Absolute Threshold

1
Absolute Threshold

• PSY 3203 - Perception Appalachian State
  University

2
Method of Limits

• A range of stimuli are selected, from below
  threshold to above threshold.
• The stimuli are ordered and presented in a
  sequence that corresponds to either an ascending
  or descending series of the amount of the
  physical dimension under investigation.
• Forced Choice The observer is to judge only
  whether the stimulus was detected (Yes) or not
  detected (No).
• The trial is ended when there is a transition.

3
Method of Limits

• The observer will need to make many judgments
  over several repetitions of the ascending and
  descending series.
• The psychophysical function is created in the
  following fashion
• X-axis contains the information about the
  physical dimension.
• Stimuli are ordered from less to more.
• Interval size reflects size of increment from
  stimulus to stimulus.
• Y-axis contains the information about the
  observers judgment of the experience.
• Responses are reported as Percent(Yes).
• Scale goes from 0.00 (0) to 1.0 (100 )

4
Lets try an example

• The following slide contains a series of music
  icons.
• Clicking on an icon will cause a 2-second tone to
  be played.
• You should record Y if you hear the tone and N if
  you do not hear the tone.
• The number on the icon indicates the amount of
  volume.
• Each increment in number is an equal increment in
  volume.

5
Lets try an example

• First, the speaker volume will be adjusted so
  that the range of stimuli goes from inaudible to
  audible.
• Second, you will make judgments with ascending
  and descending series.
• Third, we will compute the psychophysical
  function and discuss the location of the absolute
  threshold.

6
The Stimuli
15 7
14 6
13 5
12 4
11 3
10 2
9 1
8
7
Method of Constant Stimuli

• A range of stimuli are selected, from below
  threshold to above threshold.
• The stimuli are ordered and presented in a
  randomized series of the amount of the physical
  dimension under investigation.
• Forced Choice The observer is to judge only
  whether the stimulus was detected (Yes) or not
  detected (No).

8
Method of Constant Stimuli
13 15
7 6
4 11
10 14
5 3
12 2
1 9
8
9
Method of Adjustment

• Observer makes adjustment to stimulus source
  until threshold is achieved.
• A production method. (more Behavioral?)
• Method works very quickly.
• Greater variability hence less accuracy.

10
Staircase Method

• Presents and notes response (Yes or No)
• If No then increase stimulus intensity on next
  trial.
• If Yes then decrease stimulus intensity on next
  trial.
• Direction of change is reversed when answer
  changes.