Ternus illusion
illusion of apparent movement
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 brief starts where responsible research should: with the source description of “Ternus illusion” as illusion of apparent movement. Everything that follows is an evidence route, not borrowed authority.
Why this record matters
The subject matters to the history & society register because the source frames it as illusion of apparent movement. Its deeper value depends on whether names, dates, institutions and citations support that framing.
Contemporary correspondence, administrative files and participant testimony can test how later narratives organized the event or institution. The source revision retrieved here is dated Mar 14, 2025. The linked authority identifier is Q7702968. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1926, 1938 and 1917.
Official terminology may obscure informal participation, dissent or communities documented only indirectly. The lead is largely declarative, so disagreement and counter-evidence require a deliberate search beyond the opening account. Authority statements aid reconciliation but still require their own references, qualifiers and ranks to be checked.
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Questions for further research
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This entry incorporates text from “Ternus illusion” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.