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. However, it remains debated whether such interaction across dimensions, or "magnitude integration" effects, arise from low-level perceptual processes that are independent from the task performed, or whether they instead arise from high-level decision-making processes. We address this question with two experiments in which participants watched a series of dot-array stimuli modulated in numerosity, duration, and item size. In experiment 1 (task condition), the task required participants to either judge the numerosity, duration, or size of each stimulus. In experiment 2 (passive condition), instead, a separate group of participants passively watched the stimuli. The behavioral results obtained in the task show robust magnitude integration effects across all three dimensions. Then, we identify a neural signature of magnitude integration by showing that event-related potentials at several latency windows (starting at [~]100-200 ms after stimulus onset) can predict the effect measured behaviorally. In the passive condition, we demonstrate an almost identical modulation of brain responses, occurring at the same processing stages as during the task. Importantly, using a cross-condition multivariate decoding analysis, we demonstrate that brain responses to magnitude in the task condition can predict the response in the passive condition at specific latency windows. These results thus suggest that magnitude processing and integration likely occurs via automatic perceptual processes that are engaged irrespective of the task-relevance of the stimuli, and independently from decision making.