Vertical Elevation as a Key Factor for the Neural Distinction of Target Selection and Distractor Suppression in Visual Search.

Background: Directing attention to relevant visual objects while ignoring distracting stimuli is crucial for effective perception and goal-directed behavior. Event-related potential (ERP) studies using the additional-singleton paradigm have provided valuable insights into how the human brain processes competing salient stimuli by monitoring N2pc and P_D, two event-related components thought to reflect target selection and distractor suppression, respectively. However, whether these components reflect the activity of a single or distinct neural mechanism remains controversial. Herein, we investigated the neural substrate of N2pc and P_D by manipulating the vertical elevation of target and distractor relative to the visual horizontal meridian using two variants of the additional-singleton paradigm.

Methods: In Experiment 1, participants searched for a shape singleton and identified the orientation of an embedded tilted bar while ignoring a color singleton. In Experiment 2, the tilted bars were removed and participants performed a shape search while ignoring a color singleton. Electroencephalogram (EEG) recordings at posterior sites (PO7/8) measured N2pc and P_D components. Reaction times and ERP amplitudes were analyzed across conditions.

Results: The results of both Experiments 1 and 2 showed that N2pc and P_D responded in opposite ways to the manipulation of vertical elevation. N2pc was robust for targets in the lower visual hemifield and reversed in polarity (i.e., post-N2pc positivity ) for targets in the upper visual hemifield. Conversely, P_D was more pronounced for distractors in the upper visual hemifield and nil for those in the lower visual hemifield. Critically, vertical elevation did not influence psychophysical estimates of search efficiency in either experiment, suggesting that the relationship between these components and their functional significance is less straightforward than previously thought.

Conclusions: These results provide empirical support for the idea that N2pc and P_D are influenced by the retinotopic organization of the visual cortex in a manner consistent with the neural and functional dissociation of target selection and distractor suppression in visual search.