Chapter Summary

• Constructivist theories. According to constructivist theorists, perception is an active and constructive process depending on hypotheses and expectations. Evidence that perception is influenced by motivatioinal and emotional factors supports the constructivist approach. This approach has been applied to visual illusions in the misapplied size-constancy thoery, but other theories (e.g., incorrect comparison theory) have been used to explain such illusions. Constructivist theories are most applicable to the perception of degraded or briefly presented stimuli, but they seem to predict more errors in normal perception than are actually found.

• Direct perception. Gibson proposed an ecological theory of direct perception, according to which the optic array contains invariant information about the layout of objects in the visual environment. We pick up this invariant information by means of resonance, and meaning is dealt with by assuming that affordances are directly perceivable. Gibson was correct in assuming that the visual input provides a rich source of information. However, he underestimated the complexity of the processes involved in visual perception, and his notion of affordances is inadequate as a way of understanding the role of meaning in perception.

• Theoretical integration. Indirect theorists (e.g., the constructivists; Marr; Biederman) differ from direct theorists in assuming that perception involves the formation of internal representations, and that it depends on stored knowledge. A key reason why indirect and direct theories of perception are so different is because the former theories focus on perception for recognition, whereas the latter focus on perception for action. Thus, the two approaches can be regarded as complementary rather than as mutually exclusive. However, indirect theories provide a more generally adequate account.

• Motion, perception, and action. A central issue running through much research on motion, perception, and action is whether Gibson was correct in assuming that we interact with the environment in a direct way making use of invariant information. According to Helmholtz's outflow theory, movement within the retinal image is interpreted by making use of information about intended movement sent to the eye muscles. This theory explains why the visual world seems to move when the side of the eyeball is pressed.

• Visually guided action. It has been claimed that the optic flow pattern and/or the focus of expansion provide the information needed to account for accurate heading behaviour. However, movement on the retina is determined by eye and head movements as well as by the optic flow pattern. It is possible that differential motion parallax is used to determine heading behaviour. Time to contact can be assessed by using tau. However, it can also be assessed by estimating speed and distance. Studies on walking, running, and jumping are consistent with the tau hypothesis. Tau is a good example of the kind of high-level invariant emphasised by Gibson. However, there is no compelling evidence in favour of the tau hypothesis, and other factors (e.g., depth cues) have been ignored. There is also the issue of identifying the internal processes involved in the calculation of tau. Research on running to catch a ball suggests that catchers have a strategy for arriving at the right place just in time to catch the ball; this strategy may involve keeping the optical trajectory straight. The findings are consistent with the use of this strategy.

• Perception of object motion. There is evidence that tau is used to calculate time to contact when an object moves towards a more or less motionless observer. However, tau on its own is not always sufficient to assess time to contact, and other kinds of information (e.g., knowledge of familiar size) are also used. Observers can make very accurate judgements of biological movement when presented with point-Iight displays. Accurate sex judgements in studies on biological movement may depend on structural cues (e.g., the centre of moment) or an dynamic cues (e.g., body sway of the upper body and hips). Apparent motion is generally perceived in ways that would make sense in the real world. At low sample rates, decisions about correspondences or matches between successive images in apparent motion are based in part on the rules of inertia, rigidity, and occlusion. The distinction between short-range and long-range motion may be important. Perception of causality even with meaningless shapes is commonly found in certain circumstances. Michotte argued that perceived causality is innately determined, and does not depend on inferences. The factors producing perception of causality are more numerous and complex than Michotte assumed.

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