Fading

Troxler Effect

Before continuing, try the first test in . This was first described in 1804. It has long been thought to have a straightforward explanation. As you stare at the flashing dot, the red dot to the right stays on the same part of your retina, so local feature detectors (either in the retina or further into visual processing) should adapt and cease responding. In particular, the edge detectors would stop responding, followed by the background color filling in the area that was bounded by the edges (Ramachandran and Gregory, 1991). This explains why even the slightest eye movement brings back the faded dot (which then falls on a different, unadapted, part of the visual field). It also explains why the fading time is long and variable.

Now try the variations (tests B-F) in . After experiencing the fading in each case, repeat that variation while looking at the red dot to see what actually happened. These variations were developed by Kanai and Kamitani (2003) to test the edge-adaptation hypothesis. Are your results consistent with that hypothesis? Some models of visual attention postulate that objects in the visual field compete for responses of cells in visual cortex. Kanai and Kamitani (2003) suggest that the transient object in their experiment may prevent continued response to the constant target.

Further Exploration

• The competition hypothesis proposed by Kanai and Kamitani is most effective if the brightness contrast between the target and background is low. In the original test, the red dot and green background are contrasting colors but similar brightness. Try with background and target colors that differ more or less in brightness. When is the effect strongest?
• Can you induce fading in the center of your visual field? Try staring at the target dot instead of the flashing dot. Does it fade? Try again with variations on the Troxler effect. What do your results imply?

Questions

1. How do the variants presented here challenge the edge-adaptation explanation?
2. When the edges fade, the dot is filled in by the background color. Can you come up with another example of filling-in in the visual system?

References

• Kanai R, Kamitani Y (2003). Time-locked perceptual fading induced by visual transients. Journal of Cognitive Neuroscience 15:664-672.
• Troxler D (1804). In Himly K and Schmidt JA (eds.), Ophthalmologische Bibliothek, Vol. 2, (pp. 1-119), Jena, Fromann.
• Ramachandran VS, Gregory R (1991). Perceptual filling in of artificially induced scotomas in human vision. Nature 350:717-721.