Direct Perception

Gibson's direct perception approach can be regarded as a bottom-up theory: he claimed there is much more information potentially available in sensory stimulation than is generally realised. However, he emphasised the role played in perception by movement of the individual within his or her environment, so his is not a bottom-up theory in the sense of an observer passively receiving sensory stimulation. Indeed, Gibson (1979) called his theory an ecological approach to emphasise that the primary function of perception is to facilitate interactions between individual and environment. Some of Gibson's main theoretical assumptions are as follows:

• The pattern of light reaching the eye is an optic array; this structured light contains all the visual information from the environment striking the eye.

• This optic array provides unambiguous or invariant information about the layout of objects in space. This information comes in many forms, including texture gradients, optic flow patterns, and affordances (all described later).

• Perception involves "picking up" the rich information provided by the optic array directly via resonance with little or no information processing involved.

Gibson was given the task in the Second World War of preparing training films describing the problems experienced by pilots taking off and landing. This led him to wonder exactly what information pilots have available to them while performing these manoeuvres. There is an optic flow pattern (Gibson, 1950), which can be illustrated by considering a pilot approaching the landing strip. The point towards which the pilot is moving (the focus of expansion or pole) appears motionless, with the rest of the visual environment apparently moving away from that point. The further away any part of the landing strip is from that point, the greater is its apparent speed of movement. Over time, aspects of the environment at some distance from the pole pass out of the visual field and are replaced by new aspects emerging at the pole. A shift in the centre of the outflow indicates there has been a change in the direction of the plane.

According to Gibson (1950), optic flow fields provide pilots with unambiguous information about their direction, speed, and altitude. Gibson was so impressed by the wealth of sensory information available to pilots in optic flow fields that he devoted himself to an analysis of the kinds of information available in sensory data under other conditions. For example, he argued that texture gradients provide very useful information. As we saw in Chapter 2, objects slanting away from you have a gradient (rate of change) of texture density as you look from the near edge to the far edge. Gibson (1966, 1979) claimed that observers "pick up" this information from the optic array, and so some aspects of depth are perceived directly.

The optic flow pattern and texture density illustrate some of the information that provides an observer with an unambiguous spatial layout of the environment. In more general terms, Gibson (1966, 1979) argued that certain higher-order characteristics of the visual array (invariants) remain unaltered when observers move around their environment. The fact that they remain the same over different viewing angles makes invariants of particular importance. The lack of apparent movement of the point towards which we are moving is one invariant feature of the optic array. Another invariant is useful in terms of maintaining size constancy: the ratio of an object's height to the distance between its base and the horizon is invariant regardless of its distance from the viewer. This invariant is known as the horizon ratio relation. Other invariants are discussed later.

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