Colour and Motion: The Stopped Motion Illusion

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When a moving pattern is defined by colour, its apparent speed may slow or stop (Stumpf, 1912; Ramachandran & Gregory, 1978; Moreland, 1980; Cavanagh, Favreau, & Tyler, 1984; Livingstone & Hubel, 1988; Lu & Sperling, 1999). To demonstrate this effect, the colours have to have equal luminance and the transition between the colours needs to be gradual (sharp transitions create luminance artifacts). It is difficult to get equiluminance and smooth transitions without specialized monitors and control programs. The following two demonstrations may work well on some monitors but not others. The luminance pathway gives a brisk response to the motion in these displays but the opponent colour pathways do not. To the extent that your monitors can limit the response to the colour pathways, you should see slowing. Over the Internet, using QuickTime, you have limited options for adjusting the displays but luck may be with you. Happy viewing. These Quicktime movies are big (10 MB and 2 MB, respectively, so you may need some patience to load them).

The Vanishing Donut

Here is a rectangular donut that moves in one direction while the background moves in the other. The speed of motion never changes. OK, the direction reverses but the speed doesn't change. As the movie plays the relative luminance of the red and green are varied to cover a range of green with higher luminance than red to red with higher luminance than green. This happens three times (the relative luminance ratio follows the blurry cross on the top). It then stops at a value that was equiluminous for some monitor in the distant past.

As the ratio moves through equiluminance, the relative motion should become less evident and the rectangular donut may dissolve into a jelly-like tangle of non-rigid flows. Or not. If not, try adjusting the red to green ratio as described below.

If this demonstration works on your monitor, you can show that the slowing is in your perception and not in the movie by placing place a red or green filter over movie. The red and green will then have very different luminance throughout the cycle and the shape and speed should not change. Viewing through the filter, the squares will get quite dim at one end of the range of relative luminances (this is not the equiluminance point) but even then the donut should remain visible and visibly moving.

Adjusting relative luminance: You may be able to adjust your red to green ratio by saving the movie on your computer, opening it with your QuickTime Player, accessing the video controls, and adjusting the tint, brightness, or contrast settings. If you do not have access to these controls, you may change your monitor tint, brightness or contrast in the monitor controls or in the colour control section of your video card control panel. The last possibility is to set your colour resolution to 256 colours, rather than millions. This generates a speckled luminance noisiness that masks deviations from equiluminance.

The Slowed Spokes

Here are two sets of rotating spokes. On the left, the relative luminance of the two colours varies slowly over a range that might include equiluminance (more likely on a CRT than a LCD monitor). If it does, the coloured spokes will appear to slow down briefly as the ratio moves through equiluminance, silencing the luminance response. To help judge the slowing, light and dark spokes are shown in the centre, moving at the same rate as the colour spokes. The slowing effect should grow as you watch through several cycles. To observe the slowing, you have to fixate the centre of the bull's-eye. If you look at the spokes, you can of course track them and see their actual speed. On the right, only blue varies to produce the spokes. This is added on top of a uniform yellow field that switches on and off. After a few cycles, the slowing of the yellow and white bars should become evident. Again you must fixate the centre of the bull's-eye. This display works by saturating the luminance response not by equating it. There is no relative adjustment required here to find equiluminance. When the yellow is on, the spokes appear as white and yellow bars. The colour variation stimulates principally the blue-yellow opponent colour pathway. For the luminance pathway, the response to the blue variation is below threshold because of the high level of response to the uniform yellow field. Or at least, that is the idea. Compared to the technique of equating the sum of the luminance of the two colours, this saturation technique is relatively immune to poor monitor performance. The concept was developed by George Wald and its use for chromatic slowed motion is described in two papers (Cavanagh, Adelson & Heard, 1992; Cavanagh, 1991). The reason that the slowing increases over time is that a motion aftereffect is built up in response to the luminance that appears and moves in the display during parts of the cycle (when the green and purple have different luminance on the left or the yellow background is off on the right). This accentuates the slowing and also demonstrates the transfer of motion aftereffects from luminance adapting patterns to colour test patterns (Cavanagh & Favreau, 1985; Mullen & Baker, 1985; Derrington & Badcock, 1985).