Mental rotation is thought to underlie the increase in response times (RTs) for deciding whether rotated letters are normal or mirrored versions. However, mental rotation predicts a linear increase in RTs, whereas the mirror/normal letter discrimination task typically produces a curved function. Recently, Kung and Hamm suggested that this curved function results from a mixture of trials in which mental rotation is employed and trials in which it is not. The mixture ratio may vary between individuals, with some individuals relying more on mental rotation than others. There is no factor in the Kung and Hamm model that reflects such individual differences. In the present study, we suggest that a possible exponent parameter could be added to the Kung and Hamm model to capture individual differences in the mixture ratio. This exponent parameter appears to capture an individual characteristic since the value obtained correlates between the mirror/normal letter task and a left/right object facing task. The development of a quantity that represents the mixture ratio will aid further testing of processes involved in the visual imagery system.

Since first presented by Shepard and Metzler, Science 1971, 171: 701-703, mental rotation has been described as a rotary transformation of a visual stimulus allowing it to be represented in a new orientation. For a given stimulus, the transformation is thought to occur at a constant speed, though speed may vary between stimuli; three-dimensional abstract shapes made out of blocks tend to be rotated much more slowly than alphanumeric characters or line drawings of common objects. Rotation is also presumed to be performed through the shortest angle. These assumptions are based upon the fact that response times tend to increase with angle of rotation, peaking at 180° of separation for abstract block figures or from upright for common objects and alphanumeric stimuli. The symmetry about 180° provides evidence supporting rotation through the shortest angle. In order to determine the shortest direction, the current orientation of the stimulus is assumed to be known prior to mental rotation. Moreover, in order to determine the current orientation of a common object or alphanumeric stimulus, it is assumed the stimulus is identified prior to mental rotation because the current orientation is defined by what the object is. In mirror/normal discriminations or left/right facing discriminations of rotated stimuli response times are often examined by collapsing over response options as this variable is assumed to be uninteresting in terms of mental rotation. This article examines these assumptions, and suggests that many of them are not entirely safe. WIREs Cogn Sci 2017, 8:e1443. doi: 10.1002/wcs.1443 For further resources related to this article, please visit the WIREs website.

When deciding if a rotated object would face to the left or to the right, if imagined at the upright, mental rotation is typically assumed to be carried out through the shortest angular distance to the upright prior to determining the direction of facing. However, the response time functions for left-and right-facing objects are oppositely asymmetric, which is not consistent with the standard explanation. Using Searle and Hamm's individual differences adaption of Kung and Hamm's Mixture Model, the current study compares the predicted response time functions derived when assuming that objects are rotated through the shortest route to the upright with the predicted response time functions derived when assuming that objects are rotated in the direction they face. The latter model provides a better fit to the majority of the individual data. This allows us to conclude that, when deciding if rotated objects would face to the left or to the right if imagined at the upright, mental rotation is carried out in the direction that the objects face and not necessarily in the shortest direction to the upright. By comparing results for mobile and immobile object sets we can also conclude that semantic information regarding the mobility of an object does not appear to influence the speed of mental rotation, but it does appear to influence pre-rotation processes and the likelihood of employing a mental rotation strategy.