Perception Of Object Motion

Perception of object motion is important for various reasons. It allows us to avoid colliding with moving objects. However, it also facilitates detection of small or camouflaged objects, and it can also permit us to identify an object's three-dimensional shape (known as the kinetic depth effect). A simple example of the kinetic depth effect was provided by Wallach and O'Connell (1953). A wire hanger is twisted into a random three-dimensional shape, and a light shining on it produces a shadow on a piece of paper. If the wire hanger is motionless, it is impossible to work out the three-dimensional shape of the wire hanger from the shadow. However, if the hanger rotates, its three-dimensional shape is readily perceived.

Several studies of the kinetic depth effect have used random-dot surfaces taken from three-dimensional objects. Three-dimensional structures can be perceived accurately even when only two different random-dot surfaces taken from the same object are presented alternately (see Todd & Norman, 1991). This is an impressive achievement, especially as computational analyses have suggested that a minimum of three distinct views should be required to identify an object's three-dimensional structure (e.g., Huang & Lee, 1989).

The focus in the first part of this section is on the perception of objects' motion. Two issues will be addressed. The first one is how we decide when an object moving in our direction will reach us. The second issue is how we are able to perceive biological movement even when only provided with impoverished information.

In the second part of this section, we consider two illusory phenomena related to object motion. The first phenomenon is apparent movement, and occurs when movement is perceived even though the observer is presented with a series of static images. Apparent movement is seen every time you see a film. The second phenomenon is known as perceived causality. Suppose, for example, you see one square move and collide with a second square, which then starts to move away. Most people report that it looks as if the first square has caused the second one to move.

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