Intra- and inter-subject variations of EEG patterns under various conditions

Abstract

Monopolar occipital recordings were obtained in 5 adults with flickering stimuli of 8, 10, 12, 16 and 25Hz and without flickers (NF), either with eyes closed, resting (EC), mental calculation (MC), or eyes opened, resting (EO) for a total session period of 1hr, Observations were repeated several days later. Similarly, another group of 3 adults were later tested 3 times under EC and EO. Intra- and inter-subject variations of EEG patterns under varing conditions were measured in terms of rank order correlations between a set of 5 integrated band-values per 10sec, in which the respective flicker frequencies were included.When EEG patterns were computed based on group means as in ordinary experiments, their consistencies across sessions as measured by correlations were highest under EC and lowest under EO; in addition, the EEG pattern was considerably reduced with 25Hz lights under EO (Figs. 1, 2 and Table 1), When the same inter-session were computed per S and then averaged, the result was similar but, as a whole, correlations were lower compared with those based on group means (0.87 vs. 0.80) (Table 2). Moreover, these intra-subject correlations were almost identical even when they were computed within the session (Table 3).Contrary to the intra-subject correlations, the mean inter-subject correlations within the session were in average lower and in particular markedly reduced with 25Hz lights under EC than under EO (Table 4). Thus greater individual differences were suggested under EC than under EO. No group differences (groups with 5 and 3 adults) were found.Individual differences in EEG patterns especially with respects to alpha responses have been well-recognized since Walter and the present findings suggested that these individual-specific or internal factors are naturally more dominant with eyes closed (EC) than with eyes opened (EO), when external or environmental factors are more effective.The flickering stimuli had both frequency-specific photic driving effects and nonspecific alpha-blocking effects. The former effects were dominant over the latter with stimuli of 8 to 16Hz, while the reverse was true with 25Hz lights and this would explain the lowest intra-subject correlations with these high-frequenecy lights. Similarly, the intra-subject correlations would be decreased by a house light of 50Hz, or of much faster frequency, that is under the EO conditions.In addition, the depressant effects of 25Hz stimuli upon alpha responses were considerably different from individual to individual when alpha responses are otherwise not blocked, that is under EC, and thus inter-subject correlations would be greatly decreased with 25Hz flickering stimuli with eyes closed (EC).Lastly, it was interesting to note that intra-subject correlations in EEG patterns were relatively independent of time intervals between observations and that the results under MC were closer to those under EC than those under EO; thus even dark illumination (EO) was more effective in altering EEG patterns than simple mental calculation.