Colour and Motion: The Stopped Motion Illusion

When a moving pattern is defined by colour, its apparent speed may slow or stop (Stumpf, 1911; Ramachandran & Gregory, 1978; Moreland, 1980; Cavanagh, Favreau, & Tyler, 1984; Livingstone & Hubel, 1987; Lu et al., 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 demonstrations may work well on some monitors but not others. Explanation of slowing: The opponent colour pathways do not give as strong a response to motion as they do to pattern. So paradoxically, the colour stimulus can be well above its visibility threshold but below its motion threshold — clearly seen but not moving. Campbell and Maffei (1979) gave an example of these separate thresholds for pattern and motion producing stopped motion for luminance gratings in the periphery.

The Ring of Colour
This starts first with a large luminance difference between the two colors and then shifts to something near equal luminance, depending on your monitor
The patterns are always moving at the same speed, matched to that of the spokes in the center.
Then some random dark spots are added to help mask any remaining luminance artifacts. Then the dots are removed and the luminances diverge again.

The speed of the patterns is always the same over all the phases shown above, and the overlay of luminance dots is added to suppress any luminance artifacts in the rotating rings (as suggested by Tom Troscianko and acting like the luminance noise in the Ishihara plates). To help judge the slowing, light spokes are shown in the centre, moving at the same rate as the inner colour pattern (there are two rings that rotate in opposite directions). If this demonstration works on your monitor, click here for a continuous version of the matched luminance with overlying dots. There, you can show that the slowing is in your perception and not in the video by placing place a red or green filter over the video or over your eye (maybe one half of a pair of red/green anaglyph glasses). The red and green in the display will then have very different luminances and the motion should be immediately apparent.

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 on your monitor if you are able.

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 an LCD monitor). If it does, the coloured spokes will appear to slow down briefly as the ratio moves through equiluminance, silencing the luminance response. Again, 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 may 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. The same is true if you track them only with attention (Cavanagh, 1992). 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).

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