The medical definition for visual threshold is the lowest or minimal level of light intensity evoking a visual sensation. However, the absolute threshold has been redefined by signal detection theory. This theory states that visual threshold is the level at which a stimulus will be detected at least 50 percent of the time. This threshold for a person, however, can be influenced by adaption to the light stimulus, attention and expectation of the stimulus, and cognitive processes.
Anatomy and Physiology
The human eye senses light via specialized nerve cells called photoreceptor cells. There are two types of photoreceptor cells: rods and cones. Together these photoreceptor cells have the ability to sense light, color, movement, and other visual stimuli. Rods function best at low levels of light and are responsible for night (scotopic) vision. These cells are incredibly sensitive and under optimal conditions a single unit of light, or photon, may activate rod photoreceptors. This property of having high sensitivity allows rods to have fine light and motion detection. Cones process color perception and details of an object, while rods are used for night and peripheral visions. Cones are named by their size and shape, and they are more prominent in humans and other diurnal animals compared to nocturnal animals. Because cones identify details of an object, they need plenty of light to be activated. This is called photopic vision.
Testing Absolute Visual Threshold
Since vision is dependent on light stimulus, threshold of vision can be difficult to determine. For this reason, testing absolute visual threshold requires that the person must be adapted to a dark room, which will optimize the sensitivity of the individual’s rod photoreceptors. Typically it takes about 40 minutes for a person to adapt to the dark. The light stimulus is then presented to a region of the retina with the highest concentration of rods—generally the peripheral portion of the eye. The duration of the light stimulus begins at a short length (1 millisecond) and then increases until the stimulus is correctly identified 50 percent of the time.
Although this test is very accurate, it is not practical to have each person sit in a dark room for 40 minutes prior to the exam. Thus, a faster test has been developed, but instead this test determines a person’s threshold of visual processing. Absolute threshold in this test is the presentation speed where an individual correctly detects a word 50 percent of the time, which is the same as chance (the person either sees or does not see the word). Only those who can read are able to complete this exam.
Testing Absolute Visual Processing
The stimulus in this test is a word that is presented between two nonsense words. For instance, the word “garden” is presented between “wzltyp” and “qambcx.” The duration of the word presentation is extremely fast, about 33 milliseconds long. If the individual does not see the word, then a new word is presented between two nonsense words for twice as long, 66 milliseconds. The duration of the word presentation continues to increase until the individual sees the word 50 percent of the time.
Jennifer L. Hellier
See also: Cones; Optic Nerve; Retina; Rods; Visual Perception; Visual System
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