Conflict processing of symbolic and non-symbolic numerosity

It is commonly assumed that the processing of magnitudes occurs independent of modality or notation. Several studies have reported similar behavioural as well as neurophysiological responses to magnitudes presented in distinct modalities as well as notations, but a direct assessment of possible interactions between different modalities and notations, using measures of electro-cortical processing, is lacking. The present study investigates whether the neural activity underlying symbolic and non-symbolic numerosity processing interacts with the neural activity underlying physical size processing before, or proceeds independently until, selective activation of the motor system. We used a symbolic (Arabic numbers) and non-symbolic (arrays of dots) size congruency task and instructed subjects to judge either the numerical or the physical size of the stimuli, while event related potentials were recorded. Longer reaction times as well as a decrease in accuracy were obtained for incongruent compared to congruent trials. For the event related potential data, this congruency effect was also found with respect to the latency of the P3 component reflecting an interaction at the level of stimulus evaluation. Moreover, incongruence delayed the stimulus-locked but not the response-locked lateralized readiness potential. Together these results suggest that, irrespective of notation, the interaction between different magnitudes occurs before selective response activation.