Decrease of non-linear structure in the EEG of Alzheimer patients comparedto healthy controls

Objective: Non-linear EEG analysis can provide information about the functi oning of neural networks that cannot be obtained with linear analysis. The correlation dimension (D-2) is considered to be a reflection of the complex ity of the cortical dynamics underlying the EEG signal. The presence of non -linear dynamics can be determined by comparing the D-2 calculated from ori ginal EEG data with the D-2 from phase-randomized surrogate data. Methods: In a prospective study, we used this method in order to investigat e non-linear structure in the EEG of Alzheimer patients and controls. Twent y-four patients (mean age 75.6 years) with 'probable Alzheimer's disease' ( NINCDS-ADRDA criteria) and 22 controls (mean age 70.3 years) were examined. D-2 was calculated from original and surrogate data at 16 electrodes and i n three conditions: with eyes open, eyes closed and during mental arithmeti c. Results: D-2 was significantly lower in the Alzheimer patients compared to controls (P = 0.023). The difference between original and surrogate data wa s significant in both groups, implicating that non-linear dynamics play a r ole in the D-2 value. Moreover, this difference between original and surrog ate data was smaller in the patient group. D-2 increased with activation, b ut not significantly more in controls than in patients. Conclusions: In conclusion, we found decreased dimensional complexity in th e EEG of Alzheimer patients. This decrease seems to be attributable at leas t partially to different non-linear EEG dynamics. Because of this, non-line ar EEG analysis could be a useful tool to increase our insight into brain d ysfunction in Alzheimer's disease. (C) 1999 Elsevier Science Ireland Ltd. A ll rights reserved.