The final frontier

The perception of depth is essential to the generation of a three-dimensional representation of the spatial relationships in our surroundings; a representation which is essential if we are to be able to interact with our environment in any meaningful way. The visual system has two sets of depth cues: oculomotor and visual (Figure 11.1). They are termed cues because they must be learnt through association with non-visual aspects of experience. Oculomotor cues are based on the degree of convergence (a measure of the angle of alignment) of the eyes and the degree of accommodation (change in shape) of the lens. The visual cues can be both monocular and binocular. The monocular cues include interposition, relative size, perspective and motion parallax. Binocular cues are based on the disparity between the different views of the world from the two eyes. From this disparity, a three-dimensional or stereoscopic representation can be generated. The information on depth, together with information about movement and velocity, seem to be integrated with information from other sensory modalities to produce a map of perceptual spacewhich is common to all our senses. This integration seems to occur in the posterior parietal cortex. Damage to this area causes profound impairments in our perception of space, including that occupied by our own bodies.

Oculomotor cues

When you fixate an object, your eyes are accommodated and converged by an amount dependent on the distance between you and that object (Figure 11.2).