Attention and numeration are two phenomena in cognitive sciences of brain. As the effect and interaction that these two activities have on each other, the importance of detecting differences between two phenomena is shown more than ever. Someone believes that the relation between two brain cognitive activities (i.e. attention and numeration) is close to the extent that they are considered as one. In most of past works, the qualitative effect of numeration or extra mental task on attention is surveyed. The aim of this article is to clarify differences between two phenomena and the effect of extra mental activity (numeration) on visual top-down attention. For this purpose we separate P300 sources signal and their subcomponents to divide two classes (attention with numeration and pure attention). To do this, constrained blind source separation (CBSS) is used. The main method is to use Infomax algorithm that beside it, minimizing the difference between reference and estimated signal P300 and also subcomponents, P3a and P3b, are used as constrained term. The obtained result indicates that time of appearance and amplitude of estimated signal P300 and their subcomponent for two class are different. By doing paired t-test, best result for divided two cognitive activities, were obtained by amplitude of estimated p300 with P-value 0.01. To our knowledge, Seen from this aspect to this subject, is a new effort.