Psychophysical Methods Perception Lecture 2504

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Psychophysical MethodsPerception Lecture2/5/04

• Perception Lecture
• 2/5/04

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Why We Need Methods

• Our senses convey information we need to
  interpret and respond to stimuli of light,
  chemicals, mechanical pressure, and temperature.
• How can we respond so quickly to the endless flow
  of inputs from our environment?
• What can be learned by investigating our sensory
  mechanisms and their relationship to both
  physiological and psychological phenomena?

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What Methods?

• The field of psychophysics examines this
  relationship between physical stimuli in our
  environment and our psychological reactions or
  behavior to them.
• Although the physical events around us can be
  measured, they are perceived subjectively in a
  way that is difficult to measure.

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Introduction

• Psychophysical techniques have served as keys to
  unlocking the mysteries of human perceptual
  processes.
• The tools of the psychophysicist are independent
  variables (e.g., light, sound, or mechanical
  pressure), and dependent variables -- behavioral
  responses of various kinds, such as vocalizations
  (e.g., I saw it) or button presses.

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Some Methods

• To use an analogy imagine you have a complex
  machine and youd like to examine it. There are
  three major ways
• Anatomical
• Neurophysiology
• Psychophysical

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Anatomic

• Anatomic open it up and determine its structure,
  draw diagrams of the parts and their connections.
  For example, take apart a computer tower, youll
  find slots, cards, chips, etc.

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Neurophysiological

• 2. Neurophysiological determine the functions
  of the device, what each part does. For example
  take a voltmeter to the computer tower and
  measure different areas, see how much electrical
  energy goes where and in what directions.

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Psychophysical

• 3. Psychophysical determine the operation
  capabilities of the device. Input information,
  then observe and measure the changes to the
  machine. For example, a black box.

Input Output
From the input/output relationship, the
operational characteristics of the system are
determined.
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Some Key Concepts in Psychophysics

• The rest of this lecture will consist of key
  concepts in moderate detail
• These are methods used to obtain psychophysical
  data
• Enjoy

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The Concept of the Threshold

• How sensitive a sensory system is. 
• Determined by measuring how much of a particular
  stimulus is required to reliably detect that
  stimulus
• A sensory threshold represents the entrance of a
  stimulus into sensory existence. 
• The threshold for a particular light stimulus is
  that intensity which allows it to be "just seen".

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Method of Limits

• Stimulus is either gradually increased (Ascending
  Series) or decreased (Descending Series) in
  intensity
• The subject indicates on each trial (on each
  presentation) whether the stimulus was "seen" or
  "not seen"
• (or felt, or heard, or smelled, etc.)

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Method of Adjustment (MOA)

• Subject controls the intensity of the stimulus. 
• The subject adjusts the intensity until the
  stimulus is judged to be (in the case of a visual
  stimulus) "just visible"
• For example, you keep turning your stereo up,
  louder and louder untill someone complains or
  youve lost hearing. haha.

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Method of Constant Stimuli (MOCS)

• The order of presentation of the stimulus is
  randomized, so the subject cannot anticipate the
  intensity of the stimulus on any given trial.
• The percent (Y) responses can be plotted as a
  function of stimulus strength, and a psychometric
  function (math) can be described

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Forced-Choice Procedures (FC)

• Subjects are presented with two or more
  alternatives, and must select one on each trial
  even if the stimulus was not clearly seen. 
• The choice can thus be coded as a criterion-free
  "correct" or "incorrect". 
• Alternatives can be presented sequentially
  (temporal forced-choice), or can be presented
  simultaneously (spatial forced-choice).
• There must be at least two alternatives, but
  there can be up to four or five.

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Forced-Choice Procedures
Here is an example of a three-alternative spatial
forced-choice task (color discrimination). 
Since there are more than 2 choices,
forced-choice becomes an "oddity" task, that is,
the subject's taks is to choose the "odd" (i.e.,
different looking)
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Absolute Thresholds and Difference Thresholds

• Absolute Threshold- the amount of a stimulus
  required to simply detect it against a
  background (e.g., detecting light in an
  absolutely dark room). 
• Difference Threshold, defined as the size of the
  difference between two stimuli required in order
  to just tell them apart. 
• Another word for the Difference Threshold is the
  Just-Noticeable Difference (JND).

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Weber's Law

• JND is not an absolute amount of stimulus, but is
  a constant proportion of the background
  "standard" stimulus, IO.  The larger increments
  required for them to be "seen" on "standard"
  backgrounds of increasing intensity is
  illustrated in the three figures.

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Thus, the more intense (or larger) the background
stimulus, the larger the increment needed to be
in order for it to be detected on top of the
background.
Weber's Law
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Sensory Scaling

• Stimuli which exceed threshold are referred to as
  suprathreshold.  
• Measuring and expressing the relationship of
  suprathreshold stimuli to each other is  referred
  to as sensory scaling.

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Sensory Scaling

• Magnitude Estimation Subjects assign numerical
  values to the strength of stimuli
• Magnitude Production subject adjusts the
  intensity of a stimulus to equal a prescribed
  numerical value. 
• Cross-Modal Matching subject might be asked to
  adjust the brightness of a visual stimulus until
  it is judged to be as bright as an auditory
  stimulus is loud.

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Signal Detection Theory

• Signal Detection Theory analyze the performance
  of telecommunication systems (which transmit and
  receive information, as do nervous systems).  

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Want More Details?

• http//www.psychology.psych.ndsu.nodak.edu/mccourt
  /website/htdocs/HomePage/Psy460/Visual20psychophy
  sics/Visual20psychophysics.html
• Have any questions?
• svec_at_unr.nevada.edu

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The End Thank You