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This chapter examines how the human visual system responds to human actions. Section 1 starts with the examination of the human ‘praxic’ system, i.e. a high-level visual pragmatic processing of artifacts as tools. Section 2 examines what seems to be the primary level of the visual analysis of human action, namely the perception of biological motion. Sections 3 and 4 ascend from the primary level to higher levels of the visual analysis of human actions, and argue that the human visual system has two complementary specialized neural circuits for processing human actions. The chapter distinguishes between non-social motor intentions and social intentions to reflect the difference between an intention to grasp an object and an intention to affect a conspecific's behaviour, which, in the human case, may involve the intention to cause a new mental state or representation, e.g. fear or a belief, in a conspecific on the basis of visual cues.

This chapter explores a way in which visual processing may involve innate constraints and attempts to show how such processing overcomes one enduring challenge to nativism. In particular, many challenges to nativist theories in other areas of cognitive psychology (e.g., ‘theory of mind’, infant cognition) have focused on the later development of such abilities, and have argued that such development is in conflict with innate origins (since those origins would have to be somehow changed or overwritten). Innateness, in these contexts, is seen as antidevelopmental, associated instead with static processes and principles. In contrast, certain perceptual models demonstrate how the very same mental processes can both be innately specified and yet develop richly in response to experience with the environment. This process is entirely unmysterious, as shown in certain formal theories of visual perception, including those that appeal to spontaneous endogenous stimulation and those based on Bayesian inference.

The task of the dorsal pathway is to determine the spatial layout of objects by processing their respective position in the visual field. According to the system of coordinates in which this processing is effected in the dorsal system, different outcomes are obtained. When an object's position is encoded in allocentric coordinates, its spatial position is determined relative to other objects. When it is encoded in egocentric coordinates, its spatial position is determined relative to the agent's effector and the object becomes a goal for action. Spatial localization and the visuomotor transformation are thus conceived of as two complementary functions of the dorsal pathway. This chapter examines neuropsychological evidence for similar dissociations in human visual capacities based on the examination of lesions in the human visual system. It considers the effects of lesions in the primary visual cortex, the ventral pathway, and in the dorsal pathway.

This chapter considers both conceptual reasons and empirical evidence in favour of the dualistic model of the human visual system applied to the vision of objects. It examines the contribution of the visual system to a restricted class of human actions — reaching, grasping and manipulating objects in one's vicinity — and argues that one and the same visual stimulus can undergo perceptual processing or motor processing. It characterizes the major differences between these two kinds of processing of visual inputs, called ‘semantic’ and ‘pragmatic’ processing. The chapter makes the bold claim that a new kind of non-conceptual content has been discovered by the cognitive neuroscientific study of the visual system, i.e. visuomotor content.

What is the goal of color vision? How ought we to think of color appearance? Under one view, the goal of vision is to maintain a stable representation of object properties across changes in the environment. This poses a challenge to the visual system, because the sensory signal on which visual perception is based is ambiguous with respect to the physical properties of objects in the world. Thus, to maintain stable color appearance, the visual system must estimate what object was most likely to have caused the ambiguous sensory signal. This chapter presents a Bayesian approach to solving this estimation problem that relies on statistical regularities in the world to resolve the sensory ambiguity. The chapter argues that this is a sensible idea: the human visual system evolved in this world, and thus its statistical regularities are likely to be of functional importance to vision.

Since the human visual system continues to mature and to be influenced by the visual environment over at least the first two years of life, the practical benefit of studies of infant vision will be in the development of screening tools for the early identification of visual deficits that might be correctable with early intervention. Since the trichromacy of normal adult colour vision requires three classes of functioning cone photoreceptors in the retina, as well as integrity of the parvocellular stream of the visual pathway from retina through the lateral geniculate nucleus to the cortex, colour vision testing would be a valuable tool for the screening for defects in this portion of the developing visual system. Such a test would be of use in the assessment of residual function in infants with partial vision or other disabilities, as well as in identifying the approximately 8% of males who inherit an abnormality of one of the longer wavelength-sensitive cone photopigment genes that result in a red-green colour defect. While tests for screening infant colour have been proposed recently, several factors may make them more difficult to interpret than equivalent adult tests. In this chapter, some of these difficulties are reviewed and preliminary data using techniques aimed toward ameliorating them are presented.

This chapter introduces the human visual system and its physiological adaptation to ambient light ranges. This chapter also describes research studies, particularly by Edward Castle and Max Delbrück, conducted on the photo-sensory adaptation of Phycomyces. This chapter states that in Phycomyces a photo-sensory system controls growth and phototropism with ranges of photosensitivity and adaptation comparable to the visual system of humans. The threshold for phototropism and multiplicity of photoreceptive pigments in Phycomyces and plants are also presented.

This chapter analyzes the application of Benford's law to pictures taken from nature with a digital camera. Considering that many natural phenomena seem to follow Benford's law and that images are often nothing but “snapshots of nature,” it is pertinent to wonder whether images (at least those taken from nature) obey Benford's law. While the values output by the image capture device embedded in the camera, i.e., the pixels, do not follow Benford's law, this chapter shows that if they are transformed into a domain that better approximates the human visual system then the resulting values satisfy a generalized form of Benford's law. This can be used for image forensic applications, such as detecting whether an image has been modified to carry a hidden message (steganography) or has been compressed with some loss of quality.