Distractor intrusions are the result of delayed attentional engagement: a new temporal variability account of attentional selectivity in dynamic visual tasks

When observers have to identify targets among distractors in a rapid serial visual presentation (RSVP) stream, distractor intrusion errors are frequent, demonstrating the difficulty of allocating attention to the right object at the right moment in time. However, the mechanisms responsible for such intrusion errors remain disputed. We propose a new attentional engagement account of selective visual processing in RSVP tasks. Engagement is triggered by the pre-attentive detection of target-defining features. Critically, the success versus failure of target identification is determined by the speed of such engagement processes on individual trials. To test this account, we measured electrophysiological markers of attentional engagement (N2pc components) in three experiments where observers had to report the identity of a target digit in one of two lateral RSVP streams. On most trials, the target was immediately followed by a digit distractor, resulting in many post-target distractor intrusions. Critically, N2pcs components measured on distractor intrusion trials were significantly delayed relative to trials with correct target reports. This was the case regardless of whether the target was defined by a shape cue or by its colour, and even when the location of shape-defined targets was known in advance. These findings show that distractor intrusions are the result of delayed attentional engagement. They demonstrate that temporal variability in attentional selectivity across trials can strongly affect visual awareness and perceptual reports. Our temporal variability account of attentional engagement offers a new framework for assessing the temporal dynamics of attention in visual object recognition