INTRACEREBRAL POTENTIALS TO RARE TARGET AND DISTRACTER AUDITORY AND VISUAL-STIMULI .2. MEDIAL, LATERAL AND POSTERIOR TEMPORAL-LOBE

Event-related potentials were recorded from 1221 sites in the medial, lateral and posterior aspects of the temporal lobe in 39 patients. Dep th electrodes were implanted for about 4 days in order to localize sei zure origin prior to surgical treatment. Subjects received an auditory discrimination task with target and non-target rare stimuli. In some cases, the target, distracting and frequent tones were completely bala nced across blocks for pitch and volume. Some subjects also received a n analogous visual discrimination task, or auditory tasks in which the rare target event was the omission of a tone, or the repetition of a tone within a series of alternating tones. In some subjects, the same auditory stimuli were delivered but the patient ignored them while rea ding. A complex field was recorded, indicating multiple components wit h overlapping time-courses, task correlates and generators. Two genera l patterns could be distinguished on the basis of their waveforms, lat encies and task correlates. In the temporal pole and some middle tempo ral, posterior parahippocampal and fusiform gyrus sites, a sharp triph asic negative-positive-negative waveform with peaks at about 220-320-4 20 msec was usually observed. This wave was of relatively small amplit ude and diffuse, and seldom inverted in polarity. It was multimodal bu t most prominent to auditory stimuli, appeared to remain when the stim uli were ignored, and was not apparent to repeated words and faces. A second broad, often monophasic, waveform peaking at about 380 msec was generated Ln the hippocampus, a limited region of the superior tempor al sulcus, and (by inference) in the anterobasal temporal lobe (possib le rhinal cortex). This waveform was of large amplitude, often highly focal, and could invert over short distances. It was equal to visual a nd auditory stimuli, was greatly diminished when the stimuli were igno red, and was also evoked by repeating words and faces. Preceding this waveform was a non-modality-specific negativity, possibly generated in rhinal cortex, and a visual-specific negativity in inferotemporal cor tex. The early triphasic pattern may embody a diffuse non-specific ori enting response that is also reflected in the scalp P3a. The late mono phasic pattern may embody the cognitive closure that is also reflected in the scalp P3b or late positive component.