Figure 3.

A simple approach for ssVEP estimation across a single trial. Indices of power and phase-locking can be extracted from single trials of ssVEP time series (Keil et al., 2008), by applying a simple moving average technique. As shown in the schematic, a window of fixed length k, sample points, containing an integer number of ssVEP cycles (4 cycles at 10 Hz in the present example), is shifted along a single trial ssVEP signal in n steps, spaced at intervals corresponding to the duration of one ssVEP cycle (here: 100 ms, corresponding the 10 Hz ssVEP response). Equation 1. Point-wise averaging the contents of the n windows (w) yields a time-domain representation of the part of the signal that is locked to the ssVEP driving stimulus. Equation 2. Determining the complex phase φ at the ssVEP frequency (f_stim) for each window (w) by means of discrete Fourier transform or a similar method allows phase averaging: Complex Fourier components at f_stim for each w are unit normalized by dividing by the absolute value, and then averaged across all n windows. The modulus of the resulting complex number represents an intuitive measure of the stability of the phase at the ssVEP frequency across the duration of the trial. Note that application of the method requires that integer numbers of EEG sample points are available for the window length k, as well as for the step size between windows t₁(w+1)− t₁(w), achieved by following the recommendations by Bach and Meigen (1999). It should be noted that the resulting measure reflects the regular and stable alignment of the oscillatory waveform to the driving stimulus over time, but not the phase-consistency within the ssVEP signal.