Instead, the difference was found much later in a N400 component between 300 550 msec over parietal regions, suggesting that those behavioral differences may not be due to early processing of visual identification, but later processing of subitizing or accessing mental number line when lacking attentional resources.
Based on previous studies of numerical cognition [34], [58], we at first focused our analyses on two well-established ERP components, N1 (negative peak voltages between 100 200 msec) and P2p (positive peak voltages between 200 320 msec), over three posterior electrode groups (Left sites: P3, P5, PO3; Middle sites: CPz, Pz, POz; Right sites: P4, P6, PO4 [51], [52].
Myelin water fraction (MWF) was defined as the percentage of signal with T 2 < 17 <span class="lh">msec over total signal.
Twelve noncontiguous axial planes (7 mm thickness, slice skip 1 mm) parallel to the anterior commissure-posterior commissure line were collected over 1100 msec in a clustered acquisition sequence, in order to allow subjects to make overt responses in relative silence (TE = 40 msec, flip angle = 70 degrees).
The spacing of the TriGridTM electrode array was 6 mm in a diamond-shaped configuration, and the electrical field was applied at an amplitude of 250 V/cm of electrode spacing for a 40 msec total duration applied as three pulses over a 400 msec interval, resulting in brief deltoid muscle contractions.
Spectra were acquired with TR 1,500 msec, TE 144 msec, and a matrix size of 16 × 16 over an FOV of 16 cm, resulting in a scan time of 6.4 min for each region.
Then the probe number was presented for a maximum time of 15,000 msec or until a response was given, followed by an ISI of 4000 msec.
The stimulus was presented for 50 msec following a random interval between 250 1000 msec.
Each trial consisted of a sound presentation (870 msec), followed by image acquisition (2760 msec) and a brief silence (30 msec) before the next trial.
For a time window of 500 msec with a step size of 100 msec, a half bandwidth was 4 Hz.
