A general spatial transformation process? Assessing the neurophysiological evidence on the similarity of mental rotation and folding.

Mental rotation and mental folding are established as archetypal mental spatial transformation tasks. Yet, it is uncertain as to whether these tasks rely on a general spatial transformation mechanism or on dedicated processes. We scrutinized benchmark mental spatial transformation findings for and against the concept of a shared mechanism in two EEG experiments, on mental rotation and folding respectively. We found the typical increase in reaction times with greater transformation difficulties for both tasks, however there was no relationship between participants' reaction time slope for each task. Analyses of event-related potentials revealed a negative going component 400-800 ms post stimulus onset which distinguished transformation from baseline in both tasks, but only differed between difficulty levels for mental rotation. We found that this component originated from two parietal sources. A central parietal source displayed negativity in this time period distinguishing only transformation from baseline conditions in mental rotation and folding. Additionally, a left parietal source showed the same pattern for mental folding as the central source, but rotation yielded greater negativity for higher difficulties. Time-frequency analyses revealed desynchronisation in broadband alpha power throughout mental rotation and folding trials in both parietal sources, commensurate with a cognitive load being placed on transformation and visuospatial systems. We discuss a possible functional explanation for the results in line with a general spatial transformation process, for which we argue the evidence is stronger than for distinct rotation and folding mechanisms.