Differential modulation of discriminability and emotion on early and late deviant processing in the three-stimulus oddball paradigm.

The human brain rapidly detects and evaluates unexpected changes in the environment, a process often studied using the three-stimulus oddball paradigm. Deviant stimuli typically elicit two event-related potentials (ERPs): the fronto-central P3a, reflecting early attentional processing, and the parietal deviant-P3b, associated with subsequent late cognitive processing. Although the early process has been widely investigated, the nature of the later cognitive process remains unclear. This study examined how standard-target discriminability and the emotional content of deviant stimuli independently or interactively modulate these two stages of deviant processing. Participants performed a visual oddball task in which discriminability between standard and target stimuli was manipulated, while deviant images varied in emotional valence (unpleasant, neutral, pleasant). Principal component analysis was used to separate P3a and deviant-P3b. Low discriminability impaired behavioral performance and reduced target-P3b amplitude while enhancing P3a elicited by deviants, indicating intensified attentional focus on the stimulus sequence. Emotional content also modulated P3a, with both unpleasant and pleasant deviants eliciting larger amplitudes than neutral ones, suggesting an arousal-driven enhancement of early attentional processing. In contrast, deviant-P3b was selectively enlarged only for unpleasant stimuli, reflecting a negativity-biased enhancement of later cognitive processing. Although Bayesian analyses provided only weak evidence against an interaction between emotion and discriminability, these findings indicate that attentional and emotional influences on deviant processing operate through independent neural pathways. Moreover, while attentional focus increases P3a, it does not significantly facilitate subsequent deviant-related cognitive processing, highlighting a functional dissociation between P3a and deviant-P3b.