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The Ternus illusion, also commonly referred to as the 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== 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.〔 When observers are presented with two immobile stimuli, that are presented in a sequential fashion at two differing locations, the stimuli will often be perceived as a solitary object that is simply moving from a starting location to another position. This apparent motion or apparent movement is of great interest to researchers because the perceived movement does not derive strictly from the physical aspect of vision such as the stimulation caused by impingement on the retina. Instead, apparent motion appears to arise from the visual system's processing of the physical properties of the percept. It is for this reason that apparent motion is a key area of research in the domain of vision research.〔 The Ternus illusion is perhaps one of the best examples of such an effect. In order to observe the Ternus illusion/effect, observers view an apparent motion display known as a Ternus display.〔 The Ternus display features a series of frames that are separated by what is known as a blank interstimulus interval (ISI). A standard Ternus display consists of three frames, sequentially presented to the observer. As can be seen in Figure 3, Frame 1 consists of three equally spread out discs that are laid out in a horizontal fashion. Frame 2 is the blank ISI which separates Frame 1 and 3 for a variable duration. Frame 3, is simply the reverse of Frame 1 with the discs on the right hand side instead. This means that the disc on the outside of Frame 1 will now appear to be in the location that the centre disc was originally in as part of Frame 1.〔 When these three frames are quickly presented in a sequential manner, individuals report perceiving two discrete types of motion from observing the Ternus display. These different perceptions are dependent on the duration of the ISI. Numerous studies have demonstrated that short ISIs cause the observer to perceive the central elements as immobile with one outside element jumping across those elements, known as element motion. These studies also support the finding that longer ISIs create the perception that the elements are all moving as one from left to right, known as group motion and that these percepts are not capable of occurring simultaneously. 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ternus illusion」の詳細全文を読む スポンサード リンク
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