bat, one
branch of the extralemniscal pathway, also known as the "central
acoustic tract" (CAT), ascends via the external nucleus of the
inferior colliculus (ICX). The majority of neurons in the ICX
prefer upward frequency modulations (FMs) at relatively slow rates
(Gordon, 98). Upward FMs are common components of social calls
produced by this species (Kanwal et al, 94) in contrast to
exceptionally fast downward FMs present in echolocation pulses.
These reports lead us to suggest that the CAT may selectively
process FM components (including rapid formant-transitions in
speech sounds) for auditory communication. FMs are well represented
in the frontal and auditory cortical areas in this species (Kanwal,
99). Furthermore, since social calls in bats and rapid
formant-transitions in speech sounds are processed selectively in
the left hemisphere (Kanwal and Suga, 95; Belin et al., 98), this
tract may be asymmetrical at least in bats and humans. We
hypothesize therefore that there are two functionally distinct
subcortical streams for processing auditory information analogous
to those processing visual information in the primate visual cortex
(van Essen and Maunsell, 83); one to primarily localize a
sound/echo source and another to identify certain spectro-temporal
features (e.g., FMs) in communication sounds.
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