Objective: The shape of visually elicited event-related potentials (ERP) of epileptic patients during their presurgical evaluation with intracerebral electrodes was investigated in the study.
Methods: Twenty intractable epileptic patients with depth electrodes at several intracranial locations in the frontal, temporal, parietal lobes, and in the amygdalo-hippocampal complex participated in the study. To evoke the ERP, a standard visual oddball task was used with target stimuli, and frequent non-habituated and habituated stimuli. The averaged responses of the 3 groups were superimposed and visually analyzed whether the shape appeared identical or non-identical.
Results: The EEG response to target and frequent stimuli was recorded in 660 intra-cerebral sites. In 88 sites (14 different patients) localized in the amygdala, parahippocampal gyrus, superior, middle, and inferior temporal gyri, fusiform and lingual gyri, sensorimotor cortex, prefrontal cortex, hippocampus, and cingulated gyrus, the identical ERPs to target and both groups of frequent stimuli were observed. In 442 sites located in the above listed structures, and in the basal ganglia and parietal cortex, the shape of the ERP differed from 0.3 to 0.47 s on after the stimulus. The remaining 130 sites did not yield the task-specific potential change.
Conclusions: The existence of identical ERPs to target and frequent stimuli in the oddball task suggests that a part of mental operations underlying the brain engagement in this task is not dependent on the way of responding.