The Best Illusion of the Year Contest took place last week. As always, there were some cool, new illusions among the finalists—a feast for illusion-afficionados like myself. I particularly like the Honeycomb Illusion by Marco Bertamini and Nicola Bruno. Take a moment to watch (my rendition of) their illusion in its beautiful simplicity (important: for best effect, watch in full screen and HD quality):

This illusion is nothing but a honeycomb grid with little stars (or barbs) at each node. You can see this in close-up on the right.

The stars are clearly visible when you look directly at them. But, and this is where things get interesting, you don't see any stars in the parts of the grid that you don't directly look at. In other words, you get the impression that the little stars follow your eyes around, as you scan the grid with your eyes. I personally find this very compelling.

So what's going on here?

The key to this illusion is that, at any one moment, you only have a clear view of a very small part of the world: the part that falls onto the central part of your retina (the fovea). This part is about the size of your thumb at arm's length. Your peripheral vision, the things that you see from the corner of your eye, is much less sharp, and color blind. This, among other things, is why you move your eyes: You successively direct your central vision at things that …

In the animation below, you can see a Christmas tree full of baubles. The baubles are arranged in two more-or-less vertical columns, but not quite: Some are shifted a bit to the left, some a bit to the right. Or are they? In actuality, of course, the baubles are arranged in two perfectly straight columns. The apparent displacement is caused by the motion of the stripes on the baubles.

This illusion is a demonstration of motion-induced displacement: the phenomenon that the perceived position of an object is affected by its motion (or motion in the object’s environment, as I’ve shown before). This effect is particularly strong if an object’s position is ill-defined, for example because it has fuzzy edges like our baubles here.

The first to show this (that I know of) were Leonard Matin and his colleagues. They showed that two line-segments that rotated around a central dot appeared to be shifted in the direction of their movement. This is similar to what happens to the perceived position of the ‘baubles’ in the animation shown above: When the texture of the bauble is moving to the right, the position of the bauble appears to shift to the right as well. The fact that the bauble itself is not moving (just its texture) is a nuance that is lost on our visual system: Essentially any kind of nearby motion will ‘grab’ the object and perceptually drag it along.

The obvious question is whether motion-induced displacement serves a …

The video below shows large columns, made up of small line-segments. The columns appear to swing from side to side, like tilted towers. In reality, of course, the columns and line-segments are perfectly vertical. The perceived tilt is illusory.

So what's going on here?

This illusion is a nice demonstration of Gestalt theory, a German a school thought that was developed in the early 20th century by Kurt Koffka, Wolfgang Köhler, and Max Wertheimer. Theories in psychology tend to become outdated rather quickly, but Gestalt theory is a notable exception. It is as relevant today as it was a century ago.

In German, Gestalt means 'shape', but in this context it refers to something that is greater than the sum of its parts, or a 'unified whole'. Which, according to the Gestalt psychologists, is a good characterization of perception. If we see a chair, for example, we don't consider the parts of the chair separately, one at a time. ("I see a leg, uhm, another leg... and something that could be a seat.") We simply see a chair. We can see that a chair has legs, of course, but the perception of the chair as a whole comes first. In other words, we automatically group objects together into a single, coherent percept.

This tilt illusion is the result of perceptual grouping. More specifically, in this case we group those line-segments together that are adjacent (law of proximity) and have the same colour (law of similarity). You can see an illustration …

In the video below you see two lines that are alternately shifted a bit to the left and the right relative to each other. Or are they? When the lines are presented continuously, you can clearly see that they are not shifted at all. They are perfectly aligned!

This is a variation of the Flash Grab illusion, designed by Patrick Cavanagh and Stuart Anstis, the same duo that is responsible for this illusion. Below you can see another animation, which shows the same Flash Grab illusion in a very different way. This one is more like the original, in which the two lines appear to be alternately rotated slightly clockwise and counterclockwise, even though they are perfectly horizontal/ vertical.

So what's going on here? According to Cavanagh and Anstis, there are two phenomena that together result in the illusion. It's a bit of a challenge to follow the logic, but here we go.

The first factor is illusory trajectory shortening: We tend to perceive movement trajectories as being a bit shorter than they really are. In the case of the first video, this means that we perceive the background figure to reverse its direction just before it actually does. In the case of the second video, this means that the disc appears to rotate slightly less than the 90 degrees that it actually rotates.

The second factor is assimilation: The flashed lines are assimilated by the moving background figure. They somehow melt into a single percept. This effect is particularly …

In the video below, you see three rings of coloured dots. In each ring there is one gap (a missing dot), and these gaps rotate like the arms on a clock. So far nothing remarkable. But see what happens if you fixate on the central cross for 15 seconds. Illusory dots of various colours will start to appear where the gaps are!

This illusion is a demonstration of the colour after effect. After effects are very basic phenomena, and most of the video is essentially decoration, not necessary for the illusion occur. In fact, you will even get a colour after effect if you present a single coloured dot, look at it for a while, and then remove it. If the dot is green, like the inner circle in the video, you will observe an after effect in the form of an illusory pinkish dot.

So what's going on here?

Let's start with the fundamentals. Light is electromagnetic radiation, just like radio signals, WiFi, microwave radiation, etc. Different forms of electromagnetic radiation are characterized by different wavelengths, and visible light corresponds to a tiny range from roughly 390 to 750 nanometre (one billionth of a meter). Within the spectrum of visible light, different wavelengths correspond to different colours: Short wavelengths are blue(ish), long wavelengths are red(dish).

Like most people, I learned about the relationship between colour and wavelength during physics class in high school (see [1]). And I distinctly remember that I found this very puzzling. After all …