The roles of stimulus-response compatibility and mental rotation in mirror-image and left-right decisions.

People were shown rotated letters and timed as they decided (a) whether the letters were normal or backward, or (b) whether a dot was to the left or right of each letter with respect to its upright orientation. In the viewer-centered version of (b), the judgement was to be independent of whether the letter was normal or backward, so that stimulus-response (S-R) compatibility was confounded with angular orientation. In the letter-centered version, the judgement was relative to the letter's own coordinates, so that the confounding between S-R compatibility and orientation was reversed for backward letters. The functions relating RT to angular orientation were parallel across the three tasks, suggesting that S-R compatibility played no role, and that the participants mentally rotated the letters to the upright in each case. A marked increase in RT to backward letters in the letter-centered task suggested a second rotation in depth to restore the letters to normal.