In their excellent book An Ecological Approach to Perceptual Learning and Development, Eleanor Gibson and Anne Pick distinguish between two broad approaches to visual perception: enrichment theories and differentiation theories. The first theory claims that the initial sensory stream needs to be enriched because the stimulus upon the eye is too poor for accurate perception of environmental structure. It was Bishop Berkeley who first argued that perception of space is impossible without enrichment. Indeed, he says that
It is, I think, agreed by all that distance, of itself and immediately, cannot be seen. For distance being a line directed end-wise to the eye, it projects only one point in the fund of the eye, which point remains invariably the same, whether the distance be longer or shorter.
Because Berkeley assumed that the retinal or “proximal” stimulus is indeterminate in respect to the “distal” stimulus, he thought that the brain needs to make some kind of probabilistic hypothesis or interpretation in order for there to be experience of distance. Thus, our experience in three dimensions is merely the result of our brain “guessing” that the earth is 3D based off the inadequate sensory reception. In this same respect, Helmholtz’s notion of unconscious inference has recently been refined into a computational theory based on the construction of representations, as with David Marr’s influential theory. There is also a rationalist variant of enrichment theories currently in vogue. These rationalists also emphasize inference in perception, but think that the major premises for inference are evolutionarily ancient. This strong nativist view is championed by people like Chomsky and Pinker.
In contrast with enrichment theory, differentiation theories emphasize the redundancy of information available in the environment regardless of whether the perceiver is there to pay attention to it. Accordingly, differentiation theorists take a different approach to Berkeley’s problem of distance. Consider the following diagram.
This picture represents two different formulations of the distance problem. The line with the points A,B,C,D is how Berkeley set up the problem. The points cannot be discriminated in respect to distance. As J.J. Gibson said however, the distance along this line is a fact of geometry, but not one of optics or visual perception. Indeed, the points W, X, Y, Z can be discriminated by the retina. As the observer moves through the ambient field of light that has settled in the environment, there is a pattern of stimulus that transforms across the retina. Because the pattern is structured nomothetically (in a lawlike manner), it corresponds or “contains information” specific to events, objects, and layouts in the environment. Indeed, the nomothetic relation between distance and the density of optic information allows for the perception of texture gradients along the surface of the earth (Notice how points Y and Z are closer together on the retina). In order to perceive accurately then, the observer simply needs to learn how to discriminate what J.J. Gibson called the variables “invariant over transformation”.
This theory is known as the “ecological” approach to visual perception. It emphasizes that information specific to the level of reality relevant to organisms is widely available and orderly structured in ambient energy arrays. In order to perceive, the animal simply needs to discriminate the invariant patterns of transformation which arise by its movement through the ambient field of energy. This is called “sampling” the optic array. The development of perception is largely concerned with learning these discriminatory skills. Alva Noë has talked at length about these skills in terms of what he calls “sensorimotor knowledge”. Indeed, he says that
The basic claim of the enactive approach is that the perceiver’s ability to perceive is constituted (in part) by sensorimotor knowledge (i.e. by practical grasp of the way sensory stimulation varies as the perceiver moves).
Movement through the ambient array corresponds to a dynamic “optic flow field”. Transformations of this flow field contain information about both the perceiver and the environment. As E. Gibson and Pick write,
There is a second reciprocal relation implied by the affordance concept: a perception-action reciprocity. Perception guides action in accord with the environmental supports or impediments presented, and action in turn yields information for further guidance, resulting in a continuous perception-action cycle. Realization of an affordance, as this reciprocity implies, means that an animal must take into account the environment resources presented in relation to the capabilities and dimensions of its own body. Children begin learning to do this very early and continue to do so as their powers and dimensions increase and change.
As we can see then, enrichment theories and differentiation theories begin with very different assumptions about the nature of the perceptual stimulus. Whereas differentiation theorists hold that the perceptual stimulus is sufficient for the guidance of action, enrichment theorists hold that the stimulus is impoverished. But as the diagram indicates, the stimulus only appears impoverished if we view it in terms of physiological optics as opposed to ecological optics. British empiricists thought that the retinal stimulus is poor because they failed to consider the problem of perception in terms relevant to the organism’s behavioral needs. This is what happens when mathematicians reason about visual perception from a priori principles of geometry: they wind up missing the abundance of information available for attentional discrimination.