in the Ternus display being seen as “jumping over” the other two discs in the display, which are then considered to be the (inner) discs; placing itself in the right hand side location. This effect can be seen in motion in the example at the top of the page in Figure 1. According to Braddick from his research in 1980, element motion can be attributed to the low-level short range motion process, signalling a null or no-movement for the two elements in the middle of the display between Frame 1 and Frame 3 when short ISIs are shown. As a response to this the higher level long-range motion process passes a signal of element movement. This means that the outer element appears to jump across to the opposite side of the display and back again. Group motion When the ISI (Frame 2) in the Ternus motion display is shown for the relatively long interval of at least 50 milliseconds, group motion can be observed. The longer the inter-frame interval or ISI the more likely a group motion effect is to occur. Group motion gives the perceiver the illusion that all of the discs within the display are moving simultaneously to the right and then back again. As with element motion this effect can be seen in Figure 3 as well as demonstrated in Figure 2. Braddick in 1980 posited that the occurrence of group motion at longer ISIs can be attributed to the short-range motion process signalling motion in the central elements of the motion display, which in turn leads to the long-range process to signal that the three elements are moving in unison. Factors Since the discovery of the Ternus illusion there has been an abundance of research into how and why it occurs. As can be deemed from research above, one of the most critical factors appears to be the length of the ISI, as it seems to be a heavy determinant in which percept becomes apparent to the observer however there are many other factors implicated. A reasonable amount of the research in this area appears to be well empirically supported, such as the idea that lower level (short range processes) and higher level (long range processes) are involved in determining which illusion is perceived. A study by Scott-Samuel & Hess found that the perception of element motion is influenced by changes in the spatial appearance within