Asymmetries in flanker-target interference at different levels of number processing

Visual stimuli presented in peripheries can be barely recognized when they are surrounded by flankers (crowding). The target-flanker interference can be asymmetrical, and this asymmetry depends on a stimulus type. In particular, recognition of a letter or a number is more disturbed by the presence of a leftward flanker, reflecting the direction of reading. So far, such reading-related asymmetry has been observed with visual recognition tasks. In the following studies, we used numbers as stimuli to examine whether the leftward asymmetry in crowding extends to other levels of information processing, i.e. whether it is present when more abstract, semantic features are extracted. We presented participants with numerical triplets in the left or right visual field, and asked them to classify the middle number according to its magnitude (Experiment 1), physical characteristics (Experiment 2) or parity (Experiment 3). We observed that the leftward flanker interfered stronger with the target than the rightward flanker, but only when magnitude and physical characteristics were classified. Our findings suggest that the leftward asymmetry in crowding extends up to the semantic level of number processing, but only selectively, i.e. when a certain sort of information (magnitude) is extracted.