physics

Ternus illusion

Ternus illusion is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Ternus illusion rather than just read about it. In short: The 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.

Key takeaways

Ternus illusion belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ternus illusion to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ternus illusion from memory before moving on to harder problems.

Reference excerpt

The 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 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.

Element motion

Element motion can be observed when ISI’s are presented for less than 50 milliseconds. Though the most common time frame used to achieve this illusion is usually around 20 milliseconds. Element motion is characterized as the outer disc 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.

… excerpt ends here. Continue reading the full article.

Illustrations

Ternus illusion: Figure 1 – element motion in effect — Figure 1 – element motion in effect

Ternus illusion: Figure 2 – group motion in effect — Figure 2 – group motion in effect

Ternus illusion: Figure 4 – element motion — Figure 4 – element motion

Ternus illusion: Figure 5 – group motion — Figure 5 – group motion

Worked examples

Example 1 — a first encounter with Ternus illusion

Start with the simplest possible case. Write down what Ternus illusion claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ternus illusion before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ternus illusion ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ternus illusion

In research: Ternus illusion appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ternus illusion in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ternus illusion is common in secondary-school and first-year university syllabi. It links to neighbouring topics Optical illusions, so understanding it makes those chapters shorter.
In everyday life: Look for Ternus illusion outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Ternus illusion in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ternus illusion means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ternus illusion out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ternus illusion in simple terms?

Why does Ternus illusion matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ternus illusion?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ternus illusion.