Troxler's fading or Troxler's effect is a phenomenon of visual perception

Visual perception

Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...

. When one fixates a particular point, after about 20 seconds or so, a stimulus

Stimulus (physiology)

In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

 away from the fixation point, in peripheral vision

Peripheral vision

Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision...

, will fade away and disappear. The effect is enhanced if the stimulus is small, is of low contrast or equiluminant, or is blurred. The effect is enhanced the further the stimulus is away from the fixation point.

Troxler's fading was discovered by Ignaz Paul Vital Troxler

Ignaz Paul Vital Troxler

Ignaz Paul Vital Troxler was a Swiss physician, politician, and philosopher.Troxler studied in Jena and Göttingen...

 in 1804.

Troxler's fading has been attributed to adaptation

Adaptation

An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....

 of neurons in the visual system

Visual system

The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...

 vital for perceiving a stimulus. It is part of the general principle in sensory systems that an unvarying stimulus soon disappears from our awareness. For example, if a small piece of paper is dropped on the inside of one's forearm, it is felt for a few seconds, then the sensation is no longer present; this is because the tactile neurons have adapted. But if one jiggles one's arm up and down, giving varying stimulation, one continues to feel the paper until it falls off one's arm.

A similar fading can be seen of a fixated stimulus when its retinal image is made stationary on the retina, a stabilized retinal image. Stabilization can be done in at least three ways. First one can mount a tiny projector on a contact lens. The projector shines an image into the eye. As the eye moves, the contact lens moves with it, so the image is always projected onto the same part of the retina. Second, one can monitor eye movements and move the stimulus to cancel the eye movements. Third, and this is the technique most people will know, one can induce an afterimage, usually by an intense, brief flash

Strobe light

A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope...

, such as when one is photographed using a photographic flash. This causes an image to be bleached onto the retina by the strong adaptation of the rods

Rod cell

Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...

 and cones

Cone cell

Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...

. In all these cases, the stimulus fades away after a short time and disappears.

Troxler's fading can occur without any extraordinary stabilization of the retinal image in peripheral vision because the neurons in the visual system beyond the rods and cones have large receptive field

Receptive field

The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system....

s. This means that the small, involuntary eye movements made when fixating something fail to move the stimulus onto a new cell's receptive field, giving unvarying stimulation.