Alpha entrainment caused by exposure to a background stimulus
continuously flickering at a rate of 8 1/3 Hz was affected by
the appearance of a foreground target stimulus to which the
subjects were requested to press a button. With the use of
bipolar derivations (to reduce volume conduction effects), scalp
recorded EEG potentials were subjected to a continuous wavelet
transform using complex Morlet wavelets at a range of scales.
Complex Morlet wavelets were used to calculate efficiently instantaneous amplitudes and phases on a per-trial basis, rather
than using the Hilbert transform on band-pass filtered data.
Multiple scales were employed to contrast the pattern of alpha
activity with those in other bands, and to determine whether
the harmonics observed in the spectral analysis of the data were simply a result of the non-sinusoidal response to the entraining signal or a distinct neural phenomenon. We were thus able to calculate desynchronization/resynchronization for both the entrained and non-entrained alpha activity. The occurance of the target stimulus caused a sharp increase in amplitude in both the entrained and non-entrained alpha activity, followed by a sharp decrease, and then a return to baseline, over a period of 2.5 seconds. However, the entrained alpha activity showed a much more rapid recovery than non-entrained activity.