A method for the discrimination between induced and evoked gamma band activity

A method for single EEG sweep analysis is presented. It is based on signal envelope calculations by means of the Hilbert transform, and it allows the discrimination between evoked (phase-locked) and induced (non-phase-locked) event-related synchronisation/desynchronisation (ERS/ERD) responses at a given frequency band. We tested our analytical method on EEG data collected from subjects under median nerve stimulation. We selected for our analysis the gamma-band range (30 to 80 Hz). No induced ERS was found in this frequency band, but the presence of evoked activity strongly phase-locked to the stimulus was evident.