Access to numerical information is dependent on the modality of stimulus presentation in mental addition: a combined ERP and behavioral study.

We investigated whether the mental representation of numbers is abstract amodal or modality-dependent. Subjects verified simple additions. In an event-related potential (ERP) experiment, subjects added an Arabic digit (S2) to a preceding number (S1) offsetting 3 s before S2. S1 was either a visually shown Arabic digit, a written number word or an acoustically presented number word. In a behavioral experiment, we measured the speed of addition using a modified paradigm. In the ERPs to S2, the amplitude of the parietal N1, the fronto-central P2, and the late positivity between 320 and 460 ms were more positive and RTs were faster when S1 was a heard number word than when S1 was a written number word. ERP amplitudes and reaction times took intermediate positions between the other two conditions when S1 was an Arabic digit. Between the Arabic and heard number conditions, this so-called numeral modality effect (NME) was present at electrodes Pz, P4, P3 and Cz when number size was small, whereas it was significant over electrode C4 and P4 when number size was large. Our results suggest that numbers presented in different surface-formats have differential access to number representations. Conclusions for models of number processing are drawn and the possible role of parietal number representations is discussed. We replicated the N270 ERP component and elicited the ERP numerical distance effect in response to incongruent arithmetical results.