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The nervous system processes sensory information and transfers it to the brain. The absolute threshold is the lowest detectable level of a stimulus 50% of the time, influenced by external conditions. Vision and hearing have their own absolute thresholds. The difference threshold is the minimum difference between stimuli needed for the brain to detect any difference. Factors such as expectation, repeated exposure, damage to sensory organs, and mental or emotional state can influence the absolute threshold.
The nervous systems of humans process sensory information by interpreting external and internal stimuli and transferring the information to the brain for processing. Neurons in the brain receive electrochemical impulses. Like any type of sensory tool, the various afferent neurons, or receptors, have a minimal stimulus that they can actually detect. This stimulus, called the absolute threshold, varies from person to person and can be influenced by external conditions. When individuals are tested, they may or may not experience the stimulus; therefore, the absolute threshold is defined as the lowest detectable level of a stimulus 50% of the time.
Vision relies on two types of cells, rods and cones, which have different functions. Cones detect color; rods detect shapes and are better suited for seeing in the dark. The absolute threshold for vision, or the minimum number of photons the eye can detect, is generally accepted as 90 photons, of which only nine reach the retina and fovea, the most sensitive part of the eye. This measure assumes optimal conditions; that is, eyes fully adapted to darkness and to a wavelength to which rods are most sensitive.
Hearing works by transmitting electrochemical impulses through the auditory nerve as air vibrations travel from the eardrum to the cilia in the inner ear. The minimum atmospheric pressure to induce audible sound when the source is adjacent to the ear is 20 micropascals, assuming no hearing damage. The ear is meant to hear sounds in the range of 20 to 20,000 hertz, but is most sensitive when exposed to frequencies between 1,000 and 5,000 hertz.
Several factors can influence the absolute threshold reported by a person: expectation or anticipation of the stimulus, repeated exposure, damage to sensory organs, and mental or emotional state. If a person expects to receive a particular stimulus, they will naturally be more attuned to it. Repeated exposure, on the other hand, will cause the sensory nervous system to filter out that kind of input. Damage to sensory organs raises the absolute threshold, and high levels of stress will cause the body to secrete adrenaline, which makes the sensory organs more attuned to incoming stimuli and more likely to respond.
The absolute threshold is distinguished from the difference threshold. This measurement refers to the minimum difference between two or more stimuli needed for the brain to detect any difference. The body’s difference threshold can be trained; for example, wine tasters learn to distinguish between nearly identical flavors of wine.
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