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Abstract:
Debate continues whether auditory nuclei represent sensory
information beyond frequency in topologic maps. Like orientation
maps on isoposition patches in the primary visual cortex V1,
systematic changes of response properties on isofrequency areas
have been found. However, it remains largely unclear how these maps
relate to features of sound, whether they are representative to
understanding neural function, and where to expect, in the
multi-level acoustic hierarchy, analogues to features in V1. We
model spatial neural activity patterns elicited by pure tones over
one cochlear partition by outputs of a gammatone filterbank and
show that their topology in a two-dimensional sheet incorporates
two mutually orthogonal, concentric and pinwheel-like, maps of
instantaneous intensities and phases transmitted by the
best-matching frequency channel. Both maps are consistent with
peaked rate-intensity functions, synchronization to amplitude
modulation, and distribution of absolute response thresholds in
isofrequency planes of the inferior colliculus of the auditory
midbrain. We claim that maps in the auditory midbrain reach a
complexity similar to visual maps in V1. Macroscopically,
processing up to either level decomposes instantaneous spatial
patterns over one cochlear partition or one segment in the visual
scene according to two mutually orthogonal spatial templates, and
has the fit-values mapped onto two mutually orthogonal anatomical
axes of isofrequency or isoposition areas. Orientation corresponds
to instantaneous phase, and isoposition patches in V1 are the sites
of concentric arrangements of the visual analogue of sound
intensity.
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