Processing Speed
Crossed-Uncrossed Difference Task
Also called: Poffenberger paradigm, Poffenberger task, CUD, interhemispheric transfer time task, simple reaction time to lateralized flashes
Measure the extra time a key press takes when the hand is on the other side from a brief flash, the crossed-uncrossed difference of the Poffenberger paradigm, read across a group.
11-14 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. Try it here first, with no account.
- Papers, 2021 to 2025
- 12
- Evidence grade
- C
- Registered clinical studies
- 0
What it measures
Measures the extra time a key press takes when the hand is on the other side from a brief flash, the crossed-uncrossed difference of the Poffenberger paradigm. A small square flashes to the left or right of a cross, and participants press F with the left hand or J with the right, one hand per part. The difference is a few milliseconds and is read across a group, not for one person.
Constructs
- Interhemispheric transfer
- Visuomotor integration
- Simple reaction time
Research uses
- Corpus callosum and hemispheric research, at the group level
- Laterality and handedness research
- Aging and white matter research, at the group level
- Within-study group comparison
How the paradigm works
The crossed-uncrossed difference task measures how much longer a key press takes when the hand is on the other side from a flash, the crossed-uncrossed difference of the Poffenberger paradigm (Poffenberger, 1912; Berlucchi and colleagues, 1971). Each half of the brain sees the opposite side of space and moves the opposite hand, so a flash on the left reaches the right half first. A press with the left hand can start in that half (uncrossed); a press with the right hand needs the signal to cross to the other half, mostly through the corpus callosum (crossed). In healthy adults crossed presses are about 3 ms slower: the weighted mean of control samples was 2.86 ms in Westerhausen's (2023) meta-analysis, against 43.5 ms in people whose corpus callosum had been cut.
More about the paradigm
A cross marks the middle of a dark panel. After 500 to 1,500 ms a small white square flashes for 50 ms, 300 px to its left or right, and the participant presses F with the left index finger or J with the right as fast as possible, one hand per part. The screen names the hand before each part, and the side is random within a part, so the eyes gain nothing by leaving the cross. The standard session has 8 parts of 46 trials, with the hands in the order A, B, B, A, A, B, B, A and the first hand drawn from the session's seed; each part has 40 flashes, 2 fixation checks and 4 catch trials with no flash. On a fixation check a digit from 2 to 9 fills the gap in the middle of the cross while the square is shown, and afterwards the participant types it, or presses the space bar after seeing none: a small digit there is hard to name unless the eyes are on the cross. 12 practice trials with feedback come first, 6 with each hand. The displays switch on screen frames, and the measured wait and flash duration are saved.
The main score is the crossed-uncrossed difference: the mean response time of the two crossed cells (a left square with the right hand, a right square with the left hand) minus the two uncrossed cells, each cell counting equally. A faster hand or a faster side cancels out, and so do delays of the screen, the browser and the keyboard, which are the same in all four cells or belong to one key or one side. The difference is also given from medians and for each side, with the four cell means, the share of squares answered with the part's key, wrong keys, presses on catch trials and before the flash, and the share of fixation checks named. Fixation check trials are left out of the response times, and response times count presses of the part's key at or after 100 ms from the flash with no press before it.
Scores
- Crossed-Uncrossed Differencems
- Mean response time of the two crossed cells (left square with the right hand, right square with the left hand) minus the two uncrossed cells, each cell counting equally.
- Positive values mean slower crossed presses; read across a group
- Crossed-Uncrossed Difference, Mediansms
- The same from the median of each cell.
- Positive values mean slower crossed presses; read across a group
- Crossed Response Timems
- Mean of the two crossed cells' mean response times.
- Lower is faster
- Uncrossed Response Timems
- Mean of the two uncrossed cells' mean response times.
- Lower is faster
- Difference, Left Squaresms
- For squares on the left: right hand minus left hand.
- Includes any difference in speed between the hands
- Difference, Right Squaresms
- For squares on the right: left hand minus right hand.
- Includes any difference in speed between the hands
- Left Square, Left Handms
- Mean response time for a left square with the left hand.
- Lower is faster
- Left Square, Right Handms
- Mean response time for a left square with the right hand.
- Lower is faster
- Right Square, Left Handms
- Mean response time for a right square with the left hand.
- Lower is faster
- Right Square, Right Handms
- Mean response time for a right square with the right hand.
- Lower is faster
- Mean Response Timems
- Mean response time over all scored squares.
- Lower is faster
- Squares Answered%
- Share of scored squares answered with the part's key at or after 100 ms, with no press before the square.
- Higher is better
- Missed Squarescount
- Scored squares with no press within 1,000 ms.
- Lower is better
- Wrong Keyscount
- Squares answered with the other hand's key.
- Over 10% of squares is flagged
- Presses Without a Square%
- Share of catch trials with a press.
- Lower is better; over a quarter is flagged
- Presses Before the Squarecount
- Trials with a press before the square appeared, or before the moment it would have appeared on a catch trial.
- Lower is better
- Anticipationscount
- Presses under 100 ms from the square's onset.
- Lower is better
- Fixation Checks Named%
- Share of fixation checks on which the digit in the cross was typed correctly.
- Under half is flagged: the eyes may have left the cross
- Fixation Checkstrials
- Fixation checks answered.
- Informational
- Fewest Responses in a Celltrials
- Usable responses in the cell of hand and side with the fewest.
- Fewer than 20 is flagged
- Flash Durationms
- Median measured time the square was on the screen.
- Informational; the plan is 50 ms
- Frame Intervalms
- Median time between screen frames, about 16.7 ms on a 60 Hz screen.
- Informational
- Long Flashes%
- Share of flashes shown a frame or more too long, a sign of missed screen frames.
- Over 10% is flagged
- Scored Squarestrials
- Trials with a square, fixation checks left out, in the scored run.
- Informational
- Catch Trialstrials
- Trials with no square in the scored run.
- Informational
- Total Trialstrials
- Trials in the scored run.
- Informational
Use and evidence
12 papers from 2021 to 2025 name this task in their title or abstract, down 37% compared with 2016 to 2020. That ranks it 444 of 495 test families in our census. See the counts for every test.
Show the numbers
| Year | Papers |
|---|---|
| 2025 | 0 |
| 2024 | 3 |
| 2023 | 2 |
| 2022 | 5 |
| 2021 | 2 |
| 2020 | 0 |
| 2019 | 3 |
| 2018 | 5 |
| 2017 | 7 |
| 2016 | 4 |
| 2015 | 2 |
| 2014 | 2 |
| 2013 | 4 |
| 2012 | 2 |
| 2011 | 4 |
| 2010 | 7 |
| 2009 | 3 |
| 2008 | 4 |
| 2007 | 5 |
| 2006 | 8 |
| 2005 | 4 |
| 2004 | 6 |
| 2003 | 2 |
| 2002 | 5 |
| 2001 | 3 |
| 2000 | 4 |
Use grade E
Fewer than 30 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
- 6
- Registered clinical studies using it as an outcome
- 0
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 0.22.0, counted 2 November 2026 from OpenAlex and ClinicalTrials.gov.
Task design
The trial sequence as the test runs in ConductCognition.
- Display
- A cross in the middle of a dark panel, and a 40 px white square 300 px to its left or right, on the cross's row.
- Wait
- 500 to 1,500 ms of the cross alone, drawn for each trial.
- Flash
- The square for 50 ms in whole screen frames, then the cross alone until a press or 1,000 ms.
- Hands
- F with the left index finger or J with the right, one hand per part, named on the screen before it. The side is random within a part.
- Parts
- 8 parts of 46 trials (standard; 4, 6, 10 or 12 parts), half with each hand in the order A, B, B, A: in each part 40 squares, 2 fixation checks and 4 trials with no square.
- Fixation checks
- A digit from 2 to 9 fills the middle of the cross while the square is shown, and afterwards the participant types it, or presses the space bar after seeing none. These trials are left out of the score.
- Practice
- 12 trials, 6 with each hand, with feedback for 800 ms. No feedback in the test.
- Score
- Mean response time of the crossed cells minus the uncrossed cells, each of the four cells of hand and side counting equally.
The difference is about 3 ms, so one person's value is not meaningful; compare groups of about 60 or more. Delays of the screen, the browser and the keyboard are the same in all four cells or belong to one key or one side, so they cancel out of the difference; timing jitter adds a little noise. The square is shown on whatever screen the participant has, so its distance from the cross in degrees varies between people.
Quality checks
Each session is marked valid, needs review or invalid, with the reason, before you see its scores.
Scored rows: crossed_uncrossed_test from the scored run, press the part's key (F with the left hand, J with the right) for a square on either side, and nothing on the trials with no square (4 in each part of 46), within 1,000 ms. Responding mainly on trials with no square invalidates; not separating squares from empty trials is flagged. Missed responses and responses under 100 ms follow the timed-test rule. Presses before the square are held to the same limit as responses under 100 ms. Presses on over half the trials with no square invalidate and on over a quarter are flagged. Fewer than 20 usable responses for any hand and side, the other hand's key on over 10% of squares, fewer than half the numbers named on the fixation checks, and a square shown a frame or more too long on over 10% of trials are flagged.
Why these rules
Responses concentrated on trials that needed none (one-sided Fisher exact test, p < .05) mean the instructions were reversed and invalidate; responses that do not separate the two trial types are flagged. Responding is the common, prepotent response here, so missing more than max(3, 25%) of response trials signals non-engagement (the platform's timed-test rule); any miss is flagged. Missed responses and responses under 100 ms keep the platform's timed-test rule: more than max(3, 25%) missed responses or more than max(2, 20%) responses under 100 ms invalidate, and any is flagged. A response under 100 ms cannot be a reaction to the stimulus. The difference is a contrast of four cells of hand and side measured in milliseconds, so each cell needs enough responses; a press of the other key may mean the other hand was used; the numbers in the cross show whether the eyes stayed on it; and a square kept on the screen too long means the browser missed frames. The limits of 20 responses, 10% wrong keys, half the numbers and 10% of squares are ConductScience choices, and the catch-trial limits follow the inhibition of return task's.
No percentile. Scores and quality flags only.
Running this test
Settings you can change
| Setting | Standard | Allowed |
|---|---|---|
| Parts | 8 | 4, 6, 8, 10, 12 |
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 Crossed-Uncrossed Difference Task in a web browser (ConductCognition task version 1.0.0, scoring version 1.0.0; Poffenberger, 1912; Berlucchi, Heron, Hyman, Rizzolatti and Umiltà, 1971), with the standard settings (parts: 8). The main score was crossed-uncrossed difference (mean response time of the two crossed cells (left square with the right hand, right square with the left hand) minus the two uncrossed cells, each cell counting equally).
ConductCognition versions: task version 1.0.0, scoring version 1.0.0. Every export records the versions each session used, for a methods section.
- Poffenberger, A. T. (1912). Reaction time to retinal stimulation, with special reference to the time lost in conduction through nerve centers. Archives of Psychology, 23, 1-73. The first report of the crossed-uncrossed difference.
- Berlucchi, G., Heron, W., Hyman, R., Rizzolatti, G., & Umiltà, C. (1971). Simple reaction times of ipsilateral and contralateral hand to lateralized visual stimuli. Brain, 94(3), 419-430. https://doi.org/10.1093/brain/94.3.419
- Marzi, C. A., Bisiacchi, P., & Nicoletti, R. (1991). Is interhemispheric transfer of visuomotor information asymmetric? Evidence from a meta-analysis. Neuropsychologia, 29(12), 1163-1177. https://doi.org/10.1016/0028-3932(91)90031-3A meta-analysis of 16 studies, with a left side advantage and a right hand advantage.
- Iacoboni, M., & Zaidel, E. (2000). Crossed-uncrossed difference in simple reaction times to lateralized flashes: Between- and within-subjects variability. Neuropsychologia, 38(5), 535-541. https://doi.org/10.1016/S0028-3932(99)00121-9Why one session's difference does not describe one person.
- Westerhausen, R. (2023). Interhemispheric integration after callosotomy: A meta-analysis of Poffenberger and redundant-target paradigms. Neuropsychology Review, 33(4), 872-890. https://doi.org/10.1007/s11065-022-09569-wThe size of the difference: 2.86 ms in control samples and 43.5 ms after callosotomy.
- Wolski, P., & Asanowicz, D. (2023). Does CUD measure interhemispheric transfer time? The allocation of attention influences the Poffenberger effect. Neuropsychologia, 185, 108581. https://doi.org/10.1016/j.neuropsychologia.2023.108581Attention to one side changes the difference, and its reliability is too low for individual differences.
Source papers
- Westerhausen R (2023). Interhemispheric integration after callosotomy: a meta-analysis of Poffenberger and redundant-target paradigms. Neuropsychology Review, 33(4):872-890.Paradigm source, Adults
Questions
How long does the Crossed-Uncrossed Difference Task take?
About 11 to 14 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?
Crossed-Uncrossed Difference, Crossed-Uncrossed Difference, Medians, Crossed Response Time, Uncrossed Response Time, Difference, Left Squares, Difference, Right Squares, Left Square, Left Hand, Left Square, Right Hand, Right Square, Left Hand, Right Square, Right Hand, Mean Response Time, Squares Answered, Missed Squares, Wrong Keys, Presses Without a Square, Presses Before the Square, Anticipations, Fixation Checks Named, Fixation Checks, Fewest Responses in a Cell, Flash Duration, Frame Interval, Long Flashes, Scored Squares, Catch Trials and Total Trials. Each session is also marked valid, needs review or invalid, with the reason.
How widely is the Crossed-Uncrossed Difference Task used in research?
12 papers from 2021 to 2025 name it in their title or abstract, down 37% compared with 2016 to 2020. That ranks it 444 of 495 test families in the ConductCognition test census (use grade E). Source: ConductCognition test census 0.22.0, counted 2 November 2026 from OpenAlex and ClinicalTrials.gov.
How much reliability evidence does the Crossed-Uncrossed Difference Task have?
In the published research, 2 papers report its test-retest reliability and 0 registered clinical studies use 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?
Parts (4, 6, 8, 10, 12). 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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