The geometry of the Landmark Task

In the Landmark Task participants are shown a horizontal line divided into two segments by a transection mark (“landmark”), and asked to compare the lengths of the two segments [3,4]. This comparison can in principle be carried out by (i) considering the stimulus as a single object and assessing whether or not it is left-right symmetrical [1], or (ii) interpreting the two segments as separate objects, mentally translating one over the other and deciding whether they are congruent (see also [2,5,6]). When stimulus lines are rectangles, which is typically the case, the two types of process cannot be easily distinguished because symmetrical stimuli are (by definition) formed by two congruent segments and non-symmetrical stimuli are formed by non-congruent segments. The purpose of this work is to separate out the contribution of symmetry processing from that of translation-congruencyprocessing by using trapezoidal instead of rectangular shapes.