ERP Recording

EEG was recorded by 32 Ag/AgCl scalp electrodes referred to the nose and positioned over symmetrical positions on the two hemispheres as illustrated in Figure 10.

The montage was guided by a special-purpose computer-controlled system (Pastel) based on a three-dimensional digitization of the head (Echallier, Perrin, & Pernier, 1992). During recording the electrode impedance was kept below 2 k-W.

Eye movement artifacts were controlled off-line by the two prefrontal electrodes (FP1 and FP2) and an electrode placed at the outer canthus of the right eye (YH).

Trials in which the potential measured in any of those channels exceeded 150 μV were rejected. Artifacts induced by amplifiers blocking were avoided, excluding trials in which amplitudes above 250 μV were measured in any of the channels.

The EEG and electroculogram (EOG) were amplified with a bandpass of 0.03 to 320 Hz (sampling rate 1000 Hz) and stored on a computer disk for off-line analysis.

The ERPs were averaged separately for each stimulus type in each experimental session over an analysis period of 1024 msec, including 100-msec prestimulus. After averaging, frequencies lower than 0.8 Hz and higher than 16 Hz (3 dB) were digitally filtered out.