Variability in the expression of the P1 amplitude was estimated by calculating the median absolute deviation (MAD) of the P1 amplitude and latency values, normalized by dividing by the median. In addition, across-trial amplitude variability was calculated at each time-point in the epoch. Due to the inherent difficulty of calculating co-efficients of variation when the central tendency is close to 0, amplitude values were normalized prior to computing the MAD estimate by converting all of the data to a z-score.
