Revisiting the electrophysiological correlates of feature analysis during visual search

Seminal event-related potential (ERP) studies of visual search revealed the componentry associated with the analysis of target and nontarget singletons that appear in an otherwise uniform array of identical items. The ERP componentry included an anterior N2, posterior bilateral N2 (N2pb), posterior contralateral N2 (N2pc), a P3b, and an occipital P3 (later called the singleton detection positivity, SDP). At about the same time, seminal reaction time results indicated that individuals can use a feature-based strategy or a salience-based strategy to find a fixed-feature singleton. Here, we varied the task parameters to revisit the electrophysiological correlates of different visual search strategies. The search array contained 16 small vertical blue bars or 15 small vertical blue bars and one of the following singletons: a horizontal blue bar (small or large) or a vertical yellow bar (small or large). In Experiment 1, participants indicated whether one pre-specified target singleton was present (20% of trials) or absent (80%), and in Experiment 2, participants indicated whether the array contained any singleton (20%) or not (80%). Thus, only participants in Experiment 1 needed to differentiate the target from other nontarget singletons. Targets elicited an N2pc and an anterior N2 in both experiments, but the latter was larger in Experiment 2. Interestingly, the N2pb appeared to reflect volume conduction from the anterior scalp. The SDP was difficult to isolate because the low probability of target-present trials led to a large P3b at nearby electrodes. Finally, in Experiment 1, a late anterior negativity was elicited by nontarget singletons that resembled the target (e.g., large yellow bar when participants searched for a small yellow bar). No late anterior negativity was evident in response to nontarget singletons that were easily distinguished from the target, suggesting that this component is associated with conflict resolution in medial frontal cortex.