Spectral difference index: a single EEG measure of drug effect.

A single EEG measure of drug effect, the spectral difference index (SDI), was compared to other quantitative measures for its sensitivity to drugs from 7 different clinical classes. The other measures included relative power in 2 c/sec bands from 0 to 17 c/sec, and in 4c/sec bands from 18 to 29 c/sec; relative power and average frequency in the bands 1-5, 6-13 and 14-30 c/sec; and the Hjorth time domain descriptors. The SDI was the most sensitive EEG discriminator between drug and placebo sessions. It is presently used in our pharmaco-EEG studies to test whether a drug has significantly altered the EEG. Its use has been extended to interhemispheric studies.