Magnetoencephalographic measurements during two types of mental rotations of three-dimensional objects

Neural mechanisms of higher-order cognitive processes are hard to study by using nonhuman primates. Of these mechanisms, the mental rotation task is the one of the best studied. When subjects decide whether two shapes presented at various orientations are the identical or mirror images, their reaction time increases with the angle of rotation between the shapes. Recent magnetic resonance imaging (MRI) and magnetoencephalographic (MEG) studies revealed that activities of the premotor area and/or the parietal association area are related to the angular difference between two objects. However, there are two kinds of degree of difficulty in the mental rotation task of three-dimensional objects. One is based on the angular difference and the other is based on the rotation method itself. Keeping these two difficulties in mind, this paper evaluates the activities for a mental rotation task . Here, the performance of the subjects are sufficient as judged by measuring response time prior to MEG experiment. Results reveal that the activities in the right occipital area contralateral to the left visual stimulus field were found in eight out of 12 cases in the range of 150-200 ms. On the other hand, no activities were found in the left. These results are consistent with contralateral dominance of the anatomical connection. fMRI researches showed activities of both parietal association area for the mental rotation of three-dimensional object. In this study, activities of these areas were estimated in both 2-D and 3-D rotation. In addition to this result, the number of subjects, whose activities of posterior part of both parietal association areas were estimated, was increased in 3-D rotation compared with 2-D rotation. From this result, it is implied that these activities are related to the degree of difficulties of rotation method itself. In addition to these result, activities were observed in the posterior part of parietal association area ipsilateral to those in the premotor area in two out of four cases in 2-D rotation and in three out of three cases in 3-D rotation. 3-D rotation requires subjects to imagine the invisible parts of visual stimuli for judging whether the visual stimulus is an identical pair or not. It is believed that this requirement activates the fro- nto-parietal circuit which is used in visuo-motor tasks.