By the Troxler effect, medicine understands the local adaptation of the human eye. Light stimuli that remain permanently constant are perceived by the retina but do not reach the brain. In everyday life, the eye’s micromovements permanently shift the light on the retina to enable perception.
What is the Troxler effect?
With the Troxler effect, the retinal areas of the eyes adapt to constantly unchanging stimuli. The Troxler effect is a phenomenon of visual perception. The phenomenon was first described at the beginning of the 19th century. The Swiss physician and philosopher Ignaz Paul Vitalis Troxler, in whose honor the phenomenon was named, is considered the first to describe it. With the Troxler effect, the retinal areas of the eyes adapt to constantly unchanging stimuli. In this way, peripherally and centrally perceived objects disappear when they hold a constant position. For this reason, people can no longer recognize constant images on the visual image after a certain period of time. The Troxler effect is also called local adaptation. In everyday life, the phenomenon hardly occurs only because the microsaccades of the eyes know how to prevent it. These are the lightning-like rapid gaze target movements of the eyes that occur one to three times per second. Microsaccades shift the light on the retina and make vision possible in the first place. The receptors of the retina show almost exclusively a reaction to changes in the light conditions. Therefore, blindness can result from a failure of the microsaccades. Although the receptors also receive constant light stimuli, they do not necessarily transmit them to the brain.
Function and task
In the visual image of every human being, there should basically be vast numbers of fine veins because of the natural anatomy of the eye. Although these veins are seen by the eye, the visually constant stimuli do not reach the brain. The veins in the field of vision are thus resolved by the eye itself, but not perceived as such by the brain. This is the basis of the Troxler effect. Since the veins remain constant and are always unchanged at the same position in the field of vision, the person does not perceive them due to the effect: they are filtered out, so to speak. The constant perception of the anatomical structure would overshadow and alienate the perception of the surroundings. Humans belong to the eye-controlled creatures. From the point of view of evolutionary biology, this means that he relies primarily on his visual perception in order to survive. He uses his eyes to check his environment for dangers and food sources. In this context, the Troxler effect takes on special significance. In certain situations, humans can notice the fine and constant veins in the visual image. For example, if you poke a tiny hole in a piece of paper with the help of a needle and look through the hole thus made, you may notice the veins. When looking through the hole, it rotates in a circle and in a radius of about one centimeter around the center. As it rotates, the veins of the eyes cast shadows on the retina. The brain can re-perceive the veins inform the shadows as a change in the visual image. To prevent the Troxler effect in everyday life, the permanent microsaccades of the eye take place, which continuously shift the light on the retina. The Troxler effect occurs mainly with peripheral stimuli because the receptive fields in the retinal periphery are much larger than in the center. The smaller the receptive fields, the more apparent the relative effect of microsaccades.
Diseases and disorders
The receptors on the retina show predominantly a response to changes in light conditions. The Troxler effect speaks to this phenomenon. Unvarying light stimuli can thus induce a loss of vision. This loss of vision does not correspond to a total loss, but to a loss of vision due to receptor fatigue, which as a result gives an impression of intrinsic gray and thus corresponds to local adaptation. If a patient’s head is held rigidly and his eye muscles are paralyzed, temporary blindness may occur due to the Troxler effect.The micromovements of the eyes are no longer possible after paralysis of the eye muscles and also the head position cannot provide for the changes of the light stimuli in the visual image, which allows the visual perception to reach the brain first. Thus, without microsaccades and the constant shift of light to different receptors of the retina, vision is hardly possible. Peripheral vision in particular depends on microsaccades. Namely, the receptive retinal fields are too large in the peripheral area to be able to perceive a sufficient change of light by other microsaccades. Ocular muscle paralysis may be associated with various diseases. Often, eye muscle paralysis and thus the failure of microsaccades is preceded by damage to one or more nerves supplying the eye muscle. Eye muscle paralysis and failure of microsaccades can also result from a disturbance in signal transmission between the nerve and muscle. Other causes of ocular muscle paralysis or weakness may be muscle diseases or other types of muscle impairments. These other types of impairments of the eye muscles can be, for example, injuries in the course of an accident. In addition, tumors can compress the nerves of the eye muscles and thus interfere with signal transmission. Primary neurological diseases are also among the conceivable causes of ocular muscle paralysis or paresis, which can cause the mircosaccades to fail. The Troxler effect can help diagnose ocular muscle paralysis. If the patient’s head is fixed and still does not notice a loss of visual acuity, full ocular muscle paralysis is probably not present.
