The neural signature of magnitude integration between time and numerosity

Magnitude dimensions such as time and numerosity are fundamental components of our visual experience, allowing us to understand the environment and interact with it. Different magnitudes are however not processed independently from each other, but show a relationship whereby the perception of one dimension depends on the others ("magnitude integration"). In this study, we use electroencephalography (EEG) to address whether such integration may arise from a shared brain processing stage where different dimensions are integrated together, or from independent parallel processes interfering with each other. In the experiment, participants judged either the average numerosity or duration of dynamic dot-array stimuli concurrently modulated in both dimensions. First, the behavioural results show a magnitude integration effect in both tasks, with duration affecting the judgement of numerosity and vice versa. The EEG results further show that both numerosity and duration significantly modulate event-related potentials at several distinct latencies. Crucially, however, we identified a significant interaction between numerosity and duration emerging in a specific latency window (360-460 ms) irrespective of the task performed by participants. In this latency window, the modulation of ERPs provided by the interfering magnitude dimension can be predicted by the strength of the behavioural bias. Our results thus support the idea of different magnitude dimensions converging onto a shared perceptual processing stage mediating their integration. Overall, our results demonstrate a clear electrophysiological signature of magnitude integration between numerosity and time, and provide new evidence for a shared representational system encompassing different magnitude dimensions.