Revisiting the timing of salient-signal suppression

According to the signal suppression hypothesis, salient stimuli automatically trigger “attend-to-me” signals that cause these stimuli to capture attention unless they are suppressed. In support of this hypothesis, Sawaki and Luck (2010, Atten Percept Psychophys) showed that a colour-singleton distractor elicited an event-related potential (ERP) marker of suppression called the distractor positivity (PD) rather than a marker of attentional selection (N2pc) in a letter-detection task. The PD was reported to begin 100 ms after stimulus onset, which was hypothesized to be early enough to prevent the distractor from eliciting the N2pc in its conventional time range (180–300 ms). Here, we tested whether this “early PD” reflected suppression or the attend-to-me signal itself. The study was motivated by two considerations. First, it may take more than 100 ms to establish a salience-based attend-to-me signal from cortex. Even if such a fast signal were possible, no ERP activity resembling such signal was observed prior to onset of the PD. Second, salient targets sometimes elicit a positivity akin to an early PD prior to the onset of N2pc. This positivity cannot be ascribed to suppression because the subsequent N2pc demonstrates that the item was attended. We replicated Sawaki and Luck’s task and then reversed the target and distractor stimuli such that participants detected the colour singleton rather than a nonsingleton letter. The singleton target elicited a small early positivity that rapidly turned into an early N2pc due to the salience of the target. The early positivity was larger for upper-visual-field targets because the temporally overlapping N2pc is smaller for upper-field stimuli. The results indicate that the early posterior contralateral positivity (Ppc) is associated with the attend-to-me signal and the later PD is associated with suppression of that signal.