OpticalIllusions

Optical Illusions

How dependent are we on our use of sight, and how reliable is that sense?

    1. Play a few games of paperless tic-tac-toe with a partner and a scribe.

    2. After 3 or 4 games, the scribe leaves the group and plays a few games with another scribe.

    3. The remaining 2 players continue play without a scribe.

    4. After a few more games, discuss strategies you used to keep track of where the Xs and Os went.

    5. View the two optical illusions listed below.

    6. How "sure" are you that the motion that you perceive the still objects to have?

    7. Conclusion: as dependent on (and sure of) vision as we are, it is not always accurate. Vision does not equal perception.

Explanation: The effect you probably noticed is a perception of movement where there was none. There has been sparse research into the precise physiological cause of this "motion after-effect" phenomenon, but many activities have been created to educe the illusion in a variety of ways. If one stares consistently at movement in a particular direction for even a short time, subsequently viewed stationary scenes briefly appear to move in the opposite direction. This phenomenon was known to the Ancient Greeks, but Robert Addams (1834) observed the effect while viewing a waterfall at Foyers in Scotland and is often referred to as the "waterfall effect".

The spiraling after effect was first "formally" described by Joseph Plateau in 1849. The variation into 3 regions was, to my knowledge, developed by the magician Jerry Andrus. This is often explained in terms of “fatigue” of the class of neurons encoding one motion direction. It is probably more accurate to interpret this in terms of adaptation or “gain control”. For a more scientific analysis, please read the pdf from MIT in the sources list below.

Sources:

Aristotle “De Somnis” (translated by Beare JI, 1931) University of Virginia Library ← chapter 2, G5r (search for ‘river’)

Mather, Verstraten & Anstis (1998) The motion aftereffect: a modern perspective. Cambridge, Mass: MIT Press George Mather’s MAE page (part of his motion site)

John, A, and Movshon, JA (2003) Neuronal adaptation to visual motion in area MT of the macaque. Neuron 39: 681-691 [PDF]

The Motion Aftereffect homepage

Other sites for optical illusions

Grey contrast illusions: http://web.mit.edu/persci/gaz/#

Skytopia illusions: http://www.skytopia.com/project/illusion/illusion.html

Michael Bach's page: http://www.michaelbach.de/ot/