A recent article by Viatcheslav Wlassoff, PhD on Brain Blogger discusses this phenomenon. Akinetopsia is the loss of motion perception. The related Zeitruffer phenomenon is the (usually) slowed down perception of the velocity of moving objects. In the article, scientists explain the more common theories as to the etiology of such experiences.
I cannot do this article justice by explaining it any better than the author. Please go to this site for the straight scoop: