A violation of pure insertion: mental rotation and choice reaction time.

Six experiments tested the assumption that the mental rotation process is purely inserted into a mirror-normal discrimination task. In Experiment 1, subjects took significantly longer to respond to upright characters in blocks containing rotated stimuli than in blocks containing only upright stimuli. Experiments 2 and 3 showed that this "rotational uncertainty" effect was not caused by the need to determine stimulus orientation, and Experiment 4 showed that it was independent of the visual quality of the stimulus. Experiment 5 showed that the effect was greatly reduced when subjects performed a go-no-go task rather than choice reaction time (RT), and Experiment 6 showed that it was independent of the complexity of the response required in the choice task. The results suggest that response selection in a choice RT mirror-normal discrimination task is altered when mental rotation is added, violating the assumption of pure insertion.