Attention
Illusory Contours Task
Also called: Kanizsa square task, Kanizsa figures, fat/thin task, illusory shape discrimination, visual shape completion task, subjective contours, contour interpolation
Measure visual shape completion: how much better people judge a square made by notched discs, with no sides drawn, than the same discs when no shape forms.
6-8 minTimed: desktop or laptop with a mouse or trackpad
Free to start: 10 free participants per account, then $0.99 per participant for academic research, or any yearly plan. Preview any test free after you sign up.
- Papers, 2021 to 2025
- 152
- Evidence grade
- C
- Registered clinical studies
- 1
What it measures
A measure of visual shape completion. Four notched discs flash for 200 ms. In one part their notches line up into a square whose sides are not drawn, and participants judge whether it is fat or thin; in the other the notches all face down, so no square forms, and they judge whether the discs are turned left or right. The score is how much better they do when the edges complete into a shape.
Constructs
- Visual shape completion
- Illusory contour perception
- Perceptual organization
Research uses
- Schizophrenia and psychosis research
- Bipolar disorder research
- Perceptual organization research
How the paradigm works
The Illusory Contours Task measures visual shape completion: how well the visual system joins separate edges into the outline of a shape. Four discs, each with a quarter cut out, sit at the corners of a square. In the illusory part the notches face the square's center, so their edges line up into its corners and most people see a square whose sides are not drawn (Kanizsa, 1976). The discs are turned a little, so the completed sides bend: out at the left and right (fat) or in (thin), and participants judge which (Ringach and Shapley, 1996). In the fragmented part the same discs face down, so no edges line up and no square forms, and participants judge whether all four are turned left or right. Both parts use the same discs, places and turns, so they differ only in whether the edges complete into a shape (Keane and colleagues, 2014, 2019).
More about the paradigm
Each trial shows a point to look at for 1,000 ms, the discs for 200 ms, a blank for 50 ms and a mask for 300 ms, then the question, answered with an arrow key with no time limit. The discs turn by 2, 3, 4.5, 7 or 10 degrees, 10 times each in each part (standard; 6 to 16), for 100 trials in a seeded order; the two parts run in a seeded order, each after 6 practice trials with feedback. Every turn and display time is fixed before the session: there is no staircase. Displays switch on screen frames, the measured durations are saved on every trial, and trials more than one frame off plan leave the scores. Desktop and laptop only.
The main score is the completion advantage: percent correct in the illusory part minus percent correct in the fragmented part. Keane and colleagues use this difference, as thresholds or as accuracy, to separate shape completion from the orientation judgment, attention and memory that both parts share, and found it smaller in schizophrenia than in healthy controls. Accuracy at each turn in each part, and on the control at the largest turn, are also reported. The Visual Illusions Test measures how much surrounding shapes change the judged size of lines and circles; this test measures whether edges that are not drawn are completed into a shape. Scores support research and do not diagnose.
Scores
- Completion Advantage%
- Percent correct in the illusory part minus percent correct in the fragmented part, over all turns, in percentage points. Null unless each part has 20 usable trials.
- Positive values mean the completed shape helped; larger values mean a larger benefit
- Accuracy, Illusory Part%
- Share right on the fat or thin judgments.
- Higher is better
- Accuracy, Fragmented Part%
- Share right on the left or right judgments.
- Higher is better
- Illusory Accuracy by Turn%
- Share right at each turn of the discs in the illusory part, keyed by the turn in degrees.
- Rises with the turn
- Fragmented Accuracy by Turn%
- Share right at each turn of the discs in the fragmented part, keyed by the turn in degrees.
- Rises with the turn
- Control Accuracy at the Largest Turn%
- Share right in the fragmented part at the largest turn, 10 degrees; null with fewer than 6 usable trials there.
- Under 60% is invalid and under 80% is flagged for review
- Median Answer Timems
- Median time from the question to the answer.
- Informational; the answer has no time limit
- Anticipationscount
- Answers within 100 ms of the question appearing.
- Informational
- Early Key Pressescount
- Arrow keys pressed before the question appeared, which are not taken as answers.
- Informational
- Frame Intervalms
- Median time between screen frames during the trials.
- About 16.7 on a 60 Hz screen
- Refresh RateHz
- Screen refresh rate from the frame interval.
- Informational
- Displays on Time%
- Share of answered trials whose figure and figure-to-mask interval each ran within one frame of plan; only these are scored.
- Under 0.9 is flagged for review
- Completed Trialstrials
- Trials answered in the scored run.
- Fewer than planned is flagged for review
- Planned Trialstrials
- Trials the run set out to show.
- Informational
Use and evidence
152 papers from 2021 to 2025 name this task in their title or abstract, down 18% compared with 2016 to 2020. That ranks it 148 of 395 test families in our census. See the counts for every test.
Show the numbers
| Year | Papers |
|---|---|
| 2025 | 52 |
| 2024 | 26 |
| 2023 | 28 |
| 2022 | 16 |
| 2021 | 30 |
| 2020 | 35 |
| 2019 | 44 |
| 2018 | 38 |
| 2017 | 33 |
| 2016 | 36 |
| 2015 | 41 |
| 2014 | 55 |
| 2013 | 37 |
| 2012 | 39 |
| 2011 | 57 |
| 2010 | 123 |
| 2009 | 40 |
| 2008 | 60 |
| 2007 | 43 |
| 2006 | 35 |
| 2005 | 35 |
| 2004 | 45 |
| 2003 | 43 |
| 2002 | 34 |
| 2001 | 43 |
| 2000 | 40 |
Use grade C
100 to 299 papers from 2021 to 2025.
Evidence grade C
At least 2 test-retest papers, 30 reliability or validity papers or 3 registered clinical studies.
- Papers on test-retest reliability
- 2
- Papers on reliability or validity
- 42
- Registered clinical studies using it as an outcome
- 1
How the grades work
Use: A is 1,000 or more papers from 2021 to 2025, B 300 to 999, C 100 to 299, D 30 to 99 and E fewer than 30. Evidence: A means at least 25 papers report its test-retest reliability and at least 25 registered clinical studies use it as an outcome; B, at least 8 test-retest papers, and either 5 registered clinical studies or 150 reliability or validity papers; C, at least 2 test-retest papers, 30 reliability or validity papers or 3 registered clinical studies; D, little published measurement evidence found. The counts measure use. They include every paper that names the task in its title or abstract. Grades describe the published research on this task, not ConductCognition's version of it.
Source: ConductCognition test census 0.14.0, counted 2 November 2026 from OpenAlex and ClinicalTrials.gov.
Task design
The trial sequence as the test runs in ConductCognition.
- Display
- On a dark panel: a point to look at for 1,000 ms, four discs with a quarter cut out for 200 ms, a blank for 50 ms, then a mask for 300 ms: the four discs whole and 16 short lines over the square.
- Figure
- The discs sit at the corners of a square 240 px wide, and each disc is 80 px across, a third of the side. In the illusory part each notch faces the square's center, so its edges line up into the square's corners; in the fragmented part every notch faces down.
- Turns
- Every disc turns by 2, 3, 4.5, 7 or 10 degrees. In the illusory part they turn alternately clockwise and counterclockwise, so the completed square is thin or fat; in the fragmented part they all turn the same way, left or right.
- Answer
- Press the left arrow for thin or turned left and the right arrow for fat or turned right, with no time limit. Pictures of the two answers stay on the question screen. Keys pressed before the question are counted and not taken as answers.
- Trials
- 10 at each turn in each part (standard; 6 to 16): 100 trials, 50 per part, in a seeded order, with the two answers balanced at each turn. The two parts run in a seeded order.
- Practice
- 6 trials before each part, with feedback for 1,000 ms: the discs show for 800, 800, 400, 400, 200 and 200 ms, turned by 10 and then 7 degrees.
- Fixed times
- Every turn and display time is set before the session, and no answer changes it: there is no staircase. Displays are shown in whole frames of the screen's own rate.
- Score
- Percent correct in the illusory part minus percent correct in the fragmented part. Trials whose displays ran more than one frame off plan leave the score.
Ringach and Shapley, and later Keane and colleagues, measured the turn needed for a set accuracy, the latter with an adaptive procedure. This version shows five fixed turns (the method of constant stimuli) and scores accuracy, as Keane and colleagues did with two fixed turns in a scanner. The discs, the mask and the answer pictures are drawn by our code.
Quality checks
Each session is marked valid, needs review or invalid, with the reason, before you see its scores.
Scored rows: illusory_contours_test from the scored run, a choice of two answers (thin or fat, or turned left or right) after a 200 ms figure and a mask, with no time limit (chance 50%). Below chance invalidates and not above chance is flagged; one arrow on 90% of answers invalidates when accuracy is not above chance. Under 60% right on the fragmented control at the largest turn, on at least six such trials, invalidates and under 80% is flagged. No completion score, a run short of its planned trials, or fewer than 90% of displays within one frame of plan is flagged for review. Answer times are not checked: the answer comes after the mask, when it may already be decided.
Why these rules
Accuracy significantly below chance (one-sided exact test, p < .05) cannot come from guessing; it means the response mapping was reversed or the task was not followed (Slick, Sherman and Iverson 1999 treat below-chance forced-choice performance as definite response bias). Chance-level accuracy can be a real result in impaired participants, so it is only flagged. One option on at least 90% of answered trials, when the correct options are balanced and accuracy is not above chance, means the answers did not depend on the items (the task counterpart of long-string responding in surveys; Curran 2016). At the largest turn the fragmented discs are plainly turned, and that part needs no completed shape, so errors there mean the discs could not be seen or the task was not followed; a person who completes shapes poorly can still be near chance on the illusory part, so that part is not checked. The display times carry the score, so trials whose displays ran late leave it.
No percentile. Scores and quality flags only.
Running this test
Settings you can change
| Setting | Standard | Allowed |
|---|---|---|
| Trials at each turn | 10 | 6 to 16 |
Yearly plans can change settings within these limits. Free and pay-as-you-go studies use the standard settings.
Need a setting this test does not have?
Devices and languages
- Timed test: participants need a desktop or laptop with a mouse or trackpad. Participant devices
- Languages: English, Spanish, French, German, Italian, Portuguese (Brazil), Chinese (Simplified) and Japanese.
- Available on: Every plan, including the free plan.
- No percentile. Scores and quality flags only. How norms work
- Recruiting online? See the setup guides for Prolific, Sona Systems, CloudResearch Connect and Qualtrics.
Cite and describe
Participants completed the Illusory Contours Task in a web browser (ConductCognition task version 1.0.0, scoring version 1.0.0; Ringach and Shapley, 1996; Keane, Joseph and Silverstein, 2014), with the standard settings (trials at each turn: 10). The main score was completion advantage (percent correct in the illusory part minus percent correct in the fragmented part, over all turns, in percentage points).
ConductCognition versions: task version 1.0.0, scoring version 1.0.0. Every export records the versions each session used, for a methods section.
- Kanizsa, G. (1976). Subjective contours. Scientific American, 234(4), 48-52. https://doi.org/10.1038/scientificamerican0476-48Notched discs whose edges line up into a shape that is seen though it is not drawn.
- Ringach, D. L., & Shapley, R. (1996). Spatial and temporal properties of illusory contours and amodal boundary completion. Vision Research, 36(19), 3037-3050. https://doi.org/10.1016/0042-6989(96)00062-4The fat or thin judgment of a square from turned discs, against discs all facing one way, with brief displays and masks.
- Keane, B. P., Joseph, J., & Silverstein, S. M. (2014). Late, not early, stages of Kanizsa shape perception are compromised in schizophrenia. Neuropsychologia, 56, 302-311. https://doi.org/10.1016/j.neuropsychologia.2014.02.001The fragmented control with every notch facing down, discs a third of the square's side, and the 200 ms figure, 50 ms blank and 300 ms mask.
- Keane, B. P., Paterno, D., Kastner, S., Krekelberg, B., & Silverstein, S. M. (2019). Intact illusory contour formation but equivalently impaired visual shape completion in first- and later-episode schizophrenia. Journal of Abnormal Psychology, 128(1), 57-68. https://doi.org/10.1037/abn0000384Shape completion as the advantage of the illusory part over the fragmented one.
- Keane, B. P., Barch, D. M., Mill, R. D., Silverstein, S. M., Krekelberg, B., & Cole, M. W. (2021). Brain network mechanisms of visual shape completion. NeuroImage, 236, 118069. https://doi.org/10.1016/j.neuroimage.2021.118069Fixed turns of 3 and 10 degrees scored as accuracy in each part.
- Keane, B. P., Erlikhman, G., Serody, M. B., & Silverstein, S. M. (2022). A brief psychometric test reveals robust shape completion deficits in schizophrenia that are less severe in bipolar disorder. Schizophrenia Research, 240, 78-80. https://doi.org/10.1016/j.schres.2021.12.015
- Shipley, T. F., & Kellman, P. J. (1992). Strength of visual interpolation depends on the ratio of physically specified to total edge length. Perception & Psychophysics, 52(1), 97-106. https://doi.org/10.3758/BF03206762The support ratio, the share of each side the discs draw.
Source papers
- Keane BP, Paterno D, Kastner S, Krekelberg B, Silverstein SM (2019). Intact illusory contour formation but equivalently impaired visual shape completion in first- and later-episode schizophrenia. Journal of Abnormal Psychology, 128(1):57-68.Paradigm source, Adults
Questions
How long does the Illusory Contours Task take?
About 6 to 8 minutes, as listed in the test library.
What device does a participant need?
Timed test: participants need a desktop or laptop with a mouse or trackpad. The browser must be Chrome, Edge, Firefox or Safari.
What scores does it produce?
Completion Advantage, Accuracy, Illusory Part, Accuracy, Fragmented Part, Illusory Accuracy by Turn, Fragmented Accuracy by Turn, Control Accuracy at the Largest Turn, Median Answer Time, Anticipations, Early Key Presses, Frame Interval, Refresh Rate, Displays on Time, Completed Trials and Planned Trials. Each session is also marked valid, needs review or invalid, with the reason.
How widely is the Illusory Contours Task used in research?
152 papers from 2021 to 2025 name it in their title or abstract, down 18% compared with 2016 to 2020. That ranks it 148 of 395 test families in the ConductCognition test census (use grade C). Source: ConductCognition test census 0.14.0, counted 2 November 2026 from OpenAlex and ClinicalTrials.gov.
How much reliability evidence does the Illusory Contours Task have?
In the published research, 2 papers report its test-retest reliability and 1 registered clinical study uses it as an outcome (evidence grade C in the ConductCognition test census).
Does it report a percentile?
No. Scores and quality flags only.
Which settings can a researcher change?
Trials at each turn (6 to 16). Settings can be changed on yearly plans; free and pay-as-you-go studies use the standard settings.
Which languages can participants use?
Participants can take this test in English, Spanish, French, German, Italian, Portuguese (Brazil), Chinese (Simplified) and Japanese. English is on every plan, and the other languages come with yearly plans. Have a native speaker read the machine-translated text before launch.
Is this a clinical test?
No. ConductCognition is for research use only. It runs, scores and exports the test; clinical interpretation is outside the platform.
Every test: the full test list.
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