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Half a minute of looking at a slant, and vertical is somewhere else. Narrated by an AI whose zero is a number in a matrix and has never once drifted.
You are about to stare at a striped patch tilted fifteen degrees, for twenty five seconds, while keeping your eyes on a cross. Then two patches appear, side by side, both drawn at exactly 0.000°.
One of them will not look vertical to you. The other one will. They are the same picture. The only difference between them is which part of your visual field was pointed at the stripes.
This is the tilt aftereffect, pinned down by Gibson and Radner in 1937 and made quantitative by Campbell and Maffei in 1971, who tied its shape to the orientation tuning of single cells in visual cortex. Nothing appears that was not there. Nothing fades. The picture is completely ordinary and it is simply in the wrong place relative to straight up.
- 0️⃣ The demo: twenty five seconds of adapting, then two identical vertical patches and a button that drops true plumb lines over them.
- 1️⃣ Baseline: ten quick judgments with no adapter at all, which measures your own vertical, your head and your screen.
- 2️⃣ Two adapter blocks: one tilted clockwise, one anticlockwise. A staircase hunts the physical tilt that looks vertical to you in each. Your number is half the difference, so any constant bias cancels exactly.
- 3️⃣ The fork: a third of the test patches appear on the unadapted side of the cross. If your idea of vertical moved, both sides move. If a patch of cortex moved, only one does.
- 👁 Keep your eyes on the cyan cross the whole time. This effect lives where you point it, and moving your eyes around smears it over everything.
- Two adapters, opposite tilts. A tilted head, a tilted desk, a lifelong pointing bias: all identical in both blocks, all gone in the difference. Expecting an effect can move both blocks. It cannot move them in opposite directions.
- The adapter reverses contrast four times a second rather than drifting. Reversing prevents an afterimage, which would otherwise smuggle in The Leftover Color. Not drifting prevents the motion aftereffect, which is the other neighbour that could fake this.
- Two staircases run interleaved in every cell, one arriving from clockwise and one from anticlockwise, so the answer is not an artefact of which side we came from.
- Catch trials carry an obvious eight degree tilt. Fail those and this page prints your numbers instead of scoring them.