Abstract
The N2pc ERP component is a well-known index for the deployment of visual attention to the one visual hemifield. So far, it has been used with elementary visual displays made of coloured geometric shapes and the targets for attentional selection were strongly lateralized. We ask whether N2pc can track visual attention along a finely graded horizontal spatial extent made of a normally spaced (as in words) letter string. Participants were required to attend a particular letter within a row of four letters horizontally centred at fixation. The centre-to-centre distance between consecutive letters was of 1.5° of visual angle. Trials consisted of a central arrow cue indicating the letter location to attend followed 750 ms later by a string of random letters. The onset time for each letter (33 ms duration) was determined randomly and independently within a 150ms period. After a 2-second delay, participants named the letter at the cued location. Each participant completed 1200 trials, half of which consisted of the cue only. These served to subtract the EEG activity elicited by the cue from that of the whole trial. The time window of 200–400ms following the onset of the target was selected to compute the N2pc. This component is obtained by subtracting the EEG activity of the electrode (PO7/PO8) ipsilateral to the target from that of the contralateral electrode. The N2pc computed separately for each target location easily discriminated between an attention focus to the left or right visual hemifield. Moreover, the N2pc associated with an attention focus to the inner targets was more negative than that to outer targets. These findings indicate that N2pc can reliably track the location of visual attention along a horizontal spatial extent with a margin of error as small as ±.75°.