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range of acoustic stimuli, including speech sounds. Tones of different frequency induce localized changes in blood flow within subregions of HG, indicating a tonotopic organization, with higher frequency tones activating the more posterior-medial portion of HG. More complex tasks have been used in conjunction with fMRI to search for evidence of regional functional specialization within auditory cortex. Results from these investigations indicate that the right temporal lobe may play a more important role in the processing of musical stimuli (e.g., pitch and rhythmic temporal patterns) than the left. There have also been reports suggesting functional specialization of right-sided temporal (and parietal) lobe regions in processing information concerning the movement of sound sources. Both fMRI and PET methods have been used extensively to map patterns of cortical activation during speech sound processing.

Inherent limitations of these methods have prevented direct extrapolation ofdata obtained with these methods to results obtained in nonhuman primates using microelectrode recording techniques. In the future, however, the spatial resolution of fMRI may improve to the extent that the functional organization of human auditory cortex can be studied in far greater detail. Because activity-induced changes in blood flow patterns occur over a period of seconds, it is unlikely that fMRI techniques alone will be capable of delineating the fine temporal patterns of activity that characterize coding and processing of information at the level of auditory cortex.

c. Direct Recording (ECoG) and Stimulation of Auditory Structures in Human Direct electrical-stimulation mapping of the cortex during surgery to relieve medically intractable epilepsy or to remove a tumor is commonly carried out as a way of guiding the surgeon's decision on the location and extent of brain tissue to excise. When the primary auditory field on HG is stimulated, patients report hearing sounds, which are often referred to the ear contralateral to the stimulated cortex. Stimulation of the belt of cortex surrounding the primary field may result in the perception of more complex sounds, although it is now thought that some of this—especially the so-called experiential hallucinations—may be the result of spread of stimulus current to underlying limbic structures. In the caudal region of the superior temporal gyrus (Wernicke's area), and on the angular and supramarginal gyri electrical stimulation may result in the arrest of speech, which is similar to the results of stimulation of the classic Broca's area on the inferior frontal gyrus.

Direct recording from the human auditory cortex has also been carried out in neurosurgical patients both acutely in the operating room and chronically under more controlled experimental conditions. This recording is referred to as the ECoG to differentiate it from the scalp recorded EEG. Results from these experiments show acoustically evoked activity on the superior temporal plane and the lateral surface of the superior temporal gyrus. A field on HG is distinguished from a field on superior temporal cortex based on the HG tonotopic map and the properties of acoustically evoked potentials. Thus, cytoarchitec-tonic, electrophysiologic, and functional imaging data leave little doubt that the cortex on mesial HG is the primary auditory field of the human and the homolog of AI in nonhuman primates and other mammals. The cortex of the lateral surface of the superior temporal gyrus represents one or more separate auditory fields. Intraoperative electrophysiological studies show that single neurons in this cortex respond vigorously to complex sound, including speech—a finding similar to that in the rhesus monkey.

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