Model diagram.

<p>(<b>1</b>) The approximate number system produces evidence/samples at a rate R e.g. every 30 ms. (<b>2</b>) This generates a stream of evidence. The importance of each sample decays with a rate D (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0195188#pone.0195188.s001" target="_blank">S1 Fig</a>). The decay accounts for memory effects in participants. (<b>3</b>) Instead of summing the stream to a threshold, evidence is used to infer which target has the larger number. As more samples arrive the width or confidence on the estimation improves. (<b>4</b>) The index finger is placed by a motor controller that minimizes movement jerk and that is slowed down by the degree of confidence. The green and cyan arrows symbolize the hardest and easiest numerical ratio, respectively. Their length depict degree of uncertainty; e.g. harder comparisons produce stronger pulls to the other response target. The red line across panels is the veridical numerical difference.