Direct Theories of Constancy

We now know that the classical explanation of the constancies is not the whole story, because animals with no prior visual experience nevertheless display adequate perceptual competence. The perceptual approaches known as Gestalt theory and direct theory, both strongly opposed to the classical view, hold that so long as the distal stimulus is invariant, some aspects of proximal stimulation will be invariant, in spite of changing size and luminance in the retinal image. This argument had been offered earlier by Ewald Hering and Ernst Mach, and it has since formed the core of two quite different direct theories of constancy.

The first of these are what can be termed physiological theories of constancy. Hering and Mach had proposed that innate networks of lateral connections exist between the receptors, as well as in the higher levels of the nervous system, and these might contribute to some of the perceptual constancies (particularly color constancy).

Such interconnections have now been demonstrated and measured, and they must surely affect how we perceive. Put simply, a more strongly illuminated region in the retinal image reduces the response of adjacent regions of the retina through a process of lateral inhibition. When the illumination falling both on an object and on its surroundings increases, there is an increase in both the light reflected to the eye from the object and the light from its surround, so the inhibition from the latter reduces the neural response to the former. The response to the light from the object therefore normally remains relatively unchanged, despite changes in the illumination, so long as the objects' and surroundings' reflectances remain constant. (Several common illusions are covered by the same explanation, notably examples of contrast, as when a bright surround makes a gray object look darker.) L. Hurvich and D. Jameson developed and tested a quantitative model of chromatic color constancy for simple patterns, thereby testing as well their model of red/ green, yellow/blue, white/black opponent pairs, which itself was based originally on Hering's theory of color receptors.

Of course, there are reasons aside from testing underlying theories to study the constancies, like the needs of imaging technologies. For example, mathematical algorithms in the fields of color science and computer vision, notably by B. Wandell and colleagues, have recently been aimed at separately estimating surface colors and their illumination, using pictures of natural scenes. More generally, the attempt to explain constancies as direct responses to stimulus invariances avoids both questions of physiological mechanisms and questions of Helmholtzian inference. Thus, when the illumination changes, both the light reaching the eye from an object and the light from its surround change, but the ratio of their intensities remains invariant. It has

Figure 2. Adelson's impossible staircase.

Figure 2. Adelson's impossible staircase.

been recognized since Hering that if sensed lightness were a response only to such a ratio, lightness constancy (and lightness contrast) would then be a direct response to that variable of stimulation.

Most generally, J. J. Gibson argued that for most properties the changing proximal stimulus pattern received by a perceiver moving around in an evolutionarily normal environment offers invariants that reflect the structure of the physical world and therefore make any inferencelike processes wholly unnecessary.

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