Long-term intra-individual variability of the background EEG in normals

Objectives: Forty-five healthy adult volunteers underwent repeated qEEG exa minations with retest intervals 25-62 months in order to investigate the lo ng-term intra-individual variability of several qEEG features such as, abso lute and relative power, power asymmetry, coherence, mean and peak frequenc y and entropy. Prior to any computations all parameters were transformed to Z-scores on the basis of a normal database. Methods: Correlation coefficients were used to test the effect of the time on the test-retest differences. Correlation coefficients were also computed between baseline and retest values, as a measure of intra-individual stabi lity, to make our results comparable to most literature data. By computing the standard deviations for test-retest differences, the intra-individual v ariabilities of the examined parameters were obtained hi the unit of inter- individual variability of normal population. The same calculations were car ried out with values obtained from the odd and even numbered epochs of the same EEG sections. This way, that portion of the intra-individual variabili ty was estimated that might be introduced even by chance only when the epoc hs were selected randomly from the same section of EEG conforming to select ion criteria. Results: As for our results, further increase of test-retest differences wi th time after 25 months might be so insignificant that it could not be demo nstrated in our test material. The long-term intra-individual variability f or most parameters, especially for total absolute power and alpha mean freq uency, was less than the inter-individual variability in the normal populat ion. The moment-to-moment variability was least in the case of the absolute power. Conclusions: Estimates for intra-individual variability expressed this way in Z-scores might easily be used in the follow-up of patients even for a fe w years. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.