Optical Illusions (II)

Spine Drift

What to see ^^

In the image to the right you see an array of spindly thingies (“spines”), with a gap demarkating a square in the middle. Does the central square appear to float, to move relative to the background?

If so, you are perceiving seeming movement, namely Kitaoka’s “Spine Drift” illusion. It is strongly affected by eye movements, thus perceived differently by different people.

What to do ^^b

To enhance the illusion: shake your display a little (easy, of course, with a notebook or iPad). Or scroll a bit, or shake you head, or glance around the display, or use the “shake” button.

If you press the “rotate 90°” button you will find: Whenever the central and peripheral spines are parallel or at 180°, there is no illusion of differential movement; when they're at 90° –whichever way– the illusion occurs. Whith the wheel you can try out any angle.

Eye movements cause motion blur on the retina. When applying motion blur to the original spine object (below, left), the resultant retinal image differs strongly in contrast between blur directions.

Thus when the retinal motion is at 45° (or –45°), the retinal image is very poor in contrast. It is well known that reduced contrast reduces perceived speed , so when the movement is in the 45°-direction the image seems to lag. When the central square and the surround differ in their spine directions by 45°, eye-movement induced retinal motion blur (in one of the two), causing differential contrast reduction, and thus a seeming shift between central square and surround.


Contrast Gain Control

What to do

Gaze for a few seconds at the fixation cross in the centre of the neighbouring image pair. The left image is blurred, and the right has high contrast, ok.

Now move the mouse pointer over the image and judge the contrast of the two new halves.

What to observe

After the change, on the left part you will (initially) perceive high contrast, on the right markedly less contrast. This persists only for a few seconds, then it becomes apparent that the 2 halves are, as indeed the case, identical.

Contrast adaptation (slow, seconds) and contrast gain control (fast, ≈100 ms) are at work all over the place, and be it ‘just’ to make up for the inhomogeneity of retinal sampling. The basic idea is to have the contrast transfer characteristic adapt optimally to the mean contrast, and blur reduces the contrast specifically for high spatial frequencies.

Corrugated Plaid

Compare the top and bottom gray squares for their brightness. They appear (and, indeed, physically are) equal. Moving the mouse over the image removes the red bars. Now the squares appear in a different context, and lo and behold, they do not look equal anymore.

When interpreted as a 3-dimensional scene, our visual system immediately estimates a lighting vector and uses this to judge the property of the material.


These are some optical illusions. There are still some that I will show in the post next week. So, follow my post every week, and you will find something that very admirable.
Keep On Spirit!!
God Bless You All.. d^^b