Key event-related potentials (ERPs) of perceptual decision-making such as the centroparietal positivity (CPP) elucidate how evidence is accumulated towards a given choice. Furthermore, this accumulation can be impacted by visual target selection signals such as the N2 contralateral (N2c). How these underlying neural mechanisms of perceptual decision-making are influenced by the spatial congruence of distractors relative to target stimuli remains unclear. Here, we used electroencephalography (EEG) in humans of both sexes to investigate the effect of distractor spatial congruency (same vs. different hemifield relative to targets) on perceptual decision-making. We confirmed that responses for perceptual decisions were slower for spatially incongruent vs. congruent distractors of high salience. Similarly, markers of target selection (N2c peak amplitude) and evidence accumulation (CPP slope) were found to be lower when distractors were spatially incongruent vs. congruent. To evaluate the effects of congruency further, we applied drift diffusion modelling to participant responses, which showed that larger amplitudes of both ERPs were correlated with shorter non-decision times when considering the effect of congruency. The modelling also suggested that congruency’s effect on behaviour occurred prior to and during evidence accumulation when considering the effects of the N2c peak and CPP slope. These findings point to spatially incongruent distractors, relative to congruent distractors, influencing decisions as early as the initial sensory processing phase, then continuing to exert an effect as evidence is accumulated throughout the decision-making process. Overall, our findings highlight how key electrophysiological signals of perceptual decision-making are influenced by the spatial congruence of target and distractor.Significance StatementWhen making perceptual decisions, we often need to focus on task-relevant targets while ignoring irrelevant distractors. In this study, we show that salient distractors in the opposite visual hemifield capture attention with consequences for the speed and accuracy of decisions. Electrophysiological signals of perceptual decision-making were attenuated by spatially incongruent distractors relative to congruent distractors. Computational modelling suggested that the influence of spatial congruency of target and distractor on the electrophysiological markers of target selection and evidence accumulation occurred during the initial sensory processing phase preceding evidence accumulation, which continued as evidence was accumulated throughout the decision process. Our findings, therefore, suggest that the spatial congruence of target and distractor impacts the dynamics of electrophysiological signals of perceptual decision-making.
