Psychophysical Methods Perception Lecture 2/5/04

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Psychophysical Methods Perception Lecture 2/5/04

  1. 1. PsychophysicalPsychophysical MethodsMethods Perception Lecture 2/5/04 Perception Lecture 2/5/04
  2. 2. 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?
  3. 3. 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.
  4. 4. 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.
  5. 5. Some Methods  To use an analogy imagine you have a complex machine and you’d like to examine it. There are three major ways: – Anatomical – Neurophysiology – Psychophysical
  6. 6. Anatomic 1. Anatomic: open it up and determine its structure, draw diagrams of the parts and their connections. For example, take apart a computer tower, you’ll find slots, cards, chips, etc.
  7. 7. 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.
  8. 8. 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.
  9. 9. 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
  10. 10. 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".
  11. 11. 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.)
  12. 12. 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 you’ve lost hearing. haha.
  13. 13. 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
  14. 14. 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.
  15. 15. 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)
  16. 16. 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).
  17. 17. 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.
  18. 18. 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
  19. 19. 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.
  20. 20.  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. Sensory Scaling
  21. 21. Signal Detection Theory  Signal Detection Theory: analyze the performance of telecommunication systems (which transmit and receive information, as do nervous systems).
  22. 22. Want More Details?  http://www.psychology.psych.ndsu.nodak. edu/mccourt/website/htdocs/HomePage/Ps y460/Visual%20psychophysics/Visual %20psychophysics.html  Have any questions? svec@unr.nevada.edu
  23. 23. The End Thank You

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