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The Difference Threshold (JND)

Another type of threshold is the difference threshold. The difference threshold is the smallest change in a stimulus which a person can detect 50% of the time. The difference threshold is determined by changing the stimulus varying amounts to see whether a subject can perceive any difference in the stimulus. The smallest change a person can detect 50% of the time is called a difference threshold or just-noticeable difference (JND).

What is the difference threshold? The JND? Why are "catch trials" necessary? How is this illustrated by the student's essay?

In testing for just noticeable differences (JNDs) one must use frequent catch trials during which no change is made. Sometimes a subject will claim to perceive a change after a catch trial, because people often perceive what they expect to perceive. If you tell a person to listen for a change in a sound, that person might claim to hear a change even if none occurred.

A student discovered this when his roommate bought a new stereo system.

After we studied sensitivity to changes in pitch and loudness of sound I executed an experiment on my unsuspecting roommate. He recently brought a very powerful and expensive amplifier from home to attach to my stereo receiver. That night when he came in I told him it was all connected, and I turned it on. The lights all came on and the radio played. About five seconds later he remarked on how great it sounded and how much clearer it was. He was really excited. What really happened was that I never connected the amp with the speakers. They were still connected to his old system. I just plugged the new system in. All the lights and meters came on but the amp wasn't doing any work. My analysis is that he expected and wanted to hear a big difference, so he did. [Author's files]

By interspersing many catch trials with genuine changes in a stimulus, the experimenter can determine the smallest amount of change that an individual can reliably distinguish from catch trials. The amount of change a person can detect half the time is defined as the difference threshold or Just Noticeable Difference (JND).

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