Middle latency auditory evoked potential (MAEPs, latencies 10-70 msec) provide a direct measure of the electrical activity associated with the short-latency thalamic and cortical analysis of sounds in humans. However, despite increasing clinical use little is known about their most fundamental properties. For example, the cortical and subcortical structures involved in MAEP generation remain in dispute, and it is uncertain how they are organized: do they have tonotopic and/or spatiotopic organizations like auditory structures in many species? We will address these basic issues in five experiments. Two new techniques will be incorporated in all studies. First, we will record detailed scalp topographies from 32 electrodes and extract Laplacean MAEPs (LAP-MAEPs) over temporal sites. LAP-MAEPs enhance the resolution of MAEPs to focal generators in auditory cortex. Second, we will correlate MAEP distributions with the relative orientations of candidate MAEP generators measured directly from high-resolution MRI scans. We will use these improved techniques to study the organization of MAEP generators for processing the intensity (Experiment I), frequency (Experiment II), and spatial position (Experiment III) of auditory signals. Hypotheses about MAEP generators will be further tested in two experiments that take advantage of our well-delineated clinical populations. In Experiment IV, we will record MAEPs in patient groups with focal lesions of candidate MAEP generators, including the superior temporal plane and auditory cortex, dorsolateral frontal cortex, inferior parietal lobe, the posterior thalamus and basal ganglia. In Experiment V, we will compare MAEPs from the scalp and cortical surface in patient undergoing craniotomies. The experiments in normal subjects and patients form a comprehensive sequence that will provide a better understanding of the shortlatency analysis of sounds in the human brain, and enhance the clinical utility of MAEPs in neurological and neurosurgical applications.
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