Pure Meaning Concept Of


PURKINJE EFFECT/PHENOMENON/ SHIFT. In the early 1800s, the Czech-born German physiologist Jan Evangelista Purkinje (1787-1869) (also spelled "Purkyne") described the change in color sensitivity as a visual stimulus moves from the center of the visual field to the periphery - where colors become gray at the periphery of the field and different colors change at different visual field locations. In 1825, Purkinje also reported that visual accommodation is caused by changes in the shape of the eye's lens. The Purkinje effect/phenomenon/shift refers to the manner in which colors emerge from darkness at dawn: initially there is only black and gray (with red as the darkest), next the blues appear, and finally the reds appear. The Purkinje effect occurs when the illumination of objects is reduced, and the red and orange hues (at the long wavelength end of the electromagnetic spectrum) lose their perceived brightness faster than the green and blue hues (at the short wavelength end of the spectrum). Consequently, the reds are relatively bright in strong light, and the greens and blues are bright in dim light. The greens and blues in dim light are not only relatively bright but are also "whitish" because of the colorless contribution of the rods under such viewing conditions. The Purkinje shift is caused by the differential activity of the rods, which have a greater overall sensitivity than the cones: at sunset we shift from cone to rod vision, and at sunrise we shift from rod to cone vision. The Purkinje effect fails to occur under conditions where the stimulus light is confined strictly to the rod-free region of the retina and when considering cases of night blindness or nyctalopia - that is due, often, to a deficiency of vitamin A or to a congenital retinal defect. In addition to the Purkinje effect, Purkinje's name is honored in the terms Purkinje figures - the network of interwoven blood vessels of the retina that may be perceived under conditions of low ambient illumination and when a small bright light is positioned just under the eye as the person stares at a blank screen or wall; Purkinje-Sanson images - named after Purkinje and the French surgeon Louis Joseph Sanson (1790-1841), refers to the perception by one person (by looking at a second person's eye) of three separate images of an object that the second person looks at, where one image is from the surface of the cornea, one is from the back of the lens, and one is from the front of the lens; and the Purkinje afterimage -is the second positive afterimage that follows stimulation by a bright light that is a hue complement to the original stimulus [the first in the sequence of three visual afterimages following a brief exposure to a bright light is called a Hering image - named after the German physiologist and psychologist Ewald Hering (1834-1918); the second afterimage is the Purkinje image; and the third afterimage is called a Hess image - named after the German ophthalmologist Carl von Hess (1863-1923)]. Purkinje's other research in sensory psychology included work on afterimages [or Bidwell's ghost - named in honor of the English physicist Shelford Bidwell (1848-1909), and is another term often used loosely for the Purkinje afterimage], dark adaptation, the location and nature of the blind spot in the retina, a comparison of monocular and binocular vision, the "flight of colors" (i.e., the succession of colors that occurs in a visual afterimage), and the physiology of optics. The Purkinje shift led the German scientist Johannes von Kries (1853-1928) to postulate the existence of two separate visual systems (rods and cones), and the Purkinje effect helped the Austrian-American physiologist Selig Hecht (1892-1947) to establish firmly von Kries' duplicity theory (or duplexity theory) of rod and cone visual systems for modern theories of vision. See also HECHT'S COLOR VISION THEORY; IMAGERY AND MENTAL IMAGERY, THEORIES OF; VISION/SIGHT, THEORIES OF; von KRIES' COLOR VISION THEORY.

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