Electrophysiological correlates of symbolic and non-symbolic numerosity processing

In the current study, we investigated the electrophysiological correlates of magnitude representations, controlling the non-symbolic magnitudes for visual input. Contrary to former ERP studies, we used a match-to-sample task. This task has been shown to tap into magnitude representations without concurrently measuring decisional processes, as is the case in the comparison distance task (Van Opstal et al., 2008; Van Opstal & Verguts, 2011). Ten adult participants performed an adapted version of the match-to-sample task. Two stimuli were presented after each other, both for 750 ms, and participants had to respond only if the target matched the sample (10% of the trials). Next to tasks with non-symbolic samples and targets, and symbolic samples and targets, tasks with non-symbolic samples and symbolic targets and vice versa were included to investigate mapping of symbolic and non-symbolic numerosity. The results show that symbolic targets preceded by non-symbolic samples elicited early parietal distance effects. Non-symbolic targets showed a widely distributed distance effect in a later time window, around 300-400 ms. When preceded by symbolic samples, these targets also showed an earlier parieto-occipital distance effect. This shows that symbolic magnitudes are processed differently from non-symbolic magnitudes, and that non-symbolic numerosity processing depends on task requirements. The results indicate that mapping symbolic and non-symbolic numerosity onto each other may be a separate component of number sense, in addition to symbolic and non-symbolic number sense. Future ERP-research could gain insight in the development of these components at a neural level.