Attention
Visual Crowding Test
Also called: crowding task, critical spacing, Bouma's law, flanked letter identification, peripheral crowding, crowding effect
Measure crowding: how far apart neighboring shapes must be before a shape seen out of the corner of the eye can be named.
5-6 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
- 196
- Evidence grade
- C
- Registered clinical studies
- 4
What it measures
Measures how far apart neighboring shapes must be before they stop a person from naming a shape seen out of the corner of the eye. While the participant looks at a central cross, three T shapes in a row, or one alone, flash for 150 ms to its left or right, with the neighbors at one of six fixed spacings. Participants press the arrow key for the side of the middle T's bar. The score is the spacing, as a share of the distance from the cross, at which 75 percent of answers are right.
Constructs
- Visual crowding
- Peripheral object recognition
- Critical spacing
Research uses
- Reading and dyslexia research
- Amblyopia and low vision research
- Visual perception and aging research
How the paradigm works
The Visual Crowding Test measures crowding: how close neighboring shapes can come before they stop a person from naming a shape seen away from the point they look at. The participant keeps their eyes on a cross while three T shapes in a row flash for 150 ms to its left or right, the middle one 240 px from the cross on a 960 px display. Each T has its bar on one of four sides, and the participant presses the arrow key for the side of the middle T's bar, with no time limit. The neighbors sit on the same line, one nearer the cross and one farther, at one of six spacings from 0.2 to 0.8 of the middle T's distance from the cross, and their bars are never on the middle T's side. On some trials the middle T shows alone. The flash is shorter than the time the eyes need to move to the shapes, and its side changes at random; gaze is not tracked, so the test relies on the instruction to keep the eyes on the cross.
More about the paradigm
Every spacing and display time is fixed before the session: each spacing, and the middle T alone, appears 16 times (standard; 10 to 24) in a seeded order, with the sides and bars balanced, for 112 trials in about 3 to 4 minutes after 6 practice trials with feedback. Nothing a participant answers changes a spacing: there is no staircase. Flashes switch on screen frames through the frame-timed helper, 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 critical spacing: the spacing, as a share of the distance from the cross, at which 75 percent of answers are right, from a monotone fit to the share right at each spacing, interpolated on log spacing. Critical spacing grows in proportion to the distance from the point looked at (Bouma's law), so the share changes little with screen size or viewing distance; lower means less crowding. Accuracy at each spacing, with the middle T alone and on each side of the cross, and how often a wrong answer named a neighbor's bar are also reported. Scores support research and do not diagnose.
Scores
- Critical Spacingscore
- The center-to-center spacing, as a share of the middle T's distance from the cross, at which 75 percent of answers are right, from a monotone fit to the share right at each spacing, interpolated on log spacing (Bouma's factor).
- Lower is better; neighbors could come closer before they got in the way
- Threshold at Widest Spacingflag
- 1 when even the widest spacing stayed under 75 percent right, so the score is the widest spacing and only a lower bound.
- 1 is flagged for review
- Threshold at Closest Spacingflag
- 1 when the closest spacing was already at or above 75 percent right, so the score is the closest spacing and an upper bound.
- 1 is noted for review
- Accuracy by Spacing%
- Share right at each spacing, keyed by the spacing as a share of the distance from the cross.
- Rises with the spacing
- Accuracy with Neighbors%
- Share right over all trials with neighbors.
- Informational
- Accuracy with the T Alone%
- Share right when the middle T showed alone.
- Under 50% is invalid and under 80% is flagged for review
- T Alone Trials Answeredtrials
- Trials with the middle T alone that were answered and ran on time.
- Informational
- Accuracy Left of the Cross%
- Share right with neighbors when the shapes were left of the cross.
- Informational
- Accuracy Right of the Cross%
- Share right with neighbors when the shapes were right of the cross.
- Informational
- Errors Naming a Neighbor%
- Share of wrong answers with neighbors that named the side of a neighbor's bar. The neighbors' bars are two of the three wrong sides, so guessing gives 2 in 3.
- Above 2 in 3 means a neighbor's bar was taken for the middle one
- Median Answer Timems
- Median time from the question to the answer.
- Informational; the answer has no time limit
- Anticipationscount
- Answers given within 100 ms of the question.
- 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
- Flashes on Time%
- Share of answered trials whose flash 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
196 papers from 2021 to 2025 name this task in their title or abstract, up 18% compared with 2016 to 2020. That ranks it 124 of 395 test families in our census. See the counts for every test.
Show the numbers
| Year | Papers |
|---|---|
| 2025 | 49 |
| 2024 | 34 |
| 2023 | 37 |
| 2022 | 34 |
| 2021 | 42 |
| 2020 | 31 |
| 2019 | 42 |
| 2018 | 32 |
| 2017 | 29 |
| 2016 | 32 |
| 2015 | 40 |
| 2014 | 41 |
| 2013 | 27 |
| 2012 | 32 |
| 2011 | 31 |
| 2010 | 32 |
| 2009 | 14 |
| 2008 | 3 |
| 2007 | 6 |
| 2006 | 4 |
| 2005 | 4 |
| 2004 | 4 |
| 2003 | 3 |
| 2002 | 2 |
| 2001 | 4 |
| 2000 | 2 |
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
- 3
- Papers on reliability or validity
- 23
- Registered clinical studies using it as an outcome
- 4
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 31 October 2026 from OpenAlex and ClinicalTrials.gov.
Task design
The trial sequence as the test runs in ConductCognition.
- Display
- On a dark panel 960 px wide: a cross for 600 ms, then for 150 ms a row of three T shapes to its left or right, the middle one 240 px from the cross, with the cross still showing. Each T is 36 px, with its bar on one of four sides.
- Spacings
- The neighbors sit one nearer the cross and one farther, at 0.2, 0.26, 0.35, 0.46, 0.61 or 0.8 of the middle T's distance from the cross, center to center. On some trials the middle T shows alone. The neighbors' bars are on two other sides, never the middle T's.
- Answer
- Press the arrow key for the side of the middle T's bar, with no time limit. Keys pressed before the question are counted and not taken as answers.
- Trials
- 16 at each spacing and 16 with the middle T alone (standard; 10 to 24 each): 112 trials in a seeded order, with the two sides and the four bars balanced within each.
- Practice
- 6 trials, 2 with the middle T alone and 2 at each of the two widest spacings, with feedback for 1,000 ms.
- Fixed times
- Every spacing and display time is set before the session, and no answer changes it: there is no staircase. The flash is shown in whole frames of the screen's own rate.
- Gaze
- Participants are asked to keep their eyes on the cross. Gaze is not tracked: the flash is shorter than the time the eyes need to move to the shapes, and its side changes at random.
- Score
- The spacing, as a share of the distance from the cross, at which 75 percent of answers are right, from a monotone fit to the share right at each spacing, interpolated on log spacing. Trials whose flash ran more than one frame off plan leave the score.
Crowding studies use letters, T shapes or other characters, often with a staircase. This version uses T shapes in four orientations and a fixed set of spacings (the method of constant stimuli), and gives the critical spacing as a share of the distance from the cross, which changes little with screen size and viewing distance.
Quality checks
Each session is marked valid, needs review or invalid, with the reason, before you see its scores.
Scored rows: visual_crowding_test from the scored run, a choice of the four sides of the middle T's bar after a 150 ms flash, with no time limit (chance 25%). Below chance invalidates and not above chance is flagged; one arrow on 90% of answers invalidates when accuracy is not above chance. Under 50% right with the middle T alone, on at least four such trials, invalidates and under 80% is flagged. No fitted spacing, a fit at the widest spacing, a run short of its planned trials, or fewer than 90% of flashes within one frame of plan is flagged for review. Answer times are not checked: the answer comes after the flash, when it may already be decided. Gaze is not tracked.
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). With the middle T alone its bar is plain to see from the cross, so errors on those trials mean the shapes could not be seen or the task was not followed. The flash time carries the score, so trials whose flash ran late leave it.
Running this test
Settings you can change
| Setting | Standard | Allowed |
|---|---|---|
| Trials at each spacing | 16 | 10 to 24 |
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.
- Recruiting online? See the setup guides for Prolific, Sona Systems, CloudResearch Connect and Qualtrics.
Cite and describe
Participants completed the Visual Crowding Test in a web browser (ConductCognition task version 1.0.0, scoring version 1.0.0; Bouma, 1970; Toet and Levi, 1992; Pelli and Tillman, 2008), with the standard settings (trials at each spacing: 16). The main score was critical spacing (the center-to-center spacing, as a share of the middle T's distance from the cross, at which 75 percent of answers are right, from a monotone fit to the share right at each spacing, interpolated on log spacing (Bouma's factor)).
ConductCognition versions: task version 1.0.0, scoring version 1.0.0. Every export records the versions each session used, for a methods section.
- Bouma, H. (1970). Interaction effects in parafoveal letter recognition. Nature, 226(5241), 177-178. https://doi.org/10.1038/226177a0Letters flanked by letters away from the point looked at.
- Toet, A., & Levi, D. M. (1992). The two-dimensional shape of spatial interaction zones in the parafovea. Vision Research, 32(7), 1349-1357. https://doi.org/10.1016/0042-6989(92)90227-AThree T shapes in a row, the middle one named, and the center-to-center spacing for 75 percent correct.
- Kooi, F. L., Toet, A., Tripathy, S. P., & Levi, D. M. (1994). The effect of similarity and duration on spatial interaction in peripheral vision. Spatial Vision, 8(2), 255-279. https://doi.org/10.1163/156856894X00350A T in one of four orientations among other T shapes; neighbors like the target crowd most.
- Pelli, D. G., & Tillman, K. A. (2008). The uncrowded window of object recognition. Nature Neuroscience, 11(10), 1129-1135. https://doi.org/10.1038/nn.2187Bouma's law: critical spacing grows in proportion to the distance from the point looked at, whatever the object.
- Pelli, D. G., Waugh, S. J., Martelli, M., Crutch, S. J., Primativo, S., Yong, K. X., Rhodes, M., Yee, K., Wu, X., Famira, H. F., & Yiltiz, H. (2016). A clinical test for visual crowding. F1000Research, 5, 81. https://doi.org/10.12688/f1000research.7835.1Crowding in amblyopia, dyslexia and reading.
Source papers
- Pelli DG, Tillman KA (2008). The uncrowded window of object recognition. Nature Neuroscience, 11(10):1129-1135.Paradigm source, Adults
Questions
How long does the Visual Crowding Test take?
About 5 to 6 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?
Critical Spacing, Threshold at Widest Spacing, Threshold at Closest Spacing, Accuracy by Spacing, Accuracy with Neighbors, Accuracy with the T Alone, T Alone Trials Answered, Accuracy Left of the Cross, Accuracy Right of the Cross, Errors Naming a Neighbor, Median Answer Time, Anticipations, Early Key Presses, Frame Interval, Refresh Rate, Flashes on Time, Completed Trials and Planned Trials. Each session is also marked valid, needs review or invalid, with the reason.
How widely is the Visual Crowding Test used in research?
196 papers from 2021 to 2025 name it in their title or abstract, up 18% compared with 2016 to 2020. That ranks it 124 of 395 test families in the ConductCognition test census (use grade C). Source: ConductCognition test census 0.14.0, counted 31 October 2026 from OpenAlex and ClinicalTrials.gov.
How much reliability evidence does the Visual Crowding Test have?
In the published research, 3 papers report its test-retest reliability and 4 registered clinical studies use it as an outcome (evidence grade C in the ConductCognition test census).
Which settings can a researcher change?
Trials at each spacing (10 to 24). 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.
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