Electrophysiological correlates of symbolic and non-symbolic numerosity processing

Several ERP studies investigating number sense by assessing the comparison distance effect (CDE) have shown effects in diverging time windows, roughly between 100 and 500 ms. In the CDE task, one discriminates between two magnitudes that are numerically close of further apart. However, the CDE may not only represent magnitude representations, but also decisional processes. In contrast, the prime distance effect (PDE) measures only magnitude representations. ERP studies on the PDE are scarce. Moreover, part of the diverging results in the CDE, especially early effects, may be not be due to numerosity but to differences in visual input, such as luminance, dot size, and area size (Hyde & Spelke, 2009; Gebuis & Reynvoet, 2012). In the current study, we investigated the electrophysiological correlates of the PDE while controlling the non-symbolic magnitudes for visual input. Participants watched prime-target pairs and responded only when prime and target represented the same magnitude (10% of the trials). Next to symbolic and non-symbolic tasks, tasks with non-symbolic primes and symbolic targets and vice versa were also included. Preliminary results show that the symbolic targets elicited early parietal distance effects, regardless of whether the prime was symbolic or non-symbolic. Non-symbolic targets showed a widely distributed distance effect in a later time window, around 300-400 ms. When preceded by symbolic primes, these targets also showed an earlier parieto-occipital distance effect in the N1 and P2p time windows. This indicates that symbolic magnitudes are processed differently than non-symbolic magnitudes, and that non-symbolic number processing depends on task requirements.