Workers of the ant Myrmica sabuleti are known to mentally add visual cues when they are similar (have the same shape, color and size) and are simultaneously perceived. We here aimed to define the latter condition, i.e. what is the largest topographical distance between two signals that still allows ants to add them up? Ants belonging to four colonies were trained and tested over 72 hours with two cues placed on either side of their nest entrance and separated by 3.5 to 6.5 cm from each other. These ants mentally summed the cues separated by less than 5 cm from each other and not those separated by more than 5 cm. The critical distance is thus 5 cm for M. sabuleti. An additional experiment using two different kinds of cues presented at the same time, each kind of cue differently distant from the other, confirmed this critical distance measurement. This result leads to some ecological and physiological considerations, as well as to the need for further investigation.