18F-2-deoxyglucose with positron emission tomography (PET) is used to study brain activation during performance of a speech perception task. Temporal order perception is tested when two speech sounds are presented in two different orders at different inter-stimulus onset intervals. Indices of change in right-left asymmetry between activated and non-activated scans were computed for 20 cortical regions. Preliminary findings indicate that correlation coefficients of the relationship between the amount of change in the asymmetry index with activation and performance on the speech perception task, were greater than 0.80 for the left anterior temporal, posterior temporal, temporo-parietal and posterior- parietal regions. Additional normal subjects will be tested to determine whether relationships continue to be found between increases in brain metabolism in the left temporo-parietal region and speech perception performance. Studies employing cortical magnetic stimulation are planned to determine whether brain localization for language and speech can be determined through greater interference with language task performance during left than during right hemisphere stimulation.