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Chapter 5

Chapter 5: Perception

Chapter Review

FORM PERCEPTION: WHAT IS IT?

  • The recognition of forms begins with the detection of simple features; and in fact the rapid detection of features can be demonstrated in visual search tasks. However, recognition depends on more than a mere checklist of features. This point is evident, for example, in the fact that the catalog of features contained within a stimulus display depends on how the perceiver has organized the display.
  • To organize the input, the perceiver must parse the visual scene. This process involves the segregation of figure and ground.These interpretive steps seem broadly logical, and so (for example) cannot contain contradictions and cannot appeal to coincidence.

NETWORK MODELS OF PERCEPTION

  • According to many theorists, recognition depends on feature nets—networks of detectors with feature detectors serving as the initial level in the network. These networks rely on datadriven and knowledge-driven processes, and interactions within the network guarantee that the network’s output will provide the best possible compromise among the various rules governing the network’s functioning.
  • The recognition of more complex objects may require an extra layer of analysis in the feature network—a layer concerned with the identification of geons. Once the viewer has identified the geons and how they’re connected to each other, a subsequent step is needed to identify the object that’s being perceived. This last step can fail—and this is the problem in visual agnosia.

THE NEUROSCIENCE OF VISION

  • The neural processes underlying perception involve various specialized subsystems. On the retina, parvo cells are sensitive to color differences and seem crucial for the perception of pattern and form; magno cells are color blind and play an essential role in motion detection and depth perception. In the visual cortex, different types of cells respond to specific aspects of the stimulus. These different analyses go on in parallel; the cells that analyze the forms within the visual input are doing their work at the same time that other cells are analyzing the motion and still others are analyzing the colors.
  • Information from the visual cortex is transmitted to the temporal cortex, in an area called the “what” system; and to the parietal cortex, in an area called the “where” system. The “what” system is crucial for our identification of visual objects; the “where” system tells us where a stimulus is located.
  • How do we integrate the results provided by these different neural subsystems? Some evidence suggests that this binding problem is solved, in part, by neural synchrony.If, for example, the neurons detecting a vertical line are firing in synchrony with those signaling movement, then these attributes are registered as belonging to the same object. Synchronized neural firing, therefore, may be the nervous system’s way of representing the fact that different attributes are actually parts of a single object.

PERCEPTUAL CONSTANCY

  • People perceive a stable world even though changes in our viewing circumstances cause alteration in the stimuli that reach us. For example, we achieve size constancy even though the sizes of the images cast on our retinas are determined both by the size of the distal object and by viewing distance. We achieve shape constancy even though the shape of the image on our retinas depends on viewing angle.
  • Evidence suggests we achieve constancy through unconscious inference, which involves taking viewing circumstances (distance, viewing angle, illumination) into account by means of a process that performs the same function as a simple calculation.
  • The process of unconscious inference can sometimes lead us astray. If, for example, we misjudge the distance to an object, we’ll make a mistaken inference about its size—and so produce an illusion of size caused by an error in perceiving distance.

DISTANCE PERCEPTION: WHERE IS IT?

  • Our perception of depth depends on various depth cues, including binocular disparity and monocular (or pictorial) cues such as interposition and linear perspective. Another source of information is provided by the perceiver’s motion, which produces the depth cues of motion parallax and optic flow.

MOTION PERCEPTION: WHAT IS IT DOING?

  • It might seem that we perceive movement whenever an image moves across the retina—and in fact some cells in the visual cortex do respond to such movements on the retina.But retinal motion is only part of the story. In apparent movement, for example, an abrupt change in location produces a perception of movement even though there has been no actual motion (in the world or on the retina).
  • When there is motion across the retina, perceivers need to determine whether the motion was produced by movement in the environment or merely by a change in their viewing position. Further complications arise because we not only detect motion, we also interpret it—as shown by the phenomenon of induced motion. The interpretation of motion is also essential in solving the correspondence problem.

PERCEPTUAL SELECTION: ATTENTION

  • Perception is selective, and the selectivity is produced both by orienting and through central adjustments. These adjustments depend in part on our ability to prepare ourselves to perceive a particular stimulus by priming the relevant detectors and processing pathways. Thanks to this priming, perception is more efficient for the attended stimulus. Conversely, perception of unattended (and so unprimed) stimuli may be disrupted altogether, and several studies demonstrate how little we perceive of unattended stimuli.

OTHER MODALITIES

  • In this chapter, we’ve focused on vision. But similar phenomena can be demonstrated in other sense modalities, thus implying that other modalities require explanations similar to those we’ve considered for vision. Hearing, for example, also seems to involve feature analysis; but it also requires parsing and interpretation of the input. Auditory stimuli, like visual stimuli, are often ambiguous; and in hearing, just as in vision, we perceive relatively little from unattended inputs.
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