Dissociable event-related potential modulations of intrinsic and extrinsic factors in temporal integration

We investigated visual temporal integration, by which multiple stimuli appearing in rapid succession are perceived as a single event. Temporal integration not only depends intrinsically on the passage of time but also, extrinsically, on the number and distribution of successive stimuli that are presented across that time interval. Here, we used a missing element task to investigate intrinsic and extrinsic factors in temporal integration, by manipulating stimulus duration and number, respectively. We found that both contributed interactively to integration performance and that varying the information rate over time did not further modulate this pattern. Intrinsic and extrinsic factors had dissociable effects on the N1, N2, N2pc, and P3 components of the event-related potential, implicating unique contributions to perceptual discrimination, spatio-temporal grouping, attention, and response decision-making. Stimulus number-induced effects on the event-related potential also generally arose later than those of stimulus duration. The latter already modulated the amplitude of the N1 and the early phase of the N2pc, while the former did not. The collective results suggest that while both intrinsic and extrinsic factors drive temporal integration, they do so in different ways. This difference during integration may eventually be reflected in the way in which we perceive longer, episodic events. We studied for the first time the electrophysiological correlates of in- and extrinsic factors in temporal integration. We found that both the intrinsically-governed passage of time, and the extrinsically-governed number of successive stimuli affect the tendency to integrate. Despite their common effect on perception and behavior, event-related potentials showed that the two factors likely have very different underlying neural mechanisms. This difference may affect how we perceive events ranging from very brief ones of less than a second to much longer episodic events.