UC Riverside

Adaptations that allow for greater discrimination of low intensity sounds may be important in the echolocation behavior of bats. In this dissertation, we used the pallid bat, Antrozous pallidus, a gleaning bat of the western United States, as a model to study the adaptations for intensity selectivity present in the auditory cortex. We performed in-vivo extracellular recordings in the auditory cortex of the pallid bat, to determine the cortical organization and mechanisms of intensity selectivity. Downward frequency modulated (DFM) sweeps that approximated echolocation calls were used to study intensity selectivity using a behaviorally relevant sound. We also examined the distribution of Parvalbumin (PV) and Calbindin (CB) expressing cells in cortical regions and thalamic nuclei in the auditory pathway. Immunohistochemical staining was used to determine if the distribution of these calcium binding proteins in cortical and thalamic regions that have been implicated in processing of echolocation calls and how that distribution is different from non-echolocating regions. Lastly we examined the thalamocortical projections to intensity selective neurons in the echolocation region of the auditory cortex. Retrograde tracing from intensity selective and non-selective neurons was used to identify the thalamic nuclei that provided input to those neurons. We show that the region of the auditory cortex that is selective for echolocation calls contains a majority of neurons that are highly selective for low intensity sounds. We discovered that in the pallid bat intensity selectivity is enhanced by using behaviorally relevant stimuli and that high-frequency inhibition in the bat's echolocation call is responsible for this increased selectivity. This suggested a spectrotemporal integration mechanism that can shape intensity selectivity. Cortical mapping, in this study, revealed a systematic organization of intensity selectivity measures. We also discovered a differential staining pattern of calcium binding protein in the cortex as higher percentage of PV+ cells compared to CB+ cells was found in both echolocation call- and non-call regions. CB+ neurons where found in all of the regions of the medial geniculate body of the auditory thalamus, while PV staining was limited to the suprageniculate (SG), a region known to project to echolocation selective regions of the cortex. This study also confirmed previous results showing that echolocation selective regions of the cortex receive projections from all nuclei of the MGB except the lemniscal ventral nucleus, however an organization related to intensity selectivity was not able to be determined in the projections. The results from this dissertation highlight the special adaptations that are present in the auditory cortex of the pallid bat that may be important for processing the low-intensity echolocation call. Deviations from a general mammalian plan or even from other bat species in the properties that lead to these adaptations may strengthen the notion that general patterns of cortical processing and organization may be altered through evolution to support the unique behavioral needs of a species.