A saccade (/səˈkɑːd/ sə-KAHD, French for jerk) is a quick, simultaneous movement of both eyes between two or more phases of fixation in the same direction.[1] In contrast, in smooth pursuit movements, the eyes move smoothly instead of in jumps. The phenomenon can be associated with a shift in frequency of an emitted signal[clarification needed] or a movement of a body part or device.[2] Controlled cortically by the frontal eye fields (FEF), or subcortically by the superior colliculus, saccades serve as a mechanism for fixation, rapid eye movement, and the fast phase of optokinetic nystagmus.[1]