Horizontal and vertical asymmetry in visual spatial crowding effects.

The crowding effect is a ubiquitous phenomenon in spatial perception that has been extensively investigated. A notable property of spatial crowding is the radial and tangential asymmetry in which crowding is much more severe for items arranged on a radial line originating from the fixation point compared with items arranged tangentially with regard to the fixation point. Here, we report another asymmetric property of the crowding effect between horizontally and vertically arranged spatial layouts. In multiple experiments using a number of different stimulus patterns (e.g., letter Cs, letter Ts, and parallel/nonparallel line segments), results consistently showed that the crowding effect was significantly stronger when the target and distractors were horizontally rather than vertically arranged. Such a horizontal versus vertical asymmetry was found in all four quadrants of visual space. Importantly, a further visual acuity test did not reveal a horizontal versus vertical asymmetric effect.