What's special about orthographic processing? Further evidence from transposition effects in same-different matching.

We sought evidence for letter-specific processing in the same-different matching task by comparing performance to random consonant strings and either strings of symbols (Experiment 1) or strings of digits (Experiment 2). The strings could be aligned horizontally or vertically, and on "different" response trials the to-be-matched strings could differ by the transposition of two adjacent characters or by the substitution of two adjacent characters. Making a "different" response was harder when the difference involved a transposition compared with a substitution-the transposition effect. Crucially, the transposition effect was significantly greater for letters than for symbols or digits when stimuli were aligned horizontally, but did not differ significantly across stimulus type with vertically aligned strings. These results suggest that it is processing specific to horizontally aligned letter strings, a reading-specific mechanism, that causes the greater transposition effects for letter stimuli in the same-different matching task when stimuli are arranged horizontally.