The visual system is a complex assortment of neurons that allows animals and humans to see the world around them. This system is made of a series of different tracts and nuclei much like the “tracks” and “ports or stations” on a railroad system. Each part of the track is important, and when there is damage to any one part, it ultimately affects the visual field.
Hippocrates (460—377 BCE) was one of the first to recognize the visual field. The first documented testing, in fact, was performed by asking the patient to cover one eye while watching a fixed point. The tester would then ask the patient to identify objects held in the sides and outer edges of the visual field, usually at four points. This type of testing is called confrontation visual field evaluation. British ophthalmologist Jannik Bjerrum (1851—1920) expanded this testing in the 19th century. Bjerrum mapped visual fields by holding a white object in front of a black screen known as the tangent screen.
Over time, several additional tests were developed, such as the Amsler grid, which measures a person’s central visual field, to a kinetic perimetry test that tests the perimeter of the visual field. Many of these tests are still used today during a routine eye exam. Because vision is critical to humans, it is important for everyone to have an annual eye exam to maintain a healthy visual system.
Anatomy and Physiology
When a person looks at an object, the total amount of area that can be seen is known as the visual field. Health care providers test the visual field by having individuals focus on an object directly in front of them without moving their eyes or their head. Because the eye is a sphere, the resulting visual field is divided into the size of the angle seen both vertically and horizontally from a single eye. Thus, the typical dimensions for a human’s visual field is 60 degrees superiorly, 75 degrees inferiorly, 60 degrees nasally, and 100 degrees temporally.
The visual field is divided into two halves: the nasal and temporal fields. Both of these fields are then further divided into superior and inferior sections, thus giving the visual field four distinct quadrants. Because animals have two eyes, there are two visual fields.
When damage occurs to the optic pathway, an individual’s ability to see the world around him or her changes. The location in which the visual field is affected determines what portion of the vision or visual field is compromised. This may manifest as blind spots in vision, blurred or hazy vision, or even total blindness. Injury to the visual tract can be caused by infections, diabetes, and congenital defects, to name a few.
A person’s visual field can be compromised in five basic areas: (1) the optic nerve, (2) optic chiasm, (3) optic tract, (4) optic radiation, or (5) visual cortex. Starting at the optic nerve, before the optic chiasm, an injury here causes a break in the visual pathway. This stops all information from being sent to the visual cortex, resulting in monocular blindness.
The optic chiasm is where the information from both eyes meets before proceeding down the optic tract. Only the information from the nasal visual field crosses here while the temporal tract proceeds to the optic tract. The optic chiasm is positioned anterior to the pituitary gland. Thus, pituitary gland tumors can press on the optic chiasm, causing injury and loss of both temporal visual fields. This condition is called bitemporal hemianopsia. The optic chiasm splits and becomes two optic tracts. Damage to the optic tract will manifest as a condition called homonymous hemianopsia. This is due to damage of the ipsilateral nasal visual field and damage to the contralateral temporal visual field.
The optic radiation is a network of neurons that takes visual information from the lateral geniculate nucleus to the primary visual cortex. When the optic radiation is damaged, it results in a condition called quadrantanopsia. Persons with this injury show decreased visual sensation in the ipsilateral nasal field and contralateral temporal field either in the superior or inferior portion of the visual field.
Finally, the visual cortex is the end point for the visual tract. When there is damage to this brain region, it can result in a condition called homonymous hemianopsia with macular sparing. This means that all vision is lost in the ipsilateral nasal visual field and the contralateral temporal visual field, except for a small portion in the center of both visual fields that projects to the macula.
Adam K. Mills
See also: Blind Spot; Occipital Lobe; Optic Nerve; Retina; Tunnel Vision; Visual Perception; Visual System
Carroll, Joy N., & Chris A. Johnson. (2013). Visual field testing: From one medical student to another. University of Iowa Health Care Ophthalmology & Visual Sciences. Retrieved from http://EyeRounds.org/tutorials/VF-testing/
Purves, Dale, et al. (Eds.). (2001). Visual field deficits. In Neuroscience (2nd ed.). Sunderland, MA: Sinauer Associates. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK10912/