However, optical illusion caused by the ball's spinning may play an important part in what makes curveballs difficult to hit. The curveball's trajectory is smooth, however the batter perceives a sudden, dramatic change in the ball's direction. When an object that is spinning and moving through space is viewed directly, the overall motion is interpreted correctly by the brain. However, as it enters the peripheral vision, the internal spinning motion distorts how the overall motion is perceived. A curveball's trajectory begins in the center of the batter's vision, but overlaps with peripheral vision as it approaches the plate, which may explain the suddenness of the break perceived by the batter. A peer-reviewed article on this hypothesis was published in 2010.