In humans as well as in cats, alpha responses (8-15Hz oscillations) to sensory stimuli returning to baseline 200-300 ms after stimulus were described. Intracerebrally recorded alpha responses in cats were found not only in specific primary sensory areas but also in hippocampus where they were present independently on stimulus modality.
The aim of the present study was to investigate the intracerebral distribution of ERPs in a frequency range of 5.5-15 Hz in humans.
In seven patients with medically intractable epilepsies depth electrodes were implanted to localize the seizure origin prior to surgical treatment. A total of 268 sites were investigated. Visual oddball paradigm was performed. To reject long-latencies cognitive potentials the digital filter of 5.5-15 Hz was used. For each subject, approximately 50 trials to target and 100 trials to frequent stimuli were averaged. The sites with prominent ERPs were investigated.
In 98 contacts (6 patients) where the prominent ERPs were identified they were present always after both target and frequent stimuli, and they were identical in several successive components (range from 1 to 5) with the peak latency range from 45 to 339 ms. They were found in gyrus cinguli, gyrus fusiformis, hippocampus, gyrus parahippocampalis, gyrus temporalis superior, medius and inferior.
Simultaneous occurrence of identical components after target and frequent stimuli in different brain structures probably reflects the basic level of processing visual stimuli through diffusely distributed neuronal networks. The resulting morphology of the ERPs in some contacts seems to be very similar to alpha responses recorded in cats.