Abstract
An ellipse rotating at a fixed angular velocity appears to rotate faster as its aspect ratio increases. We have hypothesized that this speed illusion arises because regions of high curvature act as form-based, trackable features (TFs). When trackable feature are weak, the corresponding motion signals are weak, generating the illusion. Interestingly, this speed illusion is observed for rotating ellipses whose contours are defined by equally spaced dots even though the local motion of each dot along the contour is unambiguous. In the current work, we used such dotted ellipses as stimuli in a series of block-design fMRI experiments designed to dissociate the processing of local (motion) and global (form) sources of information in the perception of rotational motion. Methods: Participants viewed blocks of high medium or low aspect ellipses continuously rotating at the same angular velocity defined by either 32 dots, which produce the speed illusion (Experiment 1) or 12 dots, which do not (Experiment 2). In the third experiment the actual angular velocities of the 32 dot ellipses were adjusted so that the ellipses of each aspect ratio appeared to rotate at the same speed. Results: Modulations of BOLD activity in early visual areas V1, V2V, V2D and V3D reflected the processing of local dot motion. In contrast, BOLD activity in later visual areas, V3A, V3B, LOC, MT and MST reflected the combined processing of both local dot motion and properties of the emergent form of the stimuli (contour curvature). Conclusion: motion perception in general is mediated in part by the integration of local motion and global form information that likely occurs in V3A, V3B, LOC, MT and MST.
This research was supported by a predoctoral NFS fellowship 2005031192 to GPC.