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Hoffmann, Susanne (2009): The auditory cortex of the bat Phyllostomus discolor: Functional organization and processing of complex stimuli. Dissertation, LMU München: Faculty of Biology
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Abstract

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.