190px|thumbnail|Figure 1 – element motion in effect190px|thumbnail|right|Figure 2 – group motion in effectThe Ternus illusion or Ternus effect is an illusion related to human visual perception involving apparent motion. In a simplified explanation of one form of the illusion, two discs, (referred to here as L for left and C for centre) are shown side by side as the first frame in a sequence of three frames. Next a blank frame is presented for a very short, variable duration. In the final frame, two similar discs (C for centre and R for right) are then shown in a shifted position. Depending on various factors including the time intervals between frames as well as spacing and layout, observers perceive either element motion, in which L appears to move to R while C remains stationary or they report experiencing group motion, in which L and C appear to move together to C and R. Both element motion and group motion can be observed in animated examples to the right in Figures 1 and 2.
Overview
190px|thumbnail|Figure 3 – frames of a Ternus motion display – as shown in Dodd (2005)In 1926 and then again in 1938, the Gestalt psychologist Joseph Ternus observed and defined the "problem of phenomenal identity". Ternus' research was based around earlier undertakings in the domain by Pikler in 1917. This problem of phenomenal identity that Ternus had discovered occurs due to the human visual system's natural ability to establish and then preserve the entities of objects even when the defining attributes of those objects have undergone drastic changes and no longer resemble what they once did. The effect that Ternus had observed was in fact a bistable percept or perception of apparent motion which he found using a display consisting of three frames presented sequentially. Research suggests that these variations in apparent motion are achieved by grouping the visual elements in such a way that there is an intertwining of the perception of motion and the perception of the objects identity.
At intermediate ISIs, perceived motion is bistable, meaning that for the observer the perceptual experience interchanges between element motion and group motion in a spontaneous manner. While the bistability is present, observers are still capable of judging which percept leaves more of a lasting impression. As aforementioned the two percepts are never experienced simultaneously. This occurs due to intermediate ISIs yielding different percentages of group movement and element movement that are each dependent upon their exact values. 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
190px|thumbnail|Figure 5 – 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.
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.
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.
See also
- Beta movement
- Stroboscopic effect
- Apparent motion
- Persistence of vision
- Gestalt psychology