Motion

Movement Aftereffect

Before continuing, try , following the instructions in its caption. The aftereffect you saw is largely due to adaptation of motion detectors, analogous to the color afterimage you saw in . Unlike the color afterimage, which occurs during early visual processing (in the retina and lateral geniculate nucleus), the movement aftereffect seems to occur at many levels after primary visual cortex (Hautzel et al., 2001). Try several speeds and directions of movement. Does the aftereffect seem stronger for some of them than others?

Illusory Movement

You just experienced illusory movement due to adaptation to a real moving stimulus, but there are many other ways to generate the impression of movement where there is none. is an illusion discovered by Baingio Pinna (Pinna and Brelstaff, 2000). It requires real movement, but changes its perceived direction (from outward to rotation and vice versa). This illusion may actually have a simple explanation based on the “aperture problem” (Gurnsey et al., 2002). When the picture moves toward the eye, the image expands outward. To a diagonal-movement detector aimed at an edge in the image, this will appear as a diagonal movement; for all the edges together, that amounts to rotation.

There are other examples of illusory motion where no movement is required. Try , an illusion recently discovered by Akiyoshi Kitaoka (Kitaoka and Ashida, 2003). The artist Isia Leviant painted The Enigma in 1984; see for several variants of it. Like the Pinna-Brelstaff illusion, both of these illusions evoke activity in cortical area V5 (MT). However, the mechanism by which they do so is unclear. There is now evidence that the peripheral drift illusion results from differences in the timing of neurons that respond to low and high contrast (Conway et al., 2005). A similar explanation of the enigma illusion has not been found, but see Zanker and Walker (2004) for a review of attempts to explain this and other movement illusions in terms of involuntary eye movements.

Further Exploration

Use to experiment with the aftereffect. Set the adaptation period and measure the duration of the aftereffect. You also have control over the size, colors, contrast, edge sharpness, and aftereffect background. Try some of the following activities:

• Plot aftereffect duration as a function of adaptation time. How is aftereffect duration affected by bar size and movement speed? Is it affected by color?
• Is the aftereffect equally strong over the entire visual field?
• Set to stop on a blank screen (gray button to the right of “stop on”) and adapt yourself to a moving stimulus. When you stop the movement, do you perceive an aftereffect on the blank background? If so, what does this imply about processing of form and motion?
• Using the lowest contrast setting, can you still get an aftereffect?
• What happens if you close one eye during the adaptation period and then test for an effect with that eye closed and the other eye open? What does this experiment test for?
• Set the adaptation stimulus so that the left side moves up and the right side moves down. Use a pair of cardboard tubes (or roll up some sheets of paper) and look through them so that the left eye sees upward motion and the right eye sees downward motion. What aftereffect do you get with both eyes open? What aftereffect does each eye see independently?
• In what do you expect to see with a line, rather than a ring, of blobs? Try it.
• In , can you figure out what determines whether the movement goes clockwise or counterclockwise? What is the minimum number of colors required for the illusion?
• Are the colored rings necessary for ? Can you switch the direction of illusory movement from counterclockwise to clockwise and back just by thinking about it?
• Would you expect illusory motion to cause a motion aftereffect? Test your prediction using (maximize the window, set the number of circles to 1, and stare at the center of the circle).

Questions

1. Compare and contrast the negative color afterimage and movement aftereffect illusions. What does each tell us about visual processing?
2. Using , adapt yourself to vertical stripes moving left, then switch to diagonal stripes moving up and to the left. The diagonals may appear to move only upward. Why is that?
3. When in everyday life might you experience a motion aftereffect?

References

• Conway BR, Kitaoka A, Yazdanbakhsh A, Pack CC, Livingstone M (2005). Neural basis for a powerful static motion illusion. Journal of Neuroscience 25:5651-5656. []
• Gurnsey R, Sally SL, Potechin C, Mancini S (2002). Optimising the Pinna-Brelstaff illusion. Perception 31:1275-1280. []
• Hautzel H, Taylor JG, Krause BJ, Schmitz N, Tellmann L, Ziemons K, Shah NJ, Herzog H, Müller-Gärtner H-W (2001). The motion aftereffect: More than area V5/MT? Evidence from 15O-butanol PET studies. Brain Research 892:281-292. []
• Kitaoka A, Ashida H (2003). Phenomenal characteristics of the peripheral drift illusion. Vision 15(4):261-262. []
• Pinna B, Brelstaff GJ (2000). A new visual illusion of relative motion. Vision Research 40:2091-2096. []
• Zanker JM, Walker R (2004). A new look at Op art: Towards a simple explanation of illusory motion. Naturwissenschaften 91:149-156. []