Data analysis

The data were analyzed using EEGLab 6.01. They were filtered with a high-pass cut-off of 1 Hz (due to fast stimulation rate) and a low-pass cut-off of 45 Hz. Though the high-pass of 1 Hz might not be optimal for P3a, we justify this choice by the fact that identical filter settings could be used for both MMN and P3a responses. The EEG was divided into epochs from −100 to 600 ms (baseline −100 ms before sound onset at 0 ms). Extreme amplitude values were manually rejected from the data. Channels of poor quality were interpolated manually.

Independent Component Analysis (ICA) was conducted for all participants' data to further ensure that artifacts due to an eye blink or to a channel with poor contact were excluded and a rejection threshold of ±100 μV was applied. For all participants included in the analyses, at least 80% of the trials were accepted.

Finally, manual rejection removed epochs that still displayed clear EOG-fluctuations or otherwise abnormally massive waveforms related to muscle activation, shown as a hundred to a thousand times greater magnitude than the cortical signals of interest.

Amplitudes were quantified from individual difference waves using a 40 ms time window centered on the peak of the MMN and P3a components in the grand-average difference waves. Mean values from these time windows were calculated for F3, Fz, F4, C3, Cz, C4, P3, Pz, and P4.