ELAN: A Software Package for Analysis and Visualization of MEG, EEG, and LFP Signals
Figure 5
Time-Frequency analysis: dissociation of evoked and induced oscillatory activities, in an example of MEG responses to auditory stimuli. Top left: TF map obtained after averaging of wavelet transforms of single trials, which contains both evoked and induced responses. Top right: stimulus phase locking factor. For each single trial the phase of the response relative to an event (here the auditory stimulus) is computed, and the stability of the phase across trials is represented. Phase locking is close to 0 for activities not phase-locked to the stimulus also called induced activity, and larger stimulus phase locking values (up to 1) characterize stimulus phase-locked activities (evoked responses). Bottom left: TF map obtained after wavelet transform of the averaged evoked response, reflecting mostly evoked activities. Bottom right: time courses in the 20–30 Hz frequency band of the averaged oscillatory power (red), of evoked oscillatory power (blue), and of the stimulus phase-locking factor (green). The first peak of oscillatory responses at ~100 ms could be identified as evoked, whereas the second one around 500 ms is induced.
