The auditory cortex of the bat Phyllostomus discolor: Localization and organization of basic response properties

Background: The mammalian auditory cortex can be subdivided into
various fields characterized by neurophysiological and
neuroarchitectural properties and by connections with different
nuclei of the thalamus. Besides the primary auditory cortex,
echolocating bats have cortical fields for the processing of
temporal and spectral features of the echolocation pulses. This
paper reports on location, neuroarchitecture and basic functional
organization of the auditory cortex of the microchiropteran bat
Phyllostomus discolor (family: Phyllostomidae). Results: The
auditory cortical area of P. discolor is located at
parieto-temporal portions of the neocortex. It covers a
rostro-caudal range of about 4800 mu m and a medio-lateral distance
of about 7000 mu m on the flattened cortical surface. The auditory
cortices of ten adult P. discolor were electrophysiologically
mapped in detail. Responses of 849 units (single neurons and
neuronal clusters up to three neurons) to pure tone stimulation
were recorded extracellularly. Cortical units were characterized
and classified depending on their response properties such as best
frequency, auditory threshold, first spike latency, response
duration, width and shape of the frequency response area and
binaural interactions. Based on neurophysiological and
neuroanatomical criteria, the auditory cortex of P. discolor could
be subdivided into anterior and posterior ventral fields and
anterior and posterior dorsal fields. The representation of
response properties within the different auditory cortical fields
was analyzed in detail. The two ventral fields were distinguished
by their tonotopic organization with opposing frequency gradients.
The dorsal cortical fields were not tonotopically organized but
contained neurons that were responsive to high frequencies only.
Conclusion: The auditory cortex of P. discolor resembles the
auditory cortex of other phyllostomid bats in size and basic
functional organization. The tonotopically organized posterior
ventral field might represent the primary auditory cortex and the
tonotopically organized anterior ventral field seems to be similar
to the anterior auditory field of other mammals. As most energy of
the echolocation pulse of P. discolor is contained in the
high-frequency range, the non-tonotopically organized
high-frequency dorsal region seems to be particularly important for
echolocation.