The time course of planar and non-planar rotations in a letter rotation task.

Mental rotation (MR) of character letters requires participants to mentally rotate the letter in their minds' eyes through a process akin to the physical rotation of the stimulus. It has been suggested that different cognitive processes are engaged during such MR of both canonical and mirror-reversed letters. In addition to the planar rotation of the canonical letters, an additional "flip-over" process (non-planar rotation) has been assumed during the MR of mirror-reversed letters. However, the temporal relationship between planar and non-planar rotation has not been systematically investigated. In this study, the occurrence of both planar and non-planar rotations were examined through the analysis of the event-related brain potentials (ERPs) elicited by canonical or mirror-reversed letters presented at different rotation angles between 300 and 1000 ms post-stimulus onset over consecutive 50ms time-windows. For smaller rotation angles (30° and 60°), non-planar rotation preceded planar rotation. For letters rotated by 90°, planar and non-planar rotation occurred at the same time. For larger angles (120° and 150°), the letter was first rotated within the plane (planar rotation) and afterwards it was also rotated out-of-the-plane (non-planar rotation) until it was fully canonicalized. Thus, the temporal relationship between planar and non-planar rotation differed for each rotation angle, with the non-planar rotation occurring at increasingly later intervals for different points in time for progressively larger rotation angles. These findings have relevant methodological implications for studies investigating the psychophysiological correlates of the mental rotation of mirror letters.