Side flankers produce less crowding, but only for letters

Identification of isolated and crowded letter (B, D, F, G, K, N, L, S, T) and symbol stimuli (%, /, ?, @, }, <, £, §, μ) was examined across the visual field in a two-alternative forced-choice match-to-sample task (2AFC-MTS). During isolated presentation, identification accuracy did not differ between the two stimulus types. Identification rates for the central characters within the three-character strings were higher for letters than for symbols at the horizontal and vertical meridian (Experiment 1), and at diagonal locations (Experiment 2). However, this reduction of parafoveal letter crowding was present in horizontally but not in vertically oriented strings of stimuli. The same pattern of results was replicated in the periphery of the visual field (Experiment 3). The obtained results are in agreement with the proposition that the receptive fields of letter detectors are modified during reading acquisition, in order to support efficient letter identification (Tydgat & Grainger, 2009). However, the pervasive presence of the effect across the visual field suggests that it could originate from a non-retinotopic stage of visual processing.