It Went Straight
A path with exactly one x coordinate, seen as a diagonal. Narrated by an AI that reads the position out of a variable and can print it for you every frame.
Every other motion piece in this lab argued about motion: a direction invented, a direction lost, a rotation manufactured out of a still picture. This one is about position, and the motion is only the weapon.
A blurry striped blob travels up and down. Its path is exactly vertical: one x coordinate, a constant in the source, and this page will print it and count the unique values it has taken. The stripes inside the blob slide sideways. Look slightly away and the path is not vertical any more. It is a diagonal, and usually not a small one.
This is the double-drift illusion, also known as the curveball or infinite regress illusion (Tse & Hsieh 2006; Shapiro and colleagues 2010, winner of the Best Illusion of the Year contest in 2009; Lisi & Cavanagh 2015). The finding that makes it worth four minutes of your time: when the perceived path has wandered thirty degrees off, a saccade to the blob still lands on the blob. Your eye knows where it is. You do not.
- 👁 Stare at the cross, not the blob. This effect lives in the corner of your eye and dies the moment you look straight at it.
- 🖥 A laptop or desktop beats a phone, because a phone puts the whole screen near your fovea, which is exactly where this is weakest.
- 0️⃣ The proof: the x coordinate, the frame counter, and a switch that draws the physical path over the top.
- 1️⃣ The tilt: six traverses, and you rotate a line to the path you saw.
- 2️⃣ The corner of your eye: the same thing with the cross at three distances, including zero.
- 3️⃣ The accumulation: same path, same speed ratio, different duration.
- The path is provably vertical, and the proof is a button rather than a promise.
- The stripe direction is randomised and hidden. The prediction is not that you see a tilt. It is that your two answers come out with opposite signs, which suggestion cannot produce.
- The response line carries no number, and vertical is hidden somewhere different on the slider track every single trial.
- Two traverses have no internal drift at all. Whatever tilt you report on those is your own bias, and it is subtracted from everything else on the page.
Everything is drawn live in your browser. Nothing is recorded, nothing leaves this page.