The auditory cortex of the bat Phyllostomus discolor

The auditory cortex is the acoustically responsive part of the
neocortex and represents the highest level of processing of the
ascending auditory pathway. The experiments described in this
thesis were designed to study the auditory cortex of the
microchiropteran bat Phyllostomus discolor with both, simple and
complex acoustic stimuli. During the experiments, different methods
were used (e.g. psychophysics and neuroanatomy), but the main focus
was laid on the electrophysiological examination of the auditory
cortex. The first chapter covers a study that investigated the
hearing range of P. discolor by measuring neural and behavioral
audiograms in this species. This study shows that acoustic stimuli
at frequencies between 4 and 100 kHz could elicit either a neuronal
or behavioral response in P. discolor. Lowest thresholds were found
in the high frequency range above 35 kHz indicating the high
sensitivity of the auditory system of P. discolor to ultrasonic
sounds as for example contained in echolocation calls. However,
electrophysiologically and psychophysically determined hearing
thresholds lay in the range of thresholds known for other bat
species. The second chapter describes a study that determined the
location, extend, and subdivision of the auditory cortex of P.
discolor. The area that contained acoustically responsive neurons
was laterally positioned at the caudal part of the neocortex.
Within this area four major cortical subfields could be
distinguished based on neuroanatomical and neurophysiological
criteria. The two ventral fields were tonotopically organized and
were assumed to belong to the “core” region of the auditory cortex.
The posterior ventral field showed properties similar to that found
in the primary auditory cortex of other mammals, whereas, the
anterior ventral field seems to resemble the anterior auditory
field of the mammalian auditory cortex. The two dorsally located
subfields did not show a clear tonotopy, but contained neurons,
which were mainly responsive to high frequencies above 45 kHz. As
the dominant harmonics of the echolocation call of P. discolor
cover this high frequency range, the anterior and posterior dorsal
fields seem to be strongly involved in processing of information
obtained from echolocation. The third and fourth chapter describes
experiments that investigated the cortical processing of sound
parameters relevant for echolocation: echo roughness and acoustic
motion. Echo roughness as a measure for the temporal envelope
fluctuation of a signal is especially important for the
discrimination of complex targets like trees and bushes. Broad
leaved trees produce echoes with a higher degree of roughness
compared to small leafed trees, e.g. conifers. The
neurophysiological experiment described in chapter three revealed a
population of cortical neurons in the anterior part of the auditory
cortex, which encoded echo roughness in their response rate. The
response of these neurons could be correlated to the behaviorally
measured discrimination performance of P. discolor. In the
experiment described in chapter four, pairs of pure tones were used
to simulate either echoes from an object moving in azimuth or
echoes from a stationary object encountered by a bat during
approach. In the posterior dorsal field of the auditory cortex of
P. discolor a population of motion sensitive neurons was found,
which showed strong response facilitation to dynamic stimuli in
contrast to static stimulation. In a subset of motion sensitive
neurons the dynamic azimuthal response range was focused to small
areas in the frontal field at short temporal intervals between the
two components of the dynamic stimuli. The response of these
neurons might be important for the tracking of targets during an
approach by the bat. The results presented in this thesis reveal
that the auditory cortex of P. discolor is functionally parcellated
into at least four different fields. This parcellation seems to
reflect the segregated processing of behaviorally and ecologically
important echo parameters within specialized areas of the auditory
cortex.