The Difference Threshold and Signal-detection Theory

How much weight must be added to or subtracted from a stack of books for the carrier to sense that the load is heavier or lighter? The just-noticeable difference (JND) is the smallest change in sensation that a person is able to detect 50 percent of the time. The difference threshold is a measure of the smallest increase or decrease in a physical stimulus that is required to produce the JND. A person holding a 5-pound load would notice the addition of a single pound, but a person already holding 100 pounds would not be able to sense an additional pound. More than 150 years ago, researcher Ernst Weber(1795-1878)observed that the JND for all the senses depends on a proportion or percent of change rather than a fixed amount of change. This observation became known as Weber’s law. A weight being held must increase or decrease by 2 percent for the difference to be noticed. According to Weber’s law, the greater the original stimulus, the more it must be increased or decreased for the difference to be noticeable.

The difference threshold is not the same for all the senses. very large (20 percent)difference is necessary for some changes in taste to be detected, for instance. In contrast, a difference is noticeable if a musical tone becomes slightly higher or lower in pitch by only about 0.33 percent. Nor are the difference thresholds for the various senses the same for all people. In fact, there are great individual differences, often arising from experience or expertise. Professional food tasters know if a particular item or batch is a little too sweet, even if its sweetness varies by only a fraction of the 20 percent usually necessary to detect changes in taste. Furthermore, people who have lost one sensory ability often gain greater sensitivity in the other sensory abilities. Actually, Weber’s law best fits people with average sensitivities and sensory stimuli that are neither very strong nor very weak, for example, not as loud as thunder or as quiet as a faint whisper.

The classic methods in psychophysics for measuring difference thresholds focus exclusively on the physical stimulus-how strong or weak it is or how much the stimulus must change for the difference to be noticed. But even within the same individual, sensory capabilities are sharper and duller from time to time and under different conditions. Factors that affect a person’s ability to detect a sensory signal are, in addition to the strength of the stimulus, the motivation to detect it, previous experience, expectation that it will occur, and alertness or level of fatigue.

In another approach, these factors are taken into account. According to signal-detection theory the detection of a sensory stimulus involves discriminating that stimulus from background noise (all the other stimuli present in the environment) and deciding whether the stimulus is actually present. But deciding that a stimulus is present in the first place depends partly on the probability that the stimulus will occur and partly on the potential gain or loss associated with deciding that it is present or absent Suppose you were given the description of a cousin you had never seen before and were asked to pick her up at the airport. Your task would be to scan a sea of faces for someone fitting the description and then to decide which of the several people who fit the description was actually your cousin. All the other faces and objects in your field of vision would be considered background noise.How sure you would have to be before you approached someone would depend on several factors-among them the embarrassment you might feel approaching the wrong person as opposed to the distress you would feel if you failed to find your cousin.

Signal-detection theory has special relevance to people in many occupations: air-traffic controllers, police officers, military personnel on guard duty, medical professionals, and poultry inspectors, to name a few. Whether these professionals detect certain stimuli can have important consequences for the health and welfare of vast numbers of people.?

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1.How much weight must be added to or subtracted from a stack of books for the carrier to sense that the load is heavier or lighter? The just-noticeable difference (JND) is the smallest change in sensation that a person is able to detect 50 percent of the time. The difference threshold is a measure of the smallest increase or decrease in a physical stimulus that is required to produce the JND. A person holding a 5-pound load would notice the addition of a single pound, but a person already holding 100 pounds would not be able to sense an additional pound. More than 150 years ago, researcher Ernst Weber(1795-1878)observed that the JND for all the senses depends on a proportion or percent of change rather than a fixed amount of change. This observation became known as Weber’s law. A weight being held must increase or decrease by 2 percent for the difference to be noticed. According to Weber’s law, the greater the original stimulus, the more it must be increased or decreased for the difference to be noticeable.?

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