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Chapter 1: Sensation and Perception


Chapter 1: Sensation and Perception * * * * * * * * * * * * * * * * Quantitative Methods - Classical Psychophysics Absolute threshold - smallest amount of energy ... – PowerPoint PPT presentation

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Title: Chapter 1: Sensation and Perception

Chapter 1 Sensation and Perception
Overview of this Lecture
  • Why study perception?
  • What are the steps in the perceptual process
  • How is perception measured?

Why Study Perception?
  • Understanding how you perceive the world
  • Language processing
  • Color vision
  • Multitasking and attention
  • Mindfulness
  • Rehabilitation Counseling
  • Devices to assist people with vision and hearing

Science project
  • Your friend Ellen is helping you collect bugs for
    your science class.
  • Walk in the woods searching for bugs.
  • If you dont like bugs, imagine Ellen is helping
    you shop for the latest, coolest sneakers.

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

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

  • 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.

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 1.1 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.

  • Information that the perceiver brings to the
  • Knowledge can affect a number of steps in the
    perceptual process.
  • Experience helps you better identify stimuli.
  • Example Police knowing how to identify evidence.

  • Take a look at the next two slides.
  • How do you identify the stimulus?

  • Look at this drawing first, then close your eyes.

  • Did you see a rat or a man?

Demonstration 2
  • Take a look at the next two slides.
  • How do you identify the stimulus?

(No Transcript)
  • .

  • What the perceiver brings to the situation.
  • Experience and/or expectation
  • Is perception bottom-up or top-down

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

Both types interact
  • 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.

  • the image of the moth on Ellens retina initiates
    bottom-up processing and
  • (b) her prior knowledge of moths contributes to
    top-down processing.

How do we study perception?
  • Depends on the tools we have available.
  • Early psychologist could only present stimuli and
    ask subject to respond.
  • Ex. Rat/man demonstrations
  • Psychophysical approach (PP)
  • the stimulus-perception relationship
  • What you see is what you get.

PP Psychophysical Approach Present stimulus and
get subjects response of perception PH1
Physiological Approach 1 Present stimulus and
measure neurons firing (brain response). PH2
Physiological Approach 2 Present stimulus and
measure both brain response and subjects
  • .

Physiological Response
  • How does the brain respond to visual stimuli?
  • Visual cortex in back of brain.
  • Implant electrodes in visual cortex of cat.
  • Measure response from cells.
  • Nobel prize in 1981
  • Hubel and Wiesel
  • PH1 in Goldstein book

Brain scans
  • Newest techniques allow us to scan the brains of
    awake, human subjects.
  • Subject can tell us what they perceive.
  • Brain scans show us which areas are most active.
  • Assume those areas are important for perception.
  • PH2 in book

  • (PP) links stimuli and perception,
  • (PH1) links stimuli and physiological processes
    and the (PH2) links physiological processes and

Measuring Perception
  • Begin with psychophysical approaches.
  • First methods available to psychologists.
  • Dont require brain implant or scanners.

Psychophysics - Overview of Methods of Measurement
  • Qualitative Methods (identify and sort)
  • Describing
  • Recognizing
  • Quantitative Methods (ratio scales)
  • 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

Wisconsin Card Sorting Test
Where do you sort the top card on deck?
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
  • Hearing Test

  • 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.

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.)

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

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