Visual Cues and Constancies

10 Key excerpts on "Visual Cues and Constancies"

• 

  eBook - ePub

  Cognitive Psychology

  A Student's Handbook

  • Michael W. Eysenck, Mark T. Keane(Authors)
  • 2015(Publication Date)
  • Psychology Press

    (Publisher)

  Colour constancy is a complex achievement, and observers often fall well short of complete constancy. In view of its complexity, it is unsurprising the visual system adopts an “all hands on deck” approach in which several factors contribute to colour constancy. Of special importance are cone-excitation ratios that remain almost invariant across changes in illumination. In addition, top-down factors such as our memory of the familiar colours of common objects also play a role. Our understanding of the brain mechanisms underlying colour constancy has been enhanced by the discovery of cells in V4 responsive to colour constancy.

  What are the limitations of theory and research on colour constancy? First, we lack a comprehensive theory of how the various factors combine to produce colour constancy. Second, most research has focused on relatively simple visual environments and so the processes involved in trying to achieve colour constancy in more complex environments are poorly understood. Third, more research is needed to understand why the extent of colour constancy depends greatly on the precise instructions given to observers.

  Weblink: Cues to depth perception

  DEPTH PERCEPTION

  A major accomplishment of visual perception is the transformation of the two-dimensional retinal image into perception of a three-dimensional (3-D) world seen in depth. It is crucially important to us to construct 3-D representations of the world around us if we are to pick up objects, decide whether it is safe to cross the road, avoid cliff edges and so on.

  Depth perception depends heavily on numerous visual and other cues. We can define a cue as “any sensory information that gives rise to a sensory estimate” (Ernst & Bülthoff, 2004, p. 163).

  All cues provide ambiguous information, and so it would be unwise to place total reliance on any single cue. In addition, different cues often provide conflicting information. When you watch a movie at the cinema or on television, some cues (e.g., stereo ones) indicate everything you see is at the same distance. In contrast, other cues (e.g., perspective, shading) indicate some objects are closer than others.

  In real life, cues to depth are often provided by movement of the observer or objects in the visual environment. Some cues are not visual (e.g., based on touch or hearing). However, the major focus here will be on visual depth cues available even if the observer and environmental objects are static. Such cues can conveniently be divided into monocular, binocular and oculomotor cues. Monocular cues are those requiring only one eye, although they can also be used readily when someone has both eyes open. Such cues clearly exist, because the world still retains a sense of depth with one eye closed. Binocular cues are those involving both eyes being used together. Finally, oculomotor cues

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Perception

  From Sense to Object

  • John M. Wilding(Author)
  • 2017(Publication Date)
  • Routledge

    (Publisher)

  A third approach was to invoke information in memory. Knowledge of the correct size or brightness or shape or colour (squirrels are about 30 cm long, grass is a particular shade of green) could, it was argued, be used to correct the sensory information, or memories of sensations associated with walking towards an object or reaching for it could allow its distance to be estimated. What was never explained was how the sensations of the moment, which by definition did not provide the necessary information and could have arisen from a vast number of possible objects, could somehow evoke the right set of sensations in memory. Such an ability would imply pre-existing perception of distance, which again would seem to depend on familiarity with the object, and situations in which a given retinal size of image has in the past been matched with a certain reaching or moving experience. Though such a process might explain my learning of the distance of the book on my desk in front of me, it is difficult to conceive how I ever managed to calibrate the retinal size of an image of a squirrel 30 m away with the sensation of walking 30 m. Conceivably I could have learned a rule for the rate of change in the size of the retinal image as I move, and use that to calculate the size when the object is close to me, but it is not at all clear how structuralists could cope with the learning of rules. Nor is it clear how our generally correct perceptions of the size of novel objects could be explained. Thus none of these three approaches offers a satisfactory solution to the problem of explaining the constancies.

  Relational cues for perception

  With further investigation, however, many possible sources of the necessary information about size and distance have been discovered which make unconscious inferences from distance to size unnecessary. The slight differences in the views received by the two eyes (binocular disparity) were shown to be a powerful cue for distance perception with the invention of the stereoscope, which presents separately to the two eyes pictures giving slightly different views of an object. The most remarkable demonstrations of the effectiveness of this cue have been provided by Julesz (1964) using pairs of dot patterns which when viewed singly appear identical and random. Displacing an area in one of these laterally causes the displaced, and therefore binocularly disparate area, to appear floating in front of or behind the rest of the pattern when they are viewed in a stereoscope (see Frisby 1979, for some delightful examples). Some other cues for distance which do not require binocular vision are the relative blueness of objects (more distant ones appear bluer due to absorption by particles in the air of the longer wavelengths of the light they reflect), relative elevation in the field (more distant objects being higher if on the ground and lower if in the sky), overlap of more distant objects by nearer ones, and the casting of shadows. These cues are obviously different from and not readily reducible to simple sensations. They all depend on relations between different areas of the visual field. Other cues of more complex relational kinds which were later identified by James Gibson will be discussed below. Briefly, Gibson suggests that gradients of change

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Perception

  Theory, Development and Organisation

  • Paul Rookes, Jane Willson(Authors)
  • 2005(Publication Date)
  • Routledge

    (Publisher)
• Gestalt laws are rather vague and are sometimes difficult to distinguish one from another. The basic law of simplicity is quite hard to define and the simplest interpretation of an ambiguous figure is not always easy to predict.
• Many of the illustrations of organisation apply to very simple stimuli. Complex scenes involving everyday perception do not easily fit into a model based on simple two-dimensional illustrations.
• Eysenck and Keane (1995) suggest that the processes proposed by the Gestalt psychologists probably account only for the early stages of perceptual processing which will be organised later by previous experience.

Perceptual constancies

Constancy is the tendency for qualities of objects to stay the same despite changes in the way we view them. This is an important facet of visual perception. Try to imagine what it would be like if our perceptions were based purely on the image falling on the retina (the proximal stimulus). As you walked along the road towards a post box, it would appear to grow larger. As you moved your arm, your round wrist-watch would appear to change into an elliptical shape. As you moved from natural sunlight outdoors into artificial light indoors, your white shirt would appear yellow. Fortunately for us, our perceptual system seems to ensure that, in most circumstances our distal stimulus (our perceptual experience of objects ‘out there’) remains the same regardless of changes in the proximal stimulus.

Size constancy

This is our ability to see objects as remaining more or less the same size as we move closer or further away. The proximal size of an object can shrink and expand, but the distal size of the object seems to stay about the same. If a 6 ft. man walks away from you, you continue to perceive him as man-sized, regardless of his distance from you. It is interesting that this ability to maintain size constancy disappears under certain circumstances. When we look down from a plane, for example, cars and houses really do look like miniatures. When we take a holiday photo of a glorious, panoramic view, we can be quite disappointed with the result. In the photo, those majestic mountains can look quite pathetic. The camera, unlike the human perceiver, only registers angular size and does not compensate for the size changes that are related to distance changes. Our own size constancy mechanism does not appear to work as well when we view pictures as compared to real objects.