Neural mechanisms for call processing in the auditory cortex of mustached bats : frequency modulated sounds and their lateralization
Washington, Stuart Dante.
Thesis (Ph.D.)--Georgetown University, 2008.; Includes bibliographical references. Speech processing is lateralized to the left hemisphere of the human brain, with some variation between sexes. Single unit electrophysiological recordings in the Doppler-shifted constant frequency processing (DSCF) sub-region of the mustached bat primary auditory cortex (A1) has revealed a left hemispheric advantage for processing species-specific (or conspecific) calls that at least superficially resembles the hemispheric specialization observed in humans. The hemispheric specialization for speech in humans has been related to an advantage of the left auditory cortex for processing information with a high temporal resolution, and, thus, the discovery of a similar mechanism in mustached bats would further demonstrate the similarity between lateralization for communication sounds in humans and bats. The first half of the research described herein is focused on the processing of frequency modulations (FMs) within the DSCF area, which comprises 30-50% of A1 in the bat. The second half is focused on observed hemispheric differences in the FM selectivity of DSCF neurons, specifically the peak response latency and magnitude elicited by FMs of varying slope and bandwidth. Results presented here demonstrate that not only are DSCF neurons responsive to FMs but also the left DSCF area responds more quickly and robustly to steep, broad-band FMs than do right DSCF neurons. Processing of relatively steep FMs requires finer temporal resolution than processing of relatively shallow FMs. Further, on average, hemispheric differences appear to be greater in male as opposed to female bats, reflecting a pattern previously observed in humans. Since the conspecific calls of bats are largely composed of FMs that widely vary in instantaneous modulation slope, this left hemispheric advantage for processing steep FMs can be considered a mechanism for the previously observed left hemispheric advantage for processing calls.
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