Psychophysics for HF/E

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Absolute Threshold

- What is the minimum value o the physical stimulus which is detected reliably:

- By 50% of the study participants

- In 50% of the study conditions

Methods for establishing the Absolute Threshold value

- Method of adjustment - participant adjusts stimuli themselves on a continuous scale

- Until they believe it is just noticeable

- Naive observers using the method of adjustment are usually about ten times higher than thresholds estimated by the forced choice procedure. With some practice the difference gets smaller. But even for experienced observers the adjustment thresholds and forced-choice thresholds are different by about a factor of 3.

- Method of Limits - experimenter presents the stimuli to participant in progressive steps both above and below likely threshold working either up or down

- Participant is asked at each stage whether they can detect it

Advantages/Disadvantages

Subject can become habituated (for example, in auditory experiments, muscles of inner ear can adjust themselves)

Subject can anticipate stimulus to become undetectable or detectable and make a premature judgment

To avoid this: Staircase procedure (von Békésy, 1960)

Sound starts out audible and gets quieter after each of the subject's responses, until the subject does not report hearing it. At that point, the sound is made louder at each step, until the subject reports hearing it, t which point it is made quieter in steps again. This way the experimenter is able to "zero in" on the threshold.

Even more elegant, but more time-consuming: maximum-likelihood procedures

- Method of constant stimuli - experimenter presents the stimuli to participant in random steps

- Participant is asked at each stage whether they can detect it

- Reduces errors of habituation and expectation.

Difference threshold

- What is the minimum difference between two values of the physical stimulus which is detected reliably:

- By 50% of the study participants

- In 50% of the study conditions

- Participants usually presented with an unchanging standard (comparative) stimulus

- Was the stimulus the same or different from the standard stimulus?

- Same methods can be used: Methods of adjustment, Limits and Constant stimuli