Early and late selection processes have separable influences on the neural substrates of attention.

To improve our understanding of the mechanisms of target selection, we examined how the spatial separation of salient items and their similarity to a pre-defined target interact using lateralised electrophysiological correlates of visual spatial attention (N2pc component) and visual short-term memory (VSTM; SPCN component). Using these features of target selection, we sought to expand on previous work proposing a model of early and late selection, where the N2pc is suggested to reflect the selection probability of visual stimuli (Aubin and Jolicoeur, 2016). The authors suggested that early-selection processes could be enhanced when items are adjacent. In the present work, the stimuli were short oriented lines, all of which were grey except for two that were blue and hence salient. A decrease in N2pc amplitude with decreasing spatial separation between salient items was observed. The N2pc increased in amplitude with increasing similarity of salient distractors to the target template, but only in target-absent trials. There was no interaction between these two factors, suggesting that separable attentional mechanisms influenced the N2pc. The findings suggest that selection is initially based on easily-distinguished attributes (i.e., both blue items) followed by a later identification-based process (if necessary), which depends on feature similarity to a target template. For the SPCN component, the results were in line with previous work: for target-present trials, an increase in similarity of salient distractors was associated with an increase in SPCN amplitude, suggesting more information was maintained in VSTM. In sum, results suggest there is a need for further inspection of salient distractors when they are similar to the target, increasing the need for focal attention, demonstrated by an increase in N2pc amplitude, followed by a higher probability of transfer to VSTM, demonstrated by an increase in SPCN amplitude.