Our vision is a complex phenomenon that is made possible by the coordinated functions of the various structures in the eyes and brain. However, we are usually not aware of this complexity and hardly pay attention to the process until something goes wrong with our normal eyesight. Some problems with the eyesight are more discernible than others. For example, changes in visual acuity can be perceived easily as blurry vision.
However, changes in the perceived brightness of a scene may be subtle enough to be imperceptible. Dim vision is characterized by a lack of contrast in the visual scenes we perceive. This is often difficult to describe since there is no loss of focus and the scene is sharp enough. But one may sense that something is wrong due to the lack of brightness in the scene.
Perception of Brightness
The brightness of perceived visual scenes is determined by multiple internal and external factors. The intensity of lighting in an external scene is one of the major external contributing factors to the perceived visual brightness of the scene. A brightly lit object will appear brighter than a dimly lit object. Also, objects such as TV and computer screens emit their own light, which increases the visual brightness of the scenes being watched on such screens.
Apart from the raw intensity of the light, the contrast and colors present in a visual scene also influence the strength of how easily and strongly the scene is perceived. For example, under conditions of similar lighting, scenes with low contrast between objects or light hues of colors are perceived as dull when compared to scenes with higher contrast between objects and stronger hues of colors.
Factors internal to the eye and the visual part of the brain also contribute to the brightness or dimness of vision. Vision begins when light falls on the photoreceptors contained in the retinal layer of the eyes. Before reaching the retina, the light travels through different tissues and compartments in the eyes. The light enters the eyeball through a small window (known as the iris) in the outer corneal layer of the eye.
The light then passes through the fluid-filled anterior chamber and the gel-filled posterior chamber of the eyes before reaching the light-sensitive retina that is located in the back of the eye. Light falling on the macular region of the retina forms the sharpest images. The central part of the macula, known as the fovea, has the highest visual acuity.
Visual perception occurs in the brain rather than in the eyes. Stimulation of the photoreceptors in the retina results in the generation of nerve impulses that are relayed to the brain through the optic nerve. The visual cortex in the brain is responsible for visual perception. It makes sense of the nerve signals received from the eyes.
Causes of Dim Vision
Since the visual process involves multiple tissues in the eyes and brain, problems in any of the tissues that constitute the pathway to visual perception can result in vision problems, including dim vision. In many cases, dim vision is often confused with double vision or blurry vision.
However, blurry vision is not a problem with the perception of light intensity in the visual scene. Dim vision and blurry vision are two separate vision problems. The following are some of the potential causes of dim vision. These causes may either affect the light path in the eyes or visual perception in the brain.
Cataract is one of the most common causes of dim vision. Cataract is characterized by the development of cloudiness in the lens of the eye. This condition is typically seen in the elderly after the age of 60 years. Cloudiness in the lens diminishes the amount of light that enters the eye, causing dim vision. Cataract may affect the vision in either one or both the eyes.
The vision can be improved in the early stages of cataract through the use of prescription eyeglasses or by increasing the ambient lighting. As the cataract progresses, however, the lens becomes more cloudy and eventually requires surgical replacement with an artificial lens. Apart from old age, cataract can also be caused by eye injuries, chronic use of corticosteroids, and metabolic diseases such as diabetes.
Macular degeneration is characterized by a deterioration of the macula in the eye. This causes impaired visual acuity and dim vision. The colors in a visual scene also seem dull. Macular degeneration is of two types: dry macular degeneration and wet macular degeneration.
Wet macular degeneration is caused by leakage of fluid and blood from the retinal blood vessels into the chambers of the eyes. The exact cause of dry macular degeneration is not known. However, it is suspected to be caused by age-related factors. Dry macular degeneration has a higher rate of incidence than wet macular degeneration.
Stroke refers to tissue death in the brain caused by a disruption in the blood supply. Such a disruption in the blood supply to the brain tissue can occur either due to hemorrhage or ischemia. Hemorrhage refers to bleeding that is caused by a break in the walls of the blood vessels.
Ischemia refers to a block in a blood vessel that results in impaired blood flow. Hypertension, atherosclerosis, head injuries and malformed blood vessels are the main causes of stroke. Typical symptoms of stroke include mental confusion, impaired coordination and balance, muscle weakness, tingling and numbness. Visual disturbances may occur to varying degrees. Dull or dim vision is a possibility.
Treatment of Dim Vision
Effective treatment of dim vision can occur only when the underlying cause of dim vision is identified and understood. Various diagnostic investigations are usually required to arrive at the exact cause of the dim vision. Temporary measures that may improve the vision include increased ambient lighting in work areas and the use of prescription eyeglasses. However, these are usually temporary measures that do not treat the underlying cause of the dim vision.
The exact treatment of dim vision is determined based on the underlying cause. For example, dim vision caused by cataract can be corrected through surgery. However, not all causes of dim vision are treatable, and the visual damage may not be reversible. For example, death of brain tissue due to stroke cannot be reversed. Also, macular degeneration may not be preventable. In such untreatable cases, the focus of therapy shifts from trying to cure the dim vision to appropriate management of the condition.