Flanking letter effects are orthographic effects and not visual effects: evidence from same-different matching

Related flanking letters (e.g., RO ROCK CK) are known to facilitate processing of a central target word compared with unrelated flankers (e.g., PA ROCK TH) in the flanking letters lexical decision task. According to one account, these effects are driven by pre-orthographic visual features rather than orthographic information per se. Here, we tested this pre-orthographic account by comparing flanker effects with letter stimuli and flanker effects with symbol stimuli using the same-different matching task. We found significant flanker effects for letter strings in RTs and accuracy rates to “same” responses, as well as in discriminability indices (d') in a signal detection analysis. Crucially, we failed to find any significant impact of flanker symbols for responses to symbol targets, in all our dependent measures. The results contradict a low-level feature integration account of flanking letter effects and support the conclusion that they reflect the spatial integration of orthographic information.