Electroencephalographic changes in the dominant hemisphere during memorizing and reasoning.

The EEG was recorded with bipolar technique in ten normal subjects in the frontal, temporal and occipital regions of the dominant hemisphere in four situations: (1) during an auditory memory test, (2) during 'auditory rest' (listening to white noise), (3) during a visual reasoning test and (4) during 'visual rest' (watching a black dot on a white screen). Computer analysis of the EEG was made by (a) an on-line automatic EEG analyser yielding measures of mean power, mean frequency and frequency spread (complexity) and (b) an off-line period-amplitude (P-A) analysis, which gave per cent activity time and mean voltage in 21 frequency bands. As compared to auditory rest the auditory memory test gave an amplitude increase frontally in the alpha, theta, and delta bands. During the visual reasoning test there was in addition an amplitude increase in the alpha band in the temporal region. During the auditory test a tendency to a decrease of the alpha activity was seen in all three regions but this decrease occurred only occipitally during the visual test. The two types of mental activity thus induced two patterns of regional EEG changes. These showed principal similarities to regional cerebral blood flow patterns which have been recorded during visual and auditory tests of about the same types as those used in the present study.