Chapter Summary

• Perceptual organisation. The Gestaltists put forward several laws of perceptual organisation, including the law of proximity, the law of similarity, the law of good continuation, the law of closure, and the law of common fate. These laws assist in figure-ground segregation. The Gestaltists tried unsuccessfully to explain visual organisation in terms of electrical field forces in the brain. The Gestaltists provided descriptions rather than explanations, and did not manage to define precisely what is meant by a simple perceptual organisation. Their assumption that grouping of perceptual elements occurs very early in processing may be incorrect. Restle showed some of the ways in which perceptual grouping can economise on perceptual processing. The Gestaltists focused on lines and shapes, but Julesz found that perceptual grouping can also depend on brightness, colour, and granularity.

• Depth and size perception. Monocular cues to depth include linear perspective, aerial perspective, texture, shading, familiar size, and motion parallax. Convergence and accommodation are oculomotor cues, but are of limited usefulness. Stereopsis involves binocular cues, and involves establishing correspondences between the information presented to one eye and that presented to the other eye. Information from the various depth cues is generally combined in an additive way. Size constancy depends mainly on perceived distance, but familiar size is also important. When perceived distance is misjudged (e.g., the Ames room), then size judgements are inaccurate.

• Colour perception. Colour vision helps us to detect objects and to make finediscriminations among objects. According to the Young-Helmholtz theory, there are three types of nervous fibres (now known as cone receptors) differing in the light wavelengths to which they respond most strongly. This theory does not account fully for deficient colour vision or for negative' afterimages. Hering argued that there are three types of opponent processes in the visual system: green-red; blue-yellow; and white-black. A synthesis of the Young-Helmholtz and Hering theories accounts reasonably well for colour perception. According to retinex theory, colour constancy depends on comparisons between the light wavelength reflected from a surface and from its surround. However, colour constancy is often less complete than would be predicted by retinex theory. Colour constancy also depends on the fact that many objects have a familiar colour, and on chromatic adaptation.

• Brain systems. Colour, motion, and form are processed in anatomically separate parts of the visual cortex. Visual perception is based on a divide-and-conquer strategy based on functional specialisation. PET scans and studies on patients with achromatopsia raveal the key role of area V4 in colour processing. However, this may not correspond precisely with V4 in monkeys. Studies using MRI, MEG, and PET have indicated the involvement of area V5 in motion processing. This is supported by studies on patients suffering from akinetopsia, a condition that can be produced temporarily by transcortical magnetic stimulation to make V5 inactive. Area V3 is also involved in motion perception, especially processing of dynamic form and obtaining three-dimensional structure from motion. Several areas, including V3, V4, and IT, are involved in form perception. Some patients with damage to V1 show blindsight. The existence of blindsight in patients who have had an entire cerebral hemisphere removed suggests that blindsight can involve subcortical areas. The task of combining information about an object from different brain areas is a complex one, and may involve attentional processes.

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