Event-Related Variability is Modulated by Task and Development

In carefully designed experiments, cognitive scientists interpret the mean event-related potentials (ERP) in terms of cognitive operations. However, the huge signal variability from one trial to the next, questions the representability of such mean events. We explored here whether this variability is an unwanted noise, or an informative part of the neural response. We took advantage of the rapid changes in the visual system during human infancy and analyzed the variability of visual responses to central and lateralized faces in 2-to 6-month-old infants and adults using high-density electroencephalography (EEG). We observed that neural trajectories of individual trials always remain very far from ERP components, only moderately bending their direction with a substantial temporal jitter across trials. However, single trial trajectories displayed characteristic patterns of acceleration and deceleration when approaching ERP components, as if they were under the active influence of steering forces causing transient attraction and stabilization. These dynamic events could only partly be accounted for by induced microstate transitions or phase reset phenomena. Furthermore, these structured modulations of response variability, both between and within trials, had a rich sequential organization, which, in infants, was modulated by the task difficulty. Our approaches to characterize Event Related Variability (ERV) expand and reinterpret classic ERP analyses, making them compliant with pervasive neural variability and providing a more faithful description of neural events following stimulus presentation.