Predominance of lateral over vertical mirror errors in reading: A case for neuronal recycling and inhibition

We investigated whether lateral mirror errors could be more prevalent than vertical mirror errors (e.g., p/q vs. p/b confusions) because mirror generalization is harder to inhibit for the discrimination of a reversible letter and its lateral than its vertical mirror-image counterpart. Expert adult readers performed a negative priming task in which they determined on the prime whether two letters and on the probe whether two objects facing opposite directions were identical. We found in both experiments longer response times for objects facing opposite lateral orientations preceded by a reversible letter and its lateral mirror-image counterpart (e.g., p/q) than preceded by perceptually matched non-reversible letters (e.g., g/j). No negative priming effect was observed when objects that were vertical (Experiment 1 & 2) or lateral (Experiment 2) mirror images of each other were preceded by a letter and its vertical mirror-image counterpart (e.g. p/b). Finally, we observed longer response times for objects that were lateral mirror images of each other after lateral than after vertical reversible letters. These results suggest that lateral mirror errors are more prevalent than vertical ones because mirror generalization might be stronger and thus more difficult to inhibit in the context of the former than the latter.