How to play
Callback is a set of small perception tests. Each one takes about a minute. Everyone gets the same daily run, so you can compare scores with friends, and practice runs are unlimited.
Shade
One tile is a slightly different colour. Find it. Then find the next one. You have 60 seconds.
One tile in the grid is a slightly different colour. Tap it and the next level appears: the grid grows from 2×2 up to 8×8, and the colour difference shrinks. You have 60 seconds. A wrong tap costs 3 seconds.
Colour difference is measured in ΔE (CIEDE2000), the standard way to express how different two colours look to a person. A ΔE around 1 is close to the limit most people can see. Your result is the smallest difference you found. Each level scores 100 points plus up to 100 for speed. A colour-blind palette keeps the differences on the lightness axis.
Circle
Five shapes, one stroke each. The lab measures how perfect they are.
You get five prompts: a circle, a straight line, a right angle, an equilateral triangle and a square, in a different order each day. Draw each in one continuous stroke. Each shape is compared with the ideal shape fitted to your drawing, and scored from 0 to 100%. Speed does not matter, only accuracy. Circles and polygons must be closed.
Count
Five things flash on screen for a moment. Estimate them.
Five rounds: how many dots, what angle, how full a box is, how many times longer one line is than another, and how long a light stayed on. Each is shown briefly, then hidden. Type your estimate. Each round scores up to 10 points, and being 10% too high costs the same as being 10% too low.
Fair play
Every ranked run is replayed on our server with the same rules, and the server's score is the one that counts. Boards show percentiles rather than a global top 100.