In contrast, patients with temporal lobe excisions that encroached upon the gyrus of Heschl in the right hemisphere (but not in the left) showed significantly elevated thresholds when judging the direction of pitch change. Mean thresholds in the pitch discrimination task did not differ between patient groups and control subjects. Thresholds were determined by measuring the minimum pitch difference required for correct task performance. In a task requiring the judgement of direction of pitch change, subjects decided whether pitch rose or fell from the first tone to the second. In a pitch discrimination task, the subject had to decide whether two elements of a pure tone pair were the same or different. Subjects were asked to perform two different perceptual tasks on the same set of stimuli. In a subset of the patients, the lesion encroached significantly upon the gyrus of Heschl or its underlying white matter as determined from MRI analysis. In the present study, adaptive psychophysical procedures were used to determine auditory perceptual thresholds in 14 neurologically normal subjects, and in 31 patients who had undergone surgical resection from either the right or left temporal lobe for the relief of intractable epilepsy. Previous lesion and functional imaging studies in humans suggest a greater involvement of right rather than left auditory cortical areas in certain aspects of pitch processing.
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