Priming for letters and pseudoletters in mid-fusiform cortex: examining letter selectivity and case invariance

A large body of research indicates a critical role for the left mid-fusiform cortex in reading, however, the extent to which this area is dedicated exclusively to the processing of words and letters has been debated. Two questions regarding left mid-fusiform function are critical to this debate: (1) Are letters stored preferentially compared to visually equivalent non-letters (letter selectivity)? (2) Are letter representations abstract with respect to changes in letter case (e.g., A/a; case invariance)? The present study addressed these questions by comparing priming for letters and pseudoletters in left and right mid-fusiform regions using functional magnetic resonance imaging while subjects performed a same/different matching task. Results revealed priming for letters but not pseudoletters in the left mid-fusiform region, suggesting that representations are letter selective. However, no priming for different-case-primed letters was observed in this region, indicating that representations are not case invariant. In addition, priming for pseudoletters but not letters was observed in the homologous right mid-fusiform region. Overall, findings contradict strongly modular theories of letter/word processing and suggest that left and right mid-fusiform regions support generic object processes that are differentially effective for representing disparate types of visual stimuli.