Motion - Part 2

Posted by Jon on 15 April 2015 | Comments

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Whether hunting food, escaping predators or simply trying to judge a safe path from place to place motion perception is hugely important to almost all creatures which can see. We humans are no exception. A jumping squirrel, a swinging light fitting, our brains are hard-wired to respond to movement even in the periphery of our vision.

Advertisers know this too. Animated banners, video billboards or old fashioned brightly coloured bunting; all designed to grab your attention. At ScreenLab we factor in motion so you get a full picture of how users are reacting. In this blog post we're going to talk a bit about how motion is perceived and what that means for our models.

Our eyes detect light with photo-receptors bundled together in groups called ganglions. There are many such groups in the retina with a range of different sizes and behaviours. For example, some respond more strongly to edges in one particular direction. They also suffer from a recovery period or dead time after activation which is why eyes take some time to adjust to seeing in the dark when stepping from a bright room.

The combination of ganglions triggered and the order of triggering is interpreted by the visual system and the brain to form the image. If any object moves across our field of view then a series of ganglions will be triggered, one after another. However, that's not the only way to trigger a pattern registered as motion. Take the image below for example:

Anomalous motion trick image

The angled bright and dark edges trick your visual system into seeing non-existant movement as

you look across the image. We can also make abuse the recovery period of the visual system for further tricks. Try staring at the left image for 30 seconds, then look at the right image.

Moving black and white stripes

Textured background for optical illusion


As you can see, the visual system misrepresents the static image as moving in the opposite direction to the animated image. This is because the neurons responding to the motion gradually reduce their response over time, much like we get used to strong smells after a few minutes. Once the moving stimulus is taken away the neurons' adaption creates the false movement aftereffect.

Understanding how the visual system works helps explain these effects. It also enables us to make predictions about how people respond to different stimuli.

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