Events occur over some time interval, that is, events involve a change which occurs over time, a moving object, a moving observer, or both. Movement of objects is distinguished from movement of observers by differences in the optical transformations which occur in the visual field. Movement of objects within the environment cause local changes in the optic array, while observer movement yields global optic flow, and thus the deformations in the local and global optic flow provide the observer with information about events.

The necessity of optic flow for the perception of objects and events is evident in the observation that large changes go unnoticed if the optic flow is disrupted. This can be achieved experimentally by altering presentations during saccadic eye movement, or more simply, by introducing a grey blanking image between successive displays in which changes occur. (see Simons & Levin, 1997 for a review).

An example of change blindness - did you notice the change?

See the change now?.

Another demonstration of the necessity, and sufficiency of optic flow for the perception of objects and events is the point light displays first presented by Johannsson (1973). Johannsson filmed actors wearing luminous patches on joint centres under dark conditions. Presentation of stationary images of the actors conveyed little information, but rich perceptual information was obtained from even brief presentations of movement. Observers were able to distinguish between male and female walkers, determine the identify of walking friends, and the mass of weights being lifted by the actors.

A simple simple point display

More recent experiments have utilised point-light displays in complex sporting tasks to assess the role of expertise in the access to relevant information which specifies events such as the type of shot played, or about to be played, by an opponent.

A more complex point light display.

Selective attention

Both natural and artificial environments provide far more information that can be assimilated simultaneously. Selective attention is the process by which the salient cues are selected and combined to provide the information required to support the control of action.

The ability to selectively attend to saliant cues and obtain the information required develops with experience. For example, expert squash players obtain information about the subsequent flight of an approaching ball from the movement of their opponent's arm, that is, well before the ball is struck, while novices appear unable to access this information and must rely of the movement of the racquet and ball (Abernethy, 1987).

Visual search patterns have been studied in an attempt to understand expert/novices differences to obtain information from complex displays. The search patterns are in part determined by knowledge and prior experience. For example, skilled radiologists employ search patterns which roughly correspond to the probability distribution of the location of abnormalities (Kundel, 1974). Other differences appear to correspond to differences in skill. For example, novices drivers make more fixations on the road edge and speedometer, while experienced drivers spend more time looking further ahead (Mourant & Rockwell, 1972).