Introduction and Psychophysics

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PSY 3520 Sensation and Perception
Introduction and Psychophysics
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Introduction and Psychophysics Lecture Outline

• I. Sensation and Perception
• II. Approaches to Studying Perception
• A. Cognitive Approach
• B. Physiological Approach
• 1. Basic Structures of a Neuron
• 2. The Neural Impulse
• a. Axonal Conduction
• b. Synaptic Transmission
• c. Recording Electrical Signals
• 3. The Basic Structures of the Brain
• C. The Psychophysical Approach
• 1. Background
• 2. Absolute Thresholds
• a. Method of Limits
• b. Method of Constant Stimuli
• c. Method of Adjustment
• 3. Difference Thresholds

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I. Sensation and Perception
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Sensation and Perception

• ________ the first contact between the organism
  and the environment
• Example stimulation of neurons
• ________ conscious experience of objects and
  object relationships
• Example recognizing a friend

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Problems with making a distinction

• When does a sensation become a perception?
• Sensations and perceptions are inseparable in
  most everyday situations.
• We use perceptions to study sensations.
• We will focus on the perceptual process.

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II. Approaches to Studying Perception

• Cognitive Approach
• Physiological Approach
• Psychophysical Approach

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Cognitive Approach

• Demonstration

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Cognitive Approach

• Focuses on how perception is affected by
• ______________________
• 2. ______________________

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II. Approaches to Studying Perception

• Physiological Approach

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Basic Structures of a Neuron
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The Neural Impulse

• Axonal Conduction
• Synaptic Transmission
• Recording Electrical Signals

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The Neural Impulse

• Axonal Conduction
• electrical-chemical process
• Resting potential of -70 mV (polarized)
• Semi-permeable cell membrane

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Axonal Conduction

• _______ gates open with sufficient stimulation
• _______________ occurs if cell membrane potential
  reaches threshold

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Axonal Conduction

• _______________
• change in the electrical
• charge across the
• axonal membrane
• Graph membrane
• potential as a
• function of time (ms)

K pumped out
Na channels open
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The Neural Impulse

• Axonal Conduction
• electrical-chemical process
• ___________ change in the electrical charge
  across the axonal membrane
• action potential is self-propagating

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The Neural Impulse

• Synaptic Transmission
• Neurotransmitters

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Synaptic Transmission

• The post-synaptic neuron may have either an
  excitatory or inhibitory response.
• Excitatory increases the firing rate of the
  cell
• Inhibitory decreases the firing rate of the cell

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The Neural Impulse

• Recording Electrical Signals
• Single-cell Recordings
• Evoked Potentials
• PET (positron emission tomography) Scan
• fMRI (functional magnetic resonance imaging)

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Recording Electrical Signals

• Single-cell Recordings

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Recording Electrical Signals

• Evoked Potentials

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Recording Electrical Signals

• PET (positron emission tomography) Scan

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Recording Electrical Signals

• fMRI (functional magnetic resonance imaging)

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Basic Structures of the Brain
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II. Approaches to Studying Perception

• Psychophysical Approach
• Background
• Wilhelm Leibniz (1646-1716)
• Johann Herbart (1776-1841)
• Gustav Fechner (1801-1887)
• __________ the study of the relationship
  between psychological experience and variations
  in the physical world.

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Psychophysical Approach

• Problem - can only measure sensations indirectly.

External Event
Internal Event
Response
indirect measure of internal event
stimulus
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Psychophysical Approach

• 3 Methods for measuring sensory thresholds
  (Absolute and Difference Thresholds)
• Method of Limits
• Method of Constant Stimuli
• Method of Adjustment

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Absolute Thresholds

• Minimum amount of energy needed to just detect a
  stimulus 50 of the time.
• How bright does a light need to be for you to
  just notice its presence?
• How much sugar do you need in your coffee to just
  notice it has been added?
• Sensitivity is the inverse of threshold
• Sensitivity 1/absolute threshold

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Method of Limits

• Stimulus intensities are presented in ascending
  and descending series
• ascending increase intensity
• descending decrease intensity
• Observer makes a forced-choice response of yes
  or no
• A series ends at the ________ the point at
  which the observers response changes

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Method of Limits
Point of Change
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Method of Limits

• Advantages
• ____________________________________
• ____________________________________
• Disadvantages
• ____________________________________
• ____________________________________

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Method of Constant Stimuli

• Intensity levels within a series are presented in
  a pseudorandom order
• detection ( yes responses) values are
  calculated for each intensity level
• detection values are plotted and a smooth
  s-shaped function, an ogive function, is fitted
  to the data

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Method of Constant Stimuli
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Method of Constant Stimuli
Detection
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Absolute Threshold
Intensity
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Method of Constant Stimuli

• Advantages
• ____________________________________
• ____________________________________
• ____________________________________
• Disadvantages
• ____________________________________
• ____________________________________

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Method of Adjustment

• Observer goes back and forth between adjusting
  the stimulus intensity until they can just detect
  the stimulus or until they can no longer detect
  the stimulus
• The absolute threshold is the mean of these values

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Method of Adjustment

• Advantages
• ____________________________________
• ____________________________________
• Disadvantages
• ____________________________________
• ____________________________________

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Difference/Discrimination Thresholds

• The smallest intensity difference between two
  stimuli needed to detect the difference 50 of
  the time.
• Also called a jnd just noticeable difference
• Must have a standard stimulus (modulus) and a set
  of comparison stimuli

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Difference Threshold
Example Using weights with a 100 gm
modulus. Weights in each series are presented in
a pseudorandom order.
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Difference Threshold
75
Heavier
50
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PSE
91
102
Comparison Stimulus Intensity (gm)
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Difference Threshold

• PSE Point of Subjective Equality 95
• CE Constant Error 75-25
• 102-9111
• jnd CE/2 11/2 5.5 gm

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Difference Threshold

• DI jnd threshold difference between test and
  standard stimulus
• It test stimulus intensity
• Io standard stimulus intensity
• DI It- Io

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Webers Law

• Weber (1830s) looked at the relationship
  between DI (jnd) and Io (standard stimulus
  intensity)
• DI kIo
• k a constant in the equation
• k DI/Io 5.5/100 .055

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Webers Law

• We can then predict the jnd if we had a 1,000 gm
  standard
• DI kIo .055(1,000) 55 gm
• ________ sensory difference thresholds are a
  roughly constant proportion of the size of the
  original comparison stimulus

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Linking Hypotheses
Perception
Psychophysics
Linking Hypothesis
Stimulus
Neural Response
Physiology