Illusions
Stare at a shape for a few seconds and your visual system adjusts to it, so the next thing you see in that spot looks different. Here are three ways to try this on your own eyes, all built on the shape aftereffect. At the end, one slider sets it against a second illusion, the spokes, which arise at an earlier stage of vision.
Try it: the shape aftereffect
Tall or squat?
Keep your eyes on the cross in the middle. A horizontal bar sits on one side and a vertical bar on the other, and each moves a little so that no afterimage builds up. When the countdown ends, the bars are replaced by two rings for less than half a second.
What to look for: the ring where the horizontal bar was tends to look tall, and the ring where the vertical bar was tends to look squat. Press Swap sides and the two should trade. The first run after a rest usually shows it best, so give your eyes a minute between runs. This is informal, on your own screen, not a lab result.
Or make them look round
Press Make them look round and stare as before. The slider stretches one ring taller and the other squatter. After each change, press Show again for five more seconds of the bars and another glimpse with your new setting. When the two look equally round, press They look round, and the page reports the stretch you needed as a percentage of aspect ratio.
Or measure it
This uses the Psi method (Kontsevich and Tyler, 1999), the same one as the crowding sweep on my Science page. After the first stare, every trial gives you four more seconds of the bars, then two shapes that differ a little in aspect ratio, and asks which one looked taller. The method picks each difference to learn the most from your answer. After about 30 trials it estimates the difference that looks equal to you (the PSE), and the size of the PSE is your shape aftereffect. This is informal, on your own screen, not a lab result.
These work best on a laptop or a larger screen at arm's length. On a phone, hold it at a comfortable reading distance and keep it still. The size of the effect changes with screen size and distance, so compare your own runs with each other rather than with anyone else's.
The effect is tied to the spot on your eye where the bars landed. If your eyes wander off the cross, it fades.
Try it: the seam
A field of circles with a seam down the middle
The field fills with short bars scattered at random, horizontal on one half and vertical on the other. Keep your eyes on the cross. When the countdown ends, a ring appears for half a second at the spot of every bar.
What to look for: rings on the horizontal side should look tall and rings on the vertical side squat. Watch the strip down the middle, where the two sides meet. A soft seam mixes the two kinds of bar across a wider band. Touching rings let you ask whether neighbors pull each other toward one shape. This is informal, on your own screen, not a lab result.
Try it: paint your own lens
Paint a distortion, then look through it
Drag across the field to lay down short bars along the direction you move, or start from a preset. Then press Stare, keep your eyes on the cross, and wait for the grid of rings.
What to look for: each ring tends to stretch at right angles to the bars that were under it. After the spiral, the grid can seem to swirl. After the radial burst, rings can look stretched around the center, and after the ripple, rows of rings can seem to wave. On a phone, paint with one finger; the page does not scroll while you paint on the field. This is informal, on your own screen, not a lab result.
What is going on
Look at a horizontal bar for twenty seconds, then at a circle in the same place, and the circle looks a little tall. After a vertical bar it looks a little squat. The usual explanation is adaptation. Cells near that spot that respond best to horizontal edges and horizontal elongation respond less for a while, so when the circle arrives, its vertical extent wins.
Wolfgang Köhler and Hans Wallach described a large family of these figural aftereffects in 1944. Regan and Hamstra measured an aftereffect of aspect ratio itself in 1992, and showed that adaptation to height or width alone does not explain it.
For more of my work on what people can and cannot see, including crowding and spokes that leave light in their wake, see Science.
- Köhler and Wallach, 1944"Figural after-effects: an investigation of visual processes." Proceedings of the American Philosophical Society 88, 269 to 357.
- Regan and Hamstra, 1992"Shape discrimination and the judgement of perfect symmetry: dissociation of shape from size." Vision Research 32, 1845 to 1864. Read it
- Kontsevich and Tyler, 1999The Psi method used in Measure it. Vision Research 39, 2729 to 2737. Read it
Try it: spokes and shapes at equal brightness
One slider, two effects
A green disc and a white disc circle a white dot on a red field. When the green is as bright as the red, its spoke should fade while the white disc's spoke stays. A green bar at that same brightness should still bend the shape of a circle. First you find your own match, because eyes and screens differ.
Step 1. Find the green that looks as bright as the red
The flicker match flashes one small disc about 30 times a second for up to 20 seconds. If flicker bothers you, or you have photosensitive epilepsy, use the edge match, which does not flicker.
Step 2. Watch the spokes fade
What to look for: keep your eyes on the dot. At bright contrast, both discs pull a darker band toward the dot. At your match, the band from the green disc should weaken or vanish, while the one from the white disc stays. Moving the slider either way from your match should bring it back.
Or rate it, as we did in 2000
Each trial shows the discs for four seconds with the green at one of seven brightnesses around your match, in random order. Then you rate the green disc's spoke against the white disc's, from 0 (none) to 9 (as strong).
Step 3. The shape aftereffect at the same setting
Green bars on the same red, at the brightness set by the slider in step 2. Stare at the dot for twenty seconds, then two green rings flash in place of the bars.
What to look for: the ring where the horizontal bar was tends to look tall. Try one run at your match and one at bright contrast, with a minute's rest between them. If the distortion holds at your match while the spokes fade, the two effects are coming from different places in your visual system.
Spokes come from an early stage of motion processing that works on brightness, so a disc that differs from its background only in color makes little or none. The shape aftereffect builds up at a stage where color and brightness signals meet, so a bar that differs only in color should still distort the next shape. One slider pulls the two apart and shows two stages of vision at work.
The spokes part repeats our published result. The shape part is my prediction from our 1997 finding, and it has not been published as a demonstration.
Your match is kept only in this browser. The colors depend on your screen, so this is informal, not a lab result.
- Holcombe, Intriligator, and Tse, 2000"The spoke brightness illusion originates at an early motion processing stage." Perception and Psychophysics 62, 1619 to 1624. The green disc, the white disc, the red field, both matching methods, and the 0 to 9 rating come from its second experiment.
- Holcombe, Macknik, Intriligator, Seiffert, and Tse, 1999"Wakes and spokes: new motion-induced brightness illusions." Perception 28, 1231 to 1242.
- Rivest, Intriligator, Warner, and Suzuki, 1997"Color and luminance combine at a common neural site for shape distortions." Investigative Ophthalmology and Visual Science 38, s1000.
- Rivest, Intriligator, Suzuki, and Warner, 1998"A shape distortion effect that is size invariant." Investigative Ophthalmology and Visual Science 39, s853.