Alan Gilchrist

Summary Since at least the time of Helmholtz, the process of visual perception has been regarded as a two-stage affair consisting of an initial sensory stage corresponding to the proximal stimulus and a subsequent cognitive stage corresponding to the distal object. This construction amounts to an awkward mind body dualism wherein part of perception is done by the body and the other part is done by the mind. Gestalt theory rejected both raw sensations and their cognitive interpretation, offering a single unified perceptual process that responds to an extended pattern of stimulation. They proposed organizational rules that describe how objects arise from the indifferent retinal mosaic. The same grouping principles by which objects are segmented also function to segregate regions of uniform illumination. Lightness values can then be computed by comparing luminance values within each such framework of illumination, with no need for the mystical concept of taking the illumination into account.

Every perceptual experience has an objective and a subjective side. We see object size, independent of distance, but we also see that distant objects project smaller images. Early modern conceptions focused on local stimulation and thus on the subjective aspect. Helmholtz and Hering emphasized the objective aspect. Helmholtz split visual experience into two stages, with sensation representing the subjective side and perception, through cognitive processes, the objective side. Gestalt theory denied this dualism, rejecting both sensory and cognitive stages. Despite contrary evidence, the sensation/perception dualism persists, in brightness models and past experience theories. That we can detect visual angle and luminance does not mean that these are raw sensations out of which perception of size and lightness is built. Size and color constancy are truly visual, not cognitive. Instructions to subjects in vision experiments must avoid the proximal mode on the one hand and cognitive judgments on the other.