EVENT-RELATED PROLONGATION OF INDUCED EEG RHYTHMICITIES IN EXPERIMENTS WITH A COGNITIVE TASK

Since we consider evoked potentials (EPs) and event-related potentials (ERPs) as an ensemble of induced rhythmicities we investigated ''sensory'' evoked potentials (EPs) and ''cognitive'' event-related potentials (ERPs) with respect to their frequency components (around 5 Hz and around 10 Hz). ''Sensory'' EP sessions consisted in standard EP measurements with tone bursts of 2000 Hz frequency. Prior to the subsequent ''cognitive'' sessions, the subjects were instructed to expect frequent 2000 Hz tones and rare 1950 Hz tones, the rare tones thus being difficult to detect. A further instruction was to count the number of the ''rare'' stimuli. Not a single tone of different frequency, however, was presented. The single trials were then analyzed with respect to their 5 Hz and 10 Hz components. Time-locking was investigated using a modified single sweep wave identification (SSWI) method. Depending on features of the prestimulus EEG, it was observed that theta (4-7 Hz) and alpha (7-13 Hz) responses were prolonged in the case of ''cognitive'' sessions. We believe that the prolonged endogenous ERP components are induced rhythmicities triggered by additional information processing, when a high degree of uncertainty has to be resolved during our experimental conditions. For the analysis and visualization of these phenomena, the combination of digital filtering and SSWI showed relevant advantages in comparison to other methods.