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.

Comment – 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. Contrast gain control is specific for retinal location & spatial frequency as shown here (the idea was taken from Webster et al. 2002); it is also, for instance, specific for the depth plane (Aslin et al 2004).

Sources

Katsushika Hokusai “The Breaking Wave Off Kanagawa” (also called “The Great Wave”). Woodblock print (≈1831–1833) from Hokusai’s series of 36 views of Mount Fuji. The original is at the Hakone Museum in Japan.

Webster MA, Georgeson MA, Webster SM (2002) Neural adjustments to image blur. Nature Neuroscience 5:839–840

Aslin RN, Battaglia PW, Jacobs RA (2004) Depth-dependent contrast gain-control. Vision Research 44:685–693

Heinrich TS, Bach M (2001) Contrast adaptation in human retina and cortex. Invest Ophthalmol Vis Sci 42:2721–2727

Motion-Bounce Illusion

Warning: this is a subtle effect.

First test your sound output by pressing the appropriately named button ‘Test’. You must hear a clear ‘tink’ sound. If not, go to another illusion.

The phenomenon: Press buttons ‘1’ and ‘2’ repeatedly ad lib. Try to determine the path of the blue balls. Are they crossing like so ‘X’, or do they bounce off each other ‘> <’?

Chances are: When there is no accompanying sound, the crossed path is seen, with the sound a bounce (‘> <’) is perceived.

Comment

It was long known from the Gestaltists that two identical visual targets moving across each other can be perceived either to bounce off or to stream through each other (Metzger 1934). In 1997 Sekuler et al. demonstrated that a brief sound at the moment the targets coincide biases perception toward bouncing.

Sources

Metzger W (1934) Beobachtungen über phänomenale Identität. Psychologische Forschung 19:1–60

Sekuler R, Sekuler AB, R Lau (1997) Sound alters visual motion perception. Nature 385:308

Shimojo S, Shams L (2001) Sensory modalities are not separate modalities: plasticity and interactions. Curr Op Neurobiol 11:505–509

“Lilac Chaser”

What to see

On the right you see a circle of blue-violettish (=magenta) patches, one of which briefly disappears, circling around.

Let your gaze rest on the central fixation cross, but observe with your “inner eye” the patches just when they disappear. With good fixation, you should see a strong greenish colour whenever the violet patch has disappeared.

When you are fixating well, after a few cycles you will actually see a rotating green spot! If your gaze is really steady, the magenta patches will disappear, leaving only a rotating green spot (this is easier if you reduce saturation to, e.g., ≈20%); when you then make any eye movements the circle reappears.

With the “Colour±” button you can switch to other colours and observe the respective opponent colour. [Remember to fixate for a while because the adaptation to the previous colour subsists for several seconds.] It may surprise you that the complementary colour to red is not green (as mentioned in many textbooks), but blue-green. The color pickers further down let you explore the full range of colours, including the effect of the background colour.

Comment

The temporal presentation enhances the well-known afterimage in complementary colour.

Jeremy Hinton, the ‘inventor’, writes: “The illusion illustrates Troxler fading, complementary colours, negative after-effects, and is capable of showing colours outside the display gamut.”

I have been repeatedly asked to explain this in more detail, so here goes:

1. There is something called the “”. It becomes visible when one given hue stays on the same retinal position for several seconds (usually we would move our eyes typically 3 times per second, so this is no disadvantage in normal viewing). The afterimage builds up as that retinal location adapts to this special hue, and when looking at a neutral background the complementary colour is seen.
2. This is a good thing, normally, because it helps “colour constancy”, that is we see colours somewhat independent of the ambient illumination (compare the bluish glacier noon sun with a reddish tint in the evening living room by the fireside).
3. Ok, so the afterimage is “burnt in”, meaning: that retinal location is adapted. Now the magenta patch is suddenly switched to grey. Because of the adaptation, the complementary colour is now seen, which would be green for magenta, or light grey for a dark grey.
4. The retinal afterimage typically fades away rapidly (over a few seconds under normal conditions). But here this fade-out does not reduce the perception of the afterimage, because a new one is uncovered right after at the next location.
5. In addition, a Gestalt effect, here the “phi phenomenon” comes into play: the afterimage from the successive retinal locations is integrated and perceived as one single moving object, namely the green disk.
6. In summary, the following factors make this illusion rather compelling:
   • it is rather easy to steadily fixate on the centre
   • most of the time the retinal locations are re-adapted and the afterimage is uncovered only briefly
   • a Gestalt effect leads to the perception of a flying green disk.

Source

Jeremy L Hinton (, personal communication, jeremy dot hinton at bigfoot dot com)

Robert O’Shea wrote a pertinent

Fraser’s Spiral

The picture on the left depicts “Fraser’s Spiral”. But if you place the mouse over it, it becomes obvious that there are no spirals…

Sources.

Fraser J (1908) A New Visual Illusion of Direction. Brit J Psych 2:307–320

Morgan MJ, Moulden B (1986) The Münsterberg figure and twisted cords. Vision Res 26:1793–1800

Popple AV, Sagi D (2000) A Fraser illusion without local cues? Vision Res 40:873–878

Parts of Fraser’s original paper have been made available by .