Motion Induced Spatial Conflict

Download Demonstration

The demonstration programme is a windows application (sorry Mac users).

If you look at the little movies above, you may get the impression that the hearts are not moving back and forth in time with the background. This is known as the Fluttering Hearts Illusion. Originally the illusion was thought to be caused by a difference in the speed by which the visual system could respond to different colours (Helmholtz, 1962). A similar interpretation (von Grunau, 1976) suggests that the illusion is due to an inhibitory interaction with the neural response to the surround colour delaying responses to the figures (the hearts). A more recent suggestion is that the illusion occurs because of a difference in perceived speed between borders defined by high (green or red / black) and low (red / green) luminance contrasts (Nguyen-Tri & Faubert, 2003).

If, instead of jittering the stimulus, we moved it at a constant speed the first type of hypothesis (the latency difference account) suggests that the hearts should lag behind the spatial position of the surround by a constant distance. The second type of hypothesis (the perceived speed difference account) suggests that the different types of moving border should drift apart.

When we created a stimulus with high and low luminance contrast borders moving in close spatial proximity, we found that neither of these predictions was true. Instead, the spatial position of the border with the lower luminance contrast appeared to jitter rapidly . Why might this occur?

Our interpretation :
As they differ in luminance contrast, the two types of moving border are interpreted by the visual system as moving at different speeds, suggesting that they should drift apart. This creates a spatial conflict between predictive motion-based spatial coding and the spatial configuration that is sampled and processed at a later point in time. The illusory jitter is the consequence of the resolution of this conflict. The visual system periodically snaps the low luminance contrast border back into alignment with the high luminance contrast border. The characteristic rate of the jitter reflects the speed of the neural process that mediates this resolution.

You can demonstrate our findings, and see the illusion for yourself, by downloading our demonstration windows application (click on the link at the top of this page). The illusory jitter is dependent upon retinal movement. If you track a moving stimulus in one of the displays, thereby effectively nulling the retinal motion, you should not be able to see any illusory jitter. The illusion is also dependent upon there being a large difference in luminance contrast between the two types of moving border. Because of this, the major feature of the application is the ability to change the luminance of the central region of the moving elements within the various displays. You can also change the speed of the stimulus and the background colour. There are four different stimulus configurations to choose from, and in one of these you can also change the relative patterns of motion.

For a full description of this research, please read the published accounts.