Contrast threshold
Estimate the faintest striped patch for which you can still tell on which side it appeared.
Method
Procedure
- Keep your eyes on the small cross at the centre of the grey field.
- A striped patch appears for about 0.2 seconds inside the corner marks on the left or on the right.
- Say where it was. When you are not sure, guess: a guess is right half of the time.
- After two correct answers in a row the patch becomes fainter; after an error it becomes stronger. The run ends after 14 changes of direction, or 120 trials.
Responses
- F
- the patch was on the left
- J
- the patch was on the right
- Touch
- the Left and Right buttons
Measures
Your contrast threshold: the contrast at which you are right on about 70.7% of trials. Contrast here is how far the lightest and darkest parts of the stripes differ in brightness from the grey background, relative to that background, given as a percentage. A lower threshold means greater sensitivity.
Practice
Four practice trials, from a clearly visible patch to a fainter one, tell you after each trial where the patch was.
The result depends on the screen (its brightness, contrast and gamma), the viewing distance and the room lighting. It is not a clinical test of vision.
Terms
- Contrast: How much the stripes differ in brightness from the grey background, as a percentage.
- Threshold: The stimulus level at which a stimulus is detected on a set proportion of trials, here 70.7%.
- Staircase: A procedure that makes the stimulus fainter after correct answers and stronger after errors, so that it settles near the threshold.
- Reversal: A point at which the staircase changes direction, from fainter to stronger or the reverse.
- Psychometric function: The curve that relates the proportion of correct answers to the stimulus level.
- Visual cortex: The part of the cerebral cortex, at the back of the brain, that first processes visual information.
Experiment
Responses stay in this browser tab. Nothing is stored or sent, and everything is lost when the page is closed or reloaded.
Results
Interpretation
The page gives two estimates: one from the staircase reversals and one from the curve fitted to all your answers. They should be similar; a large difference suggests that the run was too short or that your answers were inconsistent. A lower value means you could locate fainter patches.
Limitations
The screen is not calibrated, so the result can be compared only with other runs on the same device under the same conditions. For the same reason, published thresholds are not given for comparison.
The result comes from one person in one session, on a device whose timing and display are not calibrated. It describes this run, not a stable characteristic of the participant, and a second run could differ by chance alone. Research studies test many participants under controlled conditions and report the average with its variability. The experiment is educational; it is not a test of ability or health.
One row per trial: contrast (%), side of the patch, your response, whether it was correct, how long the patch was shown and reaction time. In the file, 1 means yes and 0 means no. The file is created on this device and can be opened in a spreadsheet.
Background
Original findings
The patch is adjusted by a staircase. Levitt (1971) described transformed up-down methods: with the rule used here, two correct answers to make the patch fainter and one error to make it stronger, the staircase converges on the contrast at which 70.7% of answers are correct. The threshold is estimated from the contrasts at which the staircase changed direction. A psychometric function, the proportion correct against contrast, is then fitted to all trials by maximum likelihood, with the guessing rate fixed at 50% and a small rate of lapses allowed. Wichmann and Hill (2001) showed that estimates of threshold and slope can be seriously biased if such occasional lapses are not allowed for.
Neural basis
The detection of a small striped pattern of this kind is thought to depend largely on neurons in the primary visual cortex that respond to stripes of a particular orientation and width.
References
- Levitt, H. (1971). Transformed up-down methods in psychoacoustics. Journal of the Acoustical Society of America, 49(2B), 467–477. https://doi.org/10.1121/1.1912375
- Wichmann, F. A., & Hill, N. J. (2001). The psychometric function: I. Fitting, sampling, and goodness of fit. Perception & Psychophysics, 63(8), 1293–1313. https://doi.org/10.3758/BF03194544
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