Absolute threshold

Absolute threshold is a term in neuroscience and psychological research that refers to the least amount of sensory input that can be detected. Researchers usually further define the absolute threshold as the minimum intensity of a stimulus such as light, sound, or heat that can be detected at least 50 percent of the time. This allows for the variety of factors that can affect the perception of a stimulus. The threshold can apply to sound, sight, taste, or touch, and researchers can determine it for any living organism. The threshold can also be changed by exposure to certain stimuli, expectation of perceiving it, and other factors.

Background

The concept of the absolute threshold emerged during nineteenth-century psychophysics research as psychologists experimented to determine the least level of stimuli that a person could detect. The concept originated in the work of German physiologist Ernst Heinrich Weber and German experimental psychologist Gustav Fechner. Weber, who was a pioneer in psychophysics and sensory physiology, developed the concept of measuring levels of things that are perceived and determining the relationships between them. Fechner studied sensory perception and was interested in determining the full range of a type of sensation—for instance, the lowest and highest pitch of a musical note—that people could perceive.

Fechner developed the idea of the absolute threshold as the minimum intensity of a stimulus that can be consciously detected. He and Weber worked with a unit called a just noticeable difference (JND) in measuring levels of stimuli. Fechner also recognized that there could be levels of a stimulus that a person does not consciously register, but that may influence responses without conscious awareness. He did not include these in his assessment of absolute threshold. Instead, he maintained that the absolute threshold was the lowest level of stimuli that created conscious recognition in an organism.

It is important to note that absolute threshold is related to but not the same as the difference threshold. The difference threshold is the least amount of variation between two similar stimuli that can be detected, such as degrees of temperature. This concept was also part of the work of Weber and Fechner. However, while absolute threshold is always related to the least amount of a stimulus that can be noticed, the difference threshold can be determined between two variations of a stimulus at any level of detection. For example, researchers helping to design traffic signals might investigate the difference threshold between two shades of red to decide which is easiest to see by the most people. Researchers also apply threshold studies in hearing tests, warning systems, and accessibility design.

Overview

Researchers use one of several methods to determine absolute threshold. The method used depends in part on the type of stimuli being measured, other variables that could be factors, and the intended use of the results of the study. There are three classic methods used in psychophysical research, and one adaptive variation that is used most often in contemporary study.

The three classical methods are the adjustment method, the constant stimuli method, and the limits method. Another method is called the staircase method. Researchers choose a method that best suits the type of research being conducted.

In the adjustment method, the study participant is asked to bring the stimulus to a level where the participant can detect it. This might involve something like turning a knob to increase volume of a sound until it can be heard. It can also be used in reverse, turning the sound down until it can no longer be heard. The process is generally repeated multiple times, and the researcher uses an average of the results to determine the actual absolute threshold.

The method of limits and the constant stimuli method are similar. In each, the researcher chooses a number of specific levels of stimuli, such as the same sound at a number of different volumes. The levels are deliberately chosen to be below, at, and above the anticipated absolute threshold. In the limits method, the stimuli are presented in order from least to greatest or vice versa. In the constant stimuli method, the stimuli are presented randomly. In either method, the test participant is asked to indicate when the stimulus is detected. As with the adjustment method, the test is repeated and the results combined to allow for outside influences on the test process.

The staircase method was refined by American experimental psychologist Tom N. Cornsweet in 1962. This method starts with a few variations of a stimulus and then adjusts them up or down depending on the study participant’s perception. For example, a tone will be played, and if the participant indicates it is heard, the sound will be adjusted downward until it can no longer be heard. Conversely, if it is not heard, the sound will be adjusted upward in increments until it is detected. The staircase method can estimate thresholds efficiently while reducing the number of trials needed.

As with all the other methods, a researcher using the staircase method will repeat the study several times with the same participant and use a combination of the results to determine the absolute threshold. This allows for the many factors that can affect a person’s perception of a stimulus, including other stimuli in the test environment, the person’s overall level of health and well-being, and other elements. The researcher might also take into account such factors as the person’s age, since research has determined that the ability to detect certain levels of some stimuli changes with age.

Researchers also note that any individual’s absolute threshold can change from day to day. It can also be intentionally altered. This often happens by either continued exposure or expectation. The phenomenon sometimes called “nose blindness” is an example of this; people who are exposed to certain smells become oblivious to them, regardless of how noxious the odor might be, because the continued presence of the stimuli renders it less noticeable. People can also become more acutely aware of a stimulus and lower their absolute threshold by actively looking for clues to its presence. Parents who hear the faintest cry of their newborn in a noisy room are exhibiting the way paying attention can lower the absolute threshold. The study of absolute thresholds is an important part of psychology and neuroscience because it helps researchers understand how organisms detect sensory information.

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