Trials with RTs < 100 ms were excluded from analysis, resulting in a removal of 5% of trials in the endogenous predictive, 3.7% in the exogenous and 6.0% in the endogenous counter-predictive task. Electroencephalographic was recorded using 32 Ag–AgCl electrodes arranged according to the 10–20 system and referenced to the right earlobe. Horizontal electro-oculogram (HEOG) was recorded from the outer canthi of the eyes. Electrode impedance
was kept below 5 kΩ, earlobe and ground electrodes below 2 kΩ, amplifier bandpass was 0.01–100 Hz and digitization rate was 500 Hz. After recording the EEG was digitally re-referenced to the average of the left and right earlobe. The average earlobe reference is preferred with low-density recordings because an average reference (mean PD-1 inhibiton of all recorded electrodes) is not as accurate under such conditions (Handy, 2005; Nunez & Srinivasan, 2006). Data were filtered with a low-pass filter of 40 Hz. Then EEG was epoched offline into 300-ms periods starting 100 ms before and 200 ms after target onset for post-target analysis. The time window was restricted to 200 ms post-target to diminish contamination of the ERPs by behavioural responses. Baseline correction was performed in the 100-ms period preceding onset of target.
Trials with eye movements (voltage exceeding ± 40 μV relative to baseline at HEOG electrodes) or with other artefacts (voltage exceeding ± 80 μV relative to baseline at all electrodes) Selleck PR-171 were removed prior to EEG averaging. Additionally, the residual HEOG deflections were analysed to make sure no individual had a difference that exceeded 4 μV between cue-left and cue-right trials (Kennett et al., 2007). Further, all trials with behavioural Cyclooxygenase (COX) errors, as well as catch and filler trials, were excluded from EEG analysis. This resulted in subsequent ERP analysis for the endogenous predictive task and endogenous counter-predictive task being based on an average of 346 and 313 expected trials, respectively. For unexpected
predictive and counter-predictive tasks, analysis was based upon 85 and 81 trials per participant, for each task, respectively. The exogenous task analysis was based on an average of 130 cued and 128 uncued trials per participants. Event-related potential analysis epochs were averaged separately for task (endogenous predictive, exogenous and endogenous counter-predictive) and cue type (cued, uncued). ERP mean amplitudes were computed for measurement windows centred around the peak latencies (averaged across all conditions) of the somatosensory P45, N80, P100 and N140 components (38–58 ms, 68–88 ms, 90–122 ms and 130–160 ms post-stimulus, respectively). To investigate longer-latency effects of spatial attention, mean amplitudes were also computed between 160 and 200 ms (Nd) after tactile stimulus onset.