EEG amplitudes in healthy, late-middle-aged and elderly adults: normality of the distributions and correlations with age.

The interindividual distributions of EEG amplitudes were evaluated in 60 healthy subjects, aged 56-76. Skew and kurtosis were used to assess the normality of the amplitude distributions in 5 frequency bands (delta, theta, alpha and 2 beta ranges) and 28 scalp derivations while the subjects were awake and rested with their eyes closed and opened. Most of the interindividual EEG amplitude distributions were not normally distributed. Two transformations were applied to the EEG amplitudes: the square root and the natural logarithm. Evaluation of skew and kurtosis indicated that the natural logarithmic transformation was more effective than the square root in reducing the positive skew and leptokurtosis that characterized the untransformed interindividual EEG amplitude distributions. For all frequency bands except theta, the log transformation rendered the distributions approximately normal in form. Correlations between log transformed EEG amplitudes and age were not statistically significant. These findings are discussed in the context of previous research, aging, and methodological features.