the Ternus display which suggests that apparent motion is mediated entirely by a long-range motion process. Research undertaken by Kramer and Yantis in 1997 found that perceptual grouping of the elements also appears to have important implications. Kramer and his colleagues found an increase in observers perceiving group motion when the elements in the display seemed to form a logical group in contrast to when they were independently arranged. Yantis found that the perceived continuity of a briefly interrupted element in perception depends on early neural mechanisms in the visual system such as visible persistence as well as on a representation of a three-dimensional surface layout. As previously mentioned, studies have alluded to the idea that high level motion mechanisms determine the final decision in which percept shows through, however recent research by He & Ooi suggests that this final decision is also influenced by accounting for numerous grouping factors such as proximity, similarity and common surface amongst the elements in the scene. Though there are many ideas relating to causative factors, even current research seems to be lacking in a conclusive explanation for why the Ternus effect occurs and has not yet discovered exactly which mechanisms are responsible. Petersik and his team in 2006 suggested that intensive brain-imaging research on each percept is the way forward and is the most likely way to discover the answer. On the other hand, Grossberg and Rudd (1992, Psychol. Rev., 99, 78–121) have developed a neural model of motion perception that simulates many examples of long-range apparent motion, including both the Ternus and reverse-contrast Ternus illusions. References Optical illusions