Abstract
Orientation is an important spatial information for humans to manipulate objects. It has been shown that the orientation process in early visual areas is enhanced during visually guided action. However, in case that visual information of objects is occluded in action, how the orientation is represented in visual cortices remains unclear. In our study, the cortical representation of an object orientation was measured during vision and action phases by fMRI. The participants observed a real cylindrical object in the vision phase and then performed the instructed grasping task towards the visually occluded object in the action phase. The object was displayed in four different orientations with +45°/-45° rotation around roll and pitch axes. The grasping task comprised two types of action identified by hand gesture: precision or coarse grasp. We performed multivariate pattern analysis to determine the classification accuracy of object orientation in vision and action phases that cover regions of interest in visual cortices. The results showed that while V1 and V2 had high accuracy above chance level in the vision phase but not in the action phase, V3d showed significantly higher accuracy in both vision and action phases. In a previous study, V3d was shown to be activated for grasping action in darkness after object observation. Our findings corroborate the findings of the earlier study and indicate that V3d might process orientation information of the visually occluded object in the action phase. Additionally, classification results of the coarse grasp in action phase were higher than the precision grasp results. It could be implied that the large hand action occurring during coarse grasp invokes a greater orientation process than precision grasp with minor differences between orientations. This differential cortical representation may also reflect feedback from action-related processes, as the accuracy of the action changes depending on the grasp type.