Psychophysics: Definition & Fundamentals

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  • 0:02 Psychophysics
  • 1:19 Method of Limits
  • 1:51 Method of Adjustment
  • 2:23 Method of Constant Stimuli
  • 2:55 Difference Threshold
  • 5:26 Lesson Summary
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Lesson Transcript
Instructor: Gina Mitchell
When you interact with the world around you, your brain is responding to different environmental stimuli. Psychologists who study these responses, like light, sound, or tastes, are part of the field of psychophysics. In this lesson, you will get a definition of psychophysics and learn about the fundamental methods that psychologists use to study psychophysics.

Definition of Psychophysics

Have you ever wondered what happens when you eat a juicy watermelon? How does the grainy texture of that pink fruit get translated into the sensation of sweet, watery, and delicious in your brain? Scientists who study psychophysics are interested in the processes that occur to produce your response to the watermelon. Therefore, psychophysics quantitatively investigates how much of a stimuli we can detect and how we detect differences between stimuli in the environment with our sensory systems, including vision, auditory, taste, smell, and pain.

The field of psychophysics was pioneered by Gustav Fechner in the 1860s. Fechner introduced various methods for quantitatively measuring the relationship between stimuli and perception. Fechner's methods included the method of limits, the method of adjustment, and the method of constant stimuli. Each method measures our response to a stimulus in a different way. They are used to measure absolute threshold, or the smallest detectable amount of a stimulus. For example, if we're looking at your response to watermelon and want to measure your absolute threshold, we would look for the smallest piece of watermelon that you could taste. If we were looking at the visual sensation, we would look at the smallest amount of light that you could detect.

Method of Limits

To measure absolute threshold with the method of limits, a researcher would present the stimulus in either ascending or descending order to determine the smallest amount detectable. For example, you would be given a large amount of light and asked if you could see it. On the next trial, the amount of light would be decreased. This would occur until you report that you can no longer see the light. This would be the method of descending limits because we start with something big and work down. In the method of ascending limits, we would start with a small amount of light and work up.

Method of Adjustment

In the method of adjustment, the observer or the researcher continuously adjusts the stimulus until they can or cannot perceive it. This differs from the method of limits, because the individual is not giving a yes or no response on each trial. Instead, they are responsible for adjusting the stimulus themselves. For example, if you were asked to identify the smell of gym socks, you might have a knob that you can turn until you can just smell the scent. Or, you might turn the knob until you can no longer smell the scent.

Method of Constant Stimuli

The method of constant stimuli differs from the method of limits and the method of adjustment in that it presents the stimuli in a random order. For example, if I'm looking to see what is the lowest amount of sound that you can detect, I would randomly present several different tones at different sound levels. Some would be loud, while others would be quiet. As the observer, your job would be to let me know when you can or cannot hear the sounds. This method is generally considered the most accurate method, because it limits the subject from being able to predict a stimulus.

Difference Threshold

Another aspect of psychophysics and Fechner's methods is determining the smallest detectable difference between two things, or the difference threshold. For example, say that I present you with two yellow color patches. To determine your difference threshold for color perception of yellow, I would adjust the amount of yellow in one of the patches until you were just able to detect the difference. As can be expected, it is easy to detect differences when they are great. It is more difficult to detect differences when items are similar in color.

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