Go back a couple of centuries and there were no chains of shops or malls. In the high street in the UK you would have found the type of shop you were after by looking out for a sign hanging out. There were signs for pharmacists, tobacconists, pawnbrokers, whatever. Nowadays there’s just one traditional sign still sometimes to be seen – the barber’s pole, as left in the animation.
The barber’s sign shows a famous illusion. The cylinder is rotating horizontally, but the stripes look as if they are rising – which would be impossible, unless you had some long pole sliding through the cylinder.
You can begin to see why in the demo on the right: focus on the vertical slot and the grating seems to be moving vertically (as in the barber’s pole). But focus on the horizontal slot and in a moment the grating may seem to move horizontally. Behind the round hole, for me it tends to look as if moving obliquely.
It’s a demo that depends for a start on a fact I find counter-intuitive: if a grating of oblique lines passes behind a small hole in a mask, (an extensive grating, so that the edges of the grating never appear in the hole), the lines stay objectively the same as they traverse the hole, whether the grating, at the orientation shown in the animation, is moving horizontally to the left, obliquely up and to the left, or vertically upwards, (or conversely, as not shown here, horizontally to the right, obliquely down and to the right, or downwards).
That leaves the brain struggling to guess which way the grating is actually travelling, and being pretty indecisive about it. In the animation, when the mask goes a bit transparent, I find that if I focus on the vertical slot the whole grating tends to appear to move vertically. Focus on the horizontal slot and after a moment the grating moves horizontally. Behind the circular hole, the grating tends for me to move obliquely.
So behind the illusion first of all is an objective fact, that the stimulus the grating presents as it passes behind the slots remains constant even when its true direction of movement changes. But then, why does the grating appear to us to move consistently vertically in a vertical slot, and horizontally in a horizontal slot? The short answer is, nobody is quite sure. For a start, the cell groups which detect movement in the eye only sample small receptive fields, compounding the effects of the mask. So effects of movement across the visual field, even without the mask in this demo, have to be stitched together in the brain. In this illusion, the stitching seems to follow signals from the edges of the slots, where the moving ends of the hatched lines do objectively run along the orientation of the slots. But as ever, not all the effects we see in a careful study follow such a simple rule as that. Just how the brain finally puts it all together is much, much more subtle. If you like getting seriously technical, try this paper by Nick Fisher and Johannes Zanker from 2001.