Which properties of the visual stimuli predict the type of representation used in mental rotation?

Two modes of internal representation, holistic and piecemeal transformation, have been reported as a means to perform mental rotation (MR) tasks. The stimulus complexity effect has been proposed as an indicator to disentangle between these two representation types. However, the complexity effect has not been fully confirmed owing to the fact that different performances could result from different types of stimuli. Moreover, whether the non-mirror foils play a role in forcing participants to encode all the information from the stimuli in MR tasks is still under debate. This study aims at testing the association between these two common types of representation with different stimuli in MR tasks. First, the numbers of segments and vertices in polygon stimuli were manipulated to test which property of the visual stimuli is more likely to influence the representation in MR tasks. Second, the role of non-mirror foils was examined by comparing the stimulus complexity effect in both with- and without-non-mirror foils conditions. The results revealed that the segment number affected the slope of the linear function relating response times to rotation angle, but the vertex number in the polygons did not. This suggests that a holistic representation was more likely to be adopted in processing integrated objects, whereas a piecemeal transformation was at play in processing multi-part objects. In addition, the stimulus complexity effect was observed in the with-non-mirror foils condition but not in the without-non-mirror foils one, providing a direct evidence to support the role of non-mirror foils in MR tasks.