Is the lowest intensity at which a person can detect a stimulus 50% of the time?

Receptors for each sensory system are limited by the amount of stimulation necessary to elicit a sensation and by the amount of stimulus change that can be detected. In the field of psychology called psychophysics, relationships between physical stimuli and psychological experience are studied. One technique to study such relationships is called the method of constant stimuli, in which stimuli of varying intensities are presented in random order to a subject. The results are used to determine the absolute threshold—the minimum intensity detected by a subject 50% of the time. (Your dog, for instance, has a much lower absolute threshold for sound than you do and hears a car in the driveway before you hear the knock on the door.)

The difference threshold—the minimum (physical) distinction between stimulus attributes that can be detected 50% of the time—is also of concern. The difference threshold is also called a just noticeable difference (JND). Ernst Weber, a well‐known early investigator, observed that regardless of their magnitude, two stimuli must differ by a constant proportion for their difference to be detectable. His observations are formulated as Weber's law, which states that the “just noticeable difference” is a constant fraction of the stimulus intensity already present. (If a room is quiet, you can hear a faint knock at the door. But if your CD player is blaring, it takes a loud bang on the door for you to hear it.) If you are exposed to a stimulus that doesn't change over a period of time, sensory adaptation occurs, and you become less sensitive to the stimulus. If you have to study in a room with a constant noise outside, for example, you will usually eventually adapt to the noise, and it will become less offensive.

Signal detection. Factors other than the magnitude of the stimulus also affect sensory discriminations. When a discrimination—that is, the detection of a stimulus (a signal)—must be made against a background of noise, the procedure is called signal detection. Signal detection theory takes into account the fact that people are making decisions as they make sensory discriminations. When they attempt to separate a signal change from its background, they may guess, have biases in their judgments, or become less vigilant during the judging process. Knowledge of signal detection theory is useful in many situations—for instance, if one were teaching people to detect accurately small blips on radar screens in an air control tower. Another aspect of sensory perception, subliminal perception (perception without awareness), has been of interest in recent years. However, the data concerning the existence of the phenomenon are still controversial.

Sensation is the process by which physical energy from objects in the world or in the body stimulates the sense organs. The brain interprets and organizes this sensory information in a process called perception. Psychophysics is the study of how the physical properties of stimuli relate to people’s experience of stimuli. Research in psychophysics has revealed much information about the acuity of the senses.

Measuring the Senses

Psychologists assess the acuity of the senses in three ways:

1. Measuring the absolute threshold
2. Measuring the difference threshold
3. Applying signal detection theory

The absolute threshold is the minimum amount of stimulation required for a person to detect the stimulus 50 percent of the time. The difference threshold is the smallest difference in stimulation that can be detected 50 percent of the time. The difference threshold is sometimes called the just noticeable difference (jnd), and it depends on the strength of the stimulus.

Example: If someone were comparing two weak stimuli, such as two very slightly sweet liquids, he’d be able to detect quite a small difference in the amount of sweetness. However, if he were comparing two intense stimuli, such as two extremely sweet liquids, he could detect only a much bigger difference in the amount of sweetness.

Researchers use signal detection theory to predict when a weak signal will be detected. This theory considers the fact that the ability to detect a signal depends not only on the strength of the signal but also on the perceiver’s experience, motivation, expectation, and degree of alertness. Different people respond differently to the same signal, and the same person may detect a particular signal at one time but not another. Furthermore, people can often detect one type of signal in a sensory modality such as hearing or vision but be oblivious to other types of signals in the same sensory modality.

Sensory Adaptation

When people walk into a restaurant, they probably notice food smells right away. However, as they sit in the restaurant, the smells gradually become less noticeable. This phenomenon occurs because of sensory adaptation. Sensory adaptation is the decrease in sensitivity to an unchanging stimulus. The smells don’t disappear—the people just become less sensitive to them.

Development of the Senses

Babies have all the basic sensory abilities and many perceptual skills, but these abilities develop and grow more sensitive over time. Babies can recognize the difference between a human voice and other sounds, and they can locate a sound’s origin. They can recognize the difference between smells and, very early on, can recognize their mother’s particular smell. As for taste, they can differentiate between sweet and salty. Babies also have fairly adept visual abilities. Soon after birth, they can distinguish objects of different colors and sizes. When they are just a few weeks old, they begin to differentiate among contrasts, shadows, and patterns, and they can perceive depth after just a few months.

Sensitive Periods

Even innate perceptual skills need the right environment to develop properly. A lack of certain experiences during sensitive periods of development will impair a person’s ability to perceive the world.

Example: People who were born blind but regain their vision in adulthood usually find the visual world confusing. Since these adults were blind in infancy, they missed the sensory experiences necessary for their visual system to develop fully.

Is the lowest intensity of a stimulus that a person can detect 50% of the time?

The just noticeable difference (JND), also known as the difference threshold, is the minimum level of stimulation that a person can detect 50% of the time.

What is the level of stimulation required to detect 50% of the time?

An absolute threshold is the smallest amount of stimulation needed for a person to detect that stimulus 50% of the time.

What is the lowest stimulus intensity required for detection?

The lowest stimulus intensity required for detection is the absolute threshold, and the smallest noticeable difference between a standard stimulus intensity and another stimulus value is the difference threshold.