The visual field is the entire area of space an eye can see when the gaze is fixed on a central point. This area encompasses sharp central focus and wide-ranging peripheral vision. Measuring the full extent of this field, expressed in degrees, is a fundamental part of eye care and neurology. An accurate assessment establishes a baseline of normal function and detects areas of vision loss. Changes in the visual field can indicate various diseases affecting the eye, the optic nerve, or the brain’s visual processing centers.
Defining the Standard Visual Field
The normal visual field of a single eye (monocular field) is asymmetrical due to facial anatomy. When looking straight ahead, the field extends approximately 60 degrees inward toward the nose (nasal side). The field is much wider on the outer, or temporal, side, extending 100 to 110 degrees from the center. This difference occurs because the bridge of the nose obstructs the inward view, while the temporal side is relatively clear.
The vertical extent also shows slight asymmetry between the upper and lower halves. The field typically extends 60 to 70 degrees above the horizontal midline. Conversely, it reaches further down, extending approximately 75 to 80 degrees below the midline. This is largely due to the brow ridge limiting the superior field. Overall, the monocular field spans about 160 to 170 degrees horizontally and 135 to 150 degrees vertically.
The Difference Between Monocular and Binocular Vision
The binocular visual field represents the combined view from both eyes working together. The combined field is not simply double the monocular field, as a large portion of the two fields overlaps significantly in the center. This overlapping area enables stereopsis, which is the mechanism behind depth perception. The total binocular visual field is a wide, horizontal oval, extending to roughly 200 degrees laterally and about 130 degrees vertically. The area of overlap is centered, spanning approximately 120 degrees of the total horizontal field.
Assessing the Visual Field
Eye care professionals use specific methods to accurately map the visual field. One common and detailed method is automated perimetry, often performed using a machine like the Humphrey Field Analyzer. During this test, the patient stares at a central fixation light inside a concave dome while small flashes of light appear in various locations. The patient presses a button when they perceive a flash, allowing the computer to generate a precise map of light sensitivity.
Automated perimetry often focuses on the central 24 to 30 degrees of vision, as this area is frequently affected by diseases like glaucoma. For a broader, more rapid assessment, doctors may perform a confrontation visual field test. This technique involves the examiner sitting opposite the patient and moving their fingers into the patient’s peripheral vision. Confrontation testing is an effective screening tool to quickly identify large defects that warrant further investigation.
Causes and Patterns of Visual Field Loss
A reduction in the visual field indicates damage to the visual pathway, originating in the eye, the optic nerve, or the brain. Glaucoma typically causes a gradual, progressive loss of the peripheral field. The pattern of loss in glaucoma is often localized and arc-shaped, known as an arcuate scotoma, corresponding to damaged nerve fibers in the optic nerve.
Neurological events like a stroke or brain tumor can cause distinct patterns of loss that respect the vertical midline. Damage in the brain’s occipital lobe can lead to homonymous hemianopia, where the same half of the visual field is lost in both eyes. A tumor pressing on the optic chiasm may result in bitemporal hemianopia, causing loss of the outer (temporal) halves of the field in both eyes. Damage within the retina, such as from a retinal detachment, typically causes a localized shadow or a “curtain” effect.