Perception Deception

Does brightness change how you see speed?

If you see a person jogging through thick fog or at dusk, they may seem to drift slowly towards you, whereas when you see the same jogger in bright sunlight, they seem to move more quickly.

If you’re trying to judge how fast something is moving, it often depends on how bright the thing appears against its surroundings.

Let’s test this out and reveal how many others see what you see.

Look at the blue and yellow squares below. (To get a stronger effect, look at the cross so you see the squares in your peripheral vision which is more sensitive to detecting motion. Alternately, step back from your computer screen and look at it from a distance.)

When the squares are over the black stripes, are they gliding together or are they ‘stepping’ along like a pair of walking feet?

The yellow (light) and blue (dark) squares always move together at the same speed, regardless of the background.

The squares appear to move like stepping feet because they contrast so strongly against the black and white stripes.

Detecting motion and speed is particularly strong in your peripheral vision and it’s easier to notice things in your peripheral vision if they contrast strongly. Low contrast patterns generally produce a weaker response in the motion-sensitive parts of your brain.

When the blue (dark) square passes over a white stripe, they contrast strongly against each other and the blue square appears to speed up. As the blue/dark square passes over a black stripe there is low contrast between the two and the square appears to slow down.

Visual information is eventually processed by the visual cortex in your brain, but some pieces of information are processed along separate pathways before reaching your visual cortex.

One pathway is motion-sensitive (and is stimulated first) and the other pathway is colour-sensitive. This colour information is added into the visual cortex movement has been processed.

It all happens so quickly, you don’t notice the lag between each network.