Similarities and differences in the coding of numerical and alphabetical order using acoustic stimulation as revealed by event-related potentials in humans

We analyzed event-related potential (ERP) correlates of numerical and alphabetical positional judgements. Sixteen subjects classified whether acoustically presented numbers (1, 4, 6 or 9) were smaller or larger than 5 or whether phonemes (a, d, f or i) stood before or after letter 'e' in the alphabet. Stimulus non-specific effects in the amplitude of P2p and in the latency of N2f and N2p (220-255 ms) were found. In contrast, in the amplitude of the right parietal N2p and in response-locked ERPs, stimulus-specific distance effects were found. The topography of ERPs was similar in both tasks. However, stimulus-specific effects may reflect differences in the ordinal coding of numerical and alphabetical information. Similarity to former results is discussed and a possible scenario of executing magnitude comparison tasks is sketched.

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