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Chapter 1: Introduction to Perception


Chapter 1: Introduction to Perception Figure 1.1 The perceptual process. The steps in this process are arranged in a circle to emphasize that the process is dynamic ... – PowerPoint PPT presentation

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Title: Chapter 1: Introduction to Perception

Chapter 1 Introduction to Perception
  • Figure 1.1 The perceptual process. The steps in
    this process are arranged in a circle to
    emphasize that the process is dynamic and
    continually changing. See text for description
    of each step in process.

The Perceptual Process
  • Stimulus
  • All objects in the environment are available to
    the observer.
  • Observer selectively attends to objects.
  • Stimulus impinges on receptors resulting in
    internal representation.

  • Figure 1.2 (a) We take the woods as the starting
    point for our description of the perceptual
    process. Everything in the woods is the
    environmental stimulus. (b) Ellen focuses on the
    moth, which becomes the attended stimulus. (c) An
    image of the moth is formed on Ellens retina.

The Perceptual Process - continued
  • Electricity
  • Transduction occurs which changes environmental
    energy to nerve impulses
  • Transmission occurs when signals from the
    receptors travel to the brain.
  • Processing occurs during interactions among
    neurons in the brain.

  • Figure 1.3 (a) Transduction occurs when the
    receptors create electrical energy in response to
    the light. (b) Tranmission occurs as one neuron
    activates the next one. (c) This electrical
    energy is processed through networks of neurons.

  • Figure 1.4 Comparison of signal transmission by
    cell phone and the nervous system. (a) Cell
    phone 1 sends an electrical signal that stands
    for hello. The signal that reaches cell phone
    2 is the same as the signal sent from cell phone
    1. (b) The nervous system sends electrical
    signals that stand for the moth. The nervous
    system processes these electrical signals, so the
    signal responsible for perceiving the moth is
    different than the original signal sent from the

The Perceptual Process
  • Experience and Action
  • Perception occurs as a conscious experience.
  • Recognition occurs when an object is placed in a
    category giving it meaning.
  • Action occurs when the perceiver initiates motor
    activity in response to recognition.

  • Figure 1.5 (a) Ellen has conscious perception of
    the moth. (b) She recognizes the moth. (c) She
    takes action by walking toward the tree to get a
    better view.

  • Figure 1.6 Look at this drawing first, then close
    your eyes and turn the page, so you are looking
    at the same place on the page directly under this
    one. Then open and shut your eyes rapidly.
    (Adapted Bugelski D. Alampay, 1961.)

  • Figure 1.9 Did you see a rat or a man?
    Looking at the more ratlike picture in Figure
    1.11 increased the changes that you would see
    this one as a rat. But if you had first seen
    the man version (Figure 1.8), you would have been
    more likely to perceive this figure as a man.
    (Adapted Bugelski D. Alampay, 1961.)

  • Figure 1.11 Man version of the rat-man stimulus.
    (Adapted Bugelski D. Alampay, 1961.)

Two Interacting Aspects of Perception
  • Bottom-up processing
  • Processing based on incoming stimuli from the
  • Also called data-based processing
  • Top-down processing
  • Processing based on the perceivers previous
    knowledge (cognitive factors)
  • Also called knowledge-based processing

  • Figure 1.7 Perception is determined by an
    interaction between bottom-up processing, which
    starts with the image of the receptors, and
    top-down processing, which brings the observers
    knowledge into play. In this example, (a) the
    image of the moth on Ellens retina initiates
    bottom-up processing and (b) her prior knowledge
    of moths contributes to top-down processing.

Psychophysics - Overview of Methods of Measurement
  • Qualitative Methods
  • Describing
  • Recognizing
  • Quantitative Methods
  • Detecting
  • Perceiving Magnitude
  • Searching

Qualitative Methods of Psychophysical Measurement
  • Description
  • Indicating characteristics of a stimulus
  • First step in studying perception
  • Called phenomenological method
  • Recognition
  • Placing a stimulus in a category by identifying
  • Categorization of stimuli
  • Used to test patients with brain damage

Quantitative Methods - Classical Psychophysics
  • Absolute threshold - smallest amount of energy
    needed to detect a stimulus
  • Method of limits
  • Stimuli of different intensities presented in
    ascending and descending order
  • Observer responds to whether she perceived the
  • Cross-over point is the threshold

  • Figure 1.12 The results of an experiment to
    determine the threshold using the method of
    limits. The dashed lines indicate the crossover
    point for each sequence of stimuli. The
    threshold - the average of the crossover values -
    is 98.5 in this experiment.

Classical Psychophysics - continued
  • Absolute threshold (cont.)
  • Method of adjustment
  • Stimulus intensity is adjusted continuously until
    observer detects it
  • Repeated trials averaged for threshold

Classical Psychophysics - continued
  • Absolute threshold (cont.)
  • Method of constant stimuli
  • Five to nine stimuli of different intensities
    are presented in random order
  • Multiple trials are presented
  • Threshold is the intensity that results in
    detection in 50 of trials.

  • Figure 1.13 Results of a hypothetical experiment
    in which the threshold for seeing a light is
    measured by the method of constant stimuli. The
    threshold - the intensity at which the light is
    seen on half of its presentations - is 180 in
    this experiment.

Classical Psychophysics - continued
  • Difference Threshold (DL) - smallest difference
    between two stimuli a person can detect
  • Same methods can be used as for absolute
  • As magnitude of stimulus increases, so does DL
  • Webers Law explains this relationship
  • DL / S K

  • Figure 1.14 The difference threshold (DL). (a)
    The person can detect the difference between a
    100-gram standard weight and a 102-gram weight
    but cannot detect a smaller difference, so the DL
    is 2 grams. With a 200-gram standard weight, the
    comparison weight must be 204 grams before the
    person can detect the difference, so the DL is 4
    grams. The Weber fraction, which is the ratio of
    DL to the weight of the standard is constant.

  • Table 1.1 Weber fractions for a number of
    different sensory dimensions

Is There An Absolute Threshold?
  • There are differences in response criteria among
  • Liberal responder - responds yes if there is the
    slightest possibility of experiencing the
  • Conservative responder - responds yes only if he
    or she is sure that a stimulus was present
  • Each person has a different response criterion
    but the sensitivity level for both of them may be
    the same
  • Signal detection theory is used to take
    individuals response criteria into account.

  • Figure 1.17 Data from experiments in which the
    threshold for seeing a light is determined for
    Julie (green points) and Regina (red points) by
    means of the method of constant stimuli. These
    data indicate that Julies threshold is lower
    than Reginas. But is Julie really more sensitive
    to the light than Regina, or does she just appear
    to be more sensitive because she is a more
    liberal responder?

Quantitative Methods - Modern Psychophysics
  • Magnitude estimation (scaling)
  • Stimuli are above threshold.
  • Observer is given a standard stimulus and a value
    for its intensity.
  • Observer compares the standard stimulus to test
    stimuli by assigning numbers relative to the

Modern Psychophysics - continued
  • Magnitude estimation (cont.)
  • Response compression
  • As intensity increases, the perceived magnitude
    increases more slowly than the intensity.
  • Response expansion
  • As intensity increases, the perceived magnitude
    increases more quickly than the intensity.

  • Figure 1.15 The relationship between perceived
    magnitude and stimulus intensity for electric
    shock, line length, and brightness. (Adapted from
    Stevens, 1962.)

Quantitative Methods - continued
  • Magnitude estimation (cont.)
  • Relationship between intensity and perceived
    magnitude is a power function
  • Stevens Power Law
  • P KSn

  • Figure 1.16 The three functions from Figure 1.15
    plotted on log-log coordinates. Taking the
    logarithm of the magnitude estimates and the
    logarithm of the stimulus intensity turns the
    functions into straight lines. (Adapted from
    Stevens, 1962.)

Other Measurement Methods
  • Searching for stimuli
  • Visual search - observers look for one stimulus
    in a set of many stimuli
  • Reaction time (RT) - time from presentation of
    stimulus to observers response is measured