Use of non-linear EEG measures to characterize EEG changes during mental activity.

Non-linear EEG analysis provides a possibility for studying the dynamical changes in cortical networks related to mental activity. In this study the correlation dimension D2 was used to study local changes in complexity, and the mutual dimension Dm was used to assess changes in the dynamical coupling between different brain areas. EEGs were recorded in 25 healthy subjects under three conditions: (1) eyes closed, (2) eyes open, and (3) mental arithmetic with eyes closed (serial subtraction of 7s from 1000). In the eyes-closed condition, D2 was lower at parieto-occipital sites. D2 increased during the eye-open and arithmetic conditions. Contrary to the D2, the Dm showed no regional differences in the eyes closed condition. A clear increase in Dm was seen during eyes open and arithmetic. We conclude that both the correlation dimension and the mutual dimension are very sensitive to EEG changes during simple visual information processing and during mental arithmetic. However, these measures seem to be relatively non-specific, and correlate only weakly with performance on the arithmetic task.