Magnitude processing and integration entail perceptual processes independent from the task

The magnitude dimensions of visual stimuli, such as their numerosity, duration, and size, are intrinsically linked, leading to mutual interactions across them. Namely, the perception of one magnitude is influenced by the others, so that a large stimulus is perceived to last longer in time compared to a smaller one, and vice versa. The nature of such interaction is however still debated. In the present study we address whether magnitude integration could arise from “automatic” perceptual processes, independently from the task performed, or whether it arises from active decision making. In two separate experiments, participants watched a series of dot-array stimuli modulated in numerosity, duration, and item size. In one case (task condition), the task required them to judge the stimuli in each trial, with the specific dimension to judge indicated by a cue presented after each stimulus. In the other case (passive condition), instead, participants passively watched the stimuli. The behavioural results obtained in the task show robust magnitude integration effects across all three dimensions. Then, we identified a neural signature of magnitude integration by showing that relatively early event-related potentials can predict the effect measured behaviourally. Finally, we demonstrate an almost identical modulation of brain responses in passive viewing, occurring at the same processing stages linked to the behavioural effect. The results thus suggest that magnitude integration likely occurs via automatic perceptual processes that are engaged irrespective of the task-relevance of the stimuli, and independently from decision making.