The long-term objective of the proposed research is to determine how populations of cortical neurons code for sound locations. The auditory cortex is involved in sound localization behavior. There is no evidence for a systematic map of auditory space in the cortex. Moreover, cortical neurons exhibit a wide range of spatial tuning leading to suggestions that sound location is represented by population codes. The nature of these codes remains elusive. This issue is clinically relevant as humans with auditory cortex lesions show severe sound localization deficits. Relating lesions and behavioral deficits has been difficult because how cortex represents space is not clear. The proposed research will determine the distribution of cortical activity for different sound locations. The studies will be carried out in the pallid bat. The main advantage of this model system is that its auditory cortex contains at least two systematic maps of sensitivity to interaural intensity difference (IID). These maps allow specific and testable predictions to be made about how activity spreads across cortex for different sound locations. Another advantage is that the pallid bat listens passively to localize prey-generated noise (as opposed to echolocate prey), and that the IID maps are found in cortical regions involved in passive hearing. Thus the pallid bat is a passive hearing model likely to provide fundamental insight into how mammalian, including human, cortex represents space. There are two specific aims in this study.
The first aim i s to characterize the organization of IID sensitivity in a cluster of neurons that appear to be selective for the frontal sound field. These neurons respond strongly to binaural (but not monaural) stimulation with a preferred response to IIDs generated from the frontal sound field. Preliminary data show systematic changes in the preferred IID within this cluster. Using dichotic stimulation and single-unit electrophysiology, the map of IID sensitivity within this cluster will be determined. A previous study on the pallid bat cortex showed that a second cluster with binaurally inhibited neurons also exhibits a systematic map of IID sensitivity. The second specific aim will address how the two clusters with known IID sensitivity maps represent space. For this purpose, a multi-electrode array system will be used in conjunction with sequential dichotic and free-field acoustic stimulation. Dichotic stimulation will be used to measure IID selectivity of multiple cortical sites. These measurements will be used to predict how activity spreads across the cortex for azimuth locations that result in known IID values. The free-field study will measure the actual spread of activity for different azimuth locations. A comparison of the predicted and actual distribution of activity will be used to develop hypotheses about population codes for sound localization.
Lesions in the auditory cortex of humans and other mammals cause deficits in sound localization behavior, but how cortex represents auditory space is not clear. The proposed studies will determine how the cortex codes for sound location using a relatively recent technological advance that allows simultaneous recording from multiple cortical sites. These studies will provide valuable information for clinical studies relevant to restoration of hearing following brain damage.
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