A bilateral N2pc (N2pcb) component is elicited by search targets displayed on the vertical midline

Doro, Mattia; Bellini, Francesco; Brigadoi, Sabrina; Eimer, Martin; Dell'Acqua, Roberto · 2020 · Crossref

DOI: 10.1111/psyp.13512

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Summary

This study addresses a limitation in the electrophysiological measurement of visual attention: the standard N2pc component, which tracks attentional selection of lateral targets, cannot detect attention deployment to targets displayed on the vertical midline. Because midline targets project to both cerebral hemispheres, they do not produce the contralateral-ipsilateral asymmetry required for N2pc detection. The authors introduce and validate a new ERP marker, the N2pcb (bilateral N2pc), hypothesized to reflect the same attentional selection processes as the N2pc but elicited by midline targets. The researchers recorded EEG data from twelve participants performing visual search tasks. Participants searched for colored disk targets within arrays of distractors under two conditions: singleton search (salient target among uniform gray distractors) and feature search (target among heterogeneous colored distractors). Targets were presented either laterally or on the vertical midline. The N2pc was calculated by subtracting ipsilateral from contralateral ERPs for lateral targets. The N2pcb was quantified by subtracting the ipsilateral ERP of lateral targets from the bilateral ERP of midline targets. This design allowed for a direct comparison of the timing and amplitude of attentional selection for lateral versus midline targets. Behavioral results showed faster reaction times for singleton search compared to feature search, with no significant difference in speed between lateral and midline target detection. ERP analysis confirmed the presence of a reliable N2pcb component for midline targets, characterized by enhanced negativity over posterior electrodes. Crucially, the N2pcb showed no significant difference in amplitude or onset latency compared to the N2pc. Both components emerged earlier in singleton search than in feature search, and the latency difference between these search conditions was equivalent for both lateral and midline targets. Additionally, individual differences in the N2pc latency delay between search conditions strongly correlated with the N2pcb latency delay, suggesting a common neural source. The findings demonstrate that the N2pcb is a valid electrophysiological marker for attentional selection of midline targets. The similarity in timing, amplitude, and sensitivity to search difficulty between the N2pcb and N2pc indicates that they reflect the same underlying attentional mechanisms. This discovery extends the utility of ERP methods in visual search research, allowing for the tracking of attention deployment across the entire visual field, including the vertical midline, which was previously inaccessible to standard N2pc analysis.

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• visual search