The neural mechanisms underlying passive and active processing of numerosity

To investigate the difference in passive viewing and active processing of numerosity, we presented participants arrays of dots and concurrently measured their EEG. In the first condition, participants naive to the subject under study passively viewed the dot-arrays. In the second condition, the participants were informed about the changes in numerosity and had to actively process numerosity. The visual properties of the dot-arrays were controlled and could therefore not explain possible numerosity related effects. The results revealed no numerosity related effects in the passive and active conditions. Instead, when the data was reorganised according to visual cue size (surface or diameter, etc.), strong effects of the visual cues were present at lateral occipital and parietal electrode sites. These electrode sites and time windows correspond to the ERP components often suggested to support numerosity processes. Furthermore, a larger central-parietal P3 amplitude effect was present for active versus passive numerosity processing. This result was not influenced by numerosity itself and could not be explained by response processing. It therefore appears to reflect general cognitive processes. Together, our results show that we do not (automatically) extract numerosity from a visual scene during passive or active processing of numerosity. Instead, these results are consistent with the notion that we rely on the continuous sensory properties of numerosity stimuli to make numerosity judgments. (C) 2013 Elsevier Inc. All rights reserved.