Movement Perception

Our ability to perceive motion is extremely important to our ability to interact with our environments, to perceive threats, and to survive them. Because of this, our visual systems are sensitive to movement and can perceive it in four different ways: retinal motion, apparent motion, induced motion, and motion aftereffects (Goldstein 2010).

The first of these, retinal motion, is also called "real movement," and is interpreted exactly the way it sounds. Retinal motion describes the incidence of an object physically moving from one place to the next over a period of time (Hall, 2009). This could mean someone running, walking, a fly buzzing around the room, etc. This is the most basic form of movement perception and involves no optical "tricks," as the other forms of movement perception do.

The second way in which we perceive motion is apparent motion, a phenomenon in which we "perceive" movement, even when an object is stationary due to an image being placed in different places. For an example, think animation. Animation uses apparent movement by creating a series of images that change slightly, and when flipped in succession, it appears as though the object in the series of pictures is actually moving (Dursteler, 2004).

The next is induced movement, a concept in which movement in one object "tricks" the visual system into perceiving another object as moving (Hall, 2009). It is induced motion that tricks us into thinking that we're in motion when, another car next to use starts moving, but we're stationary. Hanover University has a great online demonstration of this at: http://psychlab1.hanover.edu/Classes/Sensation/induced/index.html .

Lastly, movement aftereffect. Movement aftereffect is perceiving movement in the opposite direction as you initially did. Hall (2009) notes that this generally occurs after 30 to 60 seconds of looking at an object that appears to be moving, or is moving, in one direction. When you look away the movement appears to change directions completely. This can be explained in terms of direction selective neurons. "Adapting to one direction changes the balance in the response to a stationary stimulus, causing the perception of motion in the opposite direction" (Goldstein, 2010).

On Michael Bach's website of optical illusions and visual phenomena, the example is a sort of whirlpool that appears to spin inward. After several seconds, the whirlpool is replaced with a stationary Buddha image. But instead of actually appearing stationary, the Buddha appears to be coming toward the viewer. This image can be seen here: http://www.michaelbach.de/ot/mot_adapt/index.html

References

Dursteler, J. C. (2004). Movement in Visualization. Retrieved November 15, 2009, from http://www.infovis.net/printMag.php?num=144&lang=2

Hall, D. M. (2009). Perceiving Movement. Retrieved November 16, 2009, from http://www.radford.edu/dhall/psyc377goldsteinchapter9edited.ppt#1

Goldstein, E. (2007).Sensation and Perception. Belmont: Wadsworth.