Grand average wavelet spectra. In each time-frequency plot, the interval of -800 to 2000 ms relative to cue onset (vertical line) is depicted on the x-axis. Morlet wavelet center frequencies, ranging from 4 to 30 Hz, are depicted on the y-axis. Color signifies wavelet amplitude in (a) and the phase-locking factor in (b). (a) At posterior electrode sites, three neurophysiological events can be observed, namely an early synchronization in the delta and theta bands, followed by a desynchronization and subsequent synchronization in the alpha band. (b) Phase-locking factor (PLF), specifying the amount of phase-locking to stimulus onset. Only the early synchronization in the delta and theta bands is phase-locked to stimulus onset. This supports the idea that the early component reflects the processing of the visual cue, while the alpha (de)synchronization is associated with the deployment of covert visual attention.