The visual system is responsible for the sense of sight or vision. Animals are highly visual, particularly humans and other mammals, as this sensory system helps them see their prey, see their food, and see their surroundings. The visual system is one of the most well-studied central nervous system processes as vision is extremely important to humans. Visual perception, however, consists of the psychological process of how a person sees a visual image. Because of this difference, visual perception is a separate entry in this encyclopedia.
Anatomy and Physiology
The visual system is made up of several anatomical structures: the eye—and its retina; optic nerves—the output of the retina, which are the second pair of cranial nerves; optic chiasm; optic tract; lateral geniculate nucleus (LGN); optic radiation; visual cortex; and visual association cortex. Since mammals have two eyes, all of the aforementioned structures are found in both the left and right sides of the brain, except for the optic chiasm. There is only one optic chiasm, as this is where the left and right optic nerves come together and cross some of their fibers.
The eye is a complex organ that acts like a camera as it uses the laws of optics. The eye uses light from an external object to focus an image onto its photoreceptors. These photoreceptors are found at the posterior portion of the eye in the retina. To help focus the light and project the image, the eye refracts the incoming light first through the cornea—a transparent structure at the anterior portion of the eye that covers the iris, pupil, and anterior chamber. The light then passes through the pupil and is refracted again via the lens. Together, the cornea and the lens act like a compound lens to project the image upside down on the retina.
The retina is made up of 10 distinct layers of neurons that modulate the light stimulus. The neural processing of the retina is very complex but can be reduced to four main stages: photoreception, transmission to bipolar cells, transmission to ganglion cells, and transmission to the optic nerve. Of all the different neurons within the retina, there are only two types of photoreceptor cells: the rods and the cones. The rods respond to dim light (such as moonlight) and produce black-and-white vision; the cones respond to bright light and produce color vision. Surrounding neurons within the other layers of the retina modulate the output signals from the rods and cones and then transfer this information to the bipolar cells. Further modulation of the signals is performed, and the bipolar cells transmit this information to the ganglion cells, the outermost layer of the retina. Finally, the ganglion cells’ axons produce action potentials that travel along the optic nerve into the brain.
The output of the retina consists of ganglion cells and their axons, which make up the optic nerves. The optic nerves travel along the underside of the brain. The representation of the visual world travels within the optic nerves all the way to the back of the brain. These brain regions are called the occipital lobes. But before the signals reach their final destinations, they are processed by relaying the information to other brain structures. The majority of the axons terminate within the LGN. The remaining axons travel to the superior colliculus, which is important for moving the head when something “catches the eye” and for controlling eye movements called saccades.
The optic chiasm is found at the base of the hypothalamus. It looks like a large white “X” because of the large number of myelinated axons. This crossing of information is important because it takes specific halves of the visual fields and projects them to specific halves of the brain. This means that the left halves of the visual fields of both eyes are crossed over and projected to the right cerebral hemisphere, while the right halves of the visual field of both eyes are sent to the left cerebral hemisphere. Small portions of the optic nerve axons that arrive from the centers of both visual fields are projected to both hemispheres for redundancy purposes.
The optic tract begins immediately after the optic chiasm and eventually splits into two divisions: the left and right optic tracts. The information carried within these tracts is exclusive to the same side as its visual field. This means that images seen in the left half of the visual field will flow into the left optic tract and the right visual field halves enter the right optic tract. These tracts follow the posterior portion of the thalamus ending in the LGN.
The term geniculate means to bend as this nucleus wraps to the lateral side of the posterior part of the thalamus and is ellipsoid in shape. The LGN is separated into six distinct layers with each layer receiving certain visual information. These layers have either parvocellular or magnocellular neurons, which are the target neurons for specific axons within the optic tract. Parvocellular neurons are commonly called P cells and are necessary for processing color and edges of an image; magnocellular neurons or M cells are important for depth perception and motion. Additionally, K cells (neurons within the retina that help with color vision) terminate on small neurons lying between the six layers. Layers 1, 4, and 6 receive information from the contralateral temporal visual field, while layers 2, 3, and 5 receive information from the ipsilateral nasal visual field. The output of the P, M, and K cells make the optic radiation and end in the primary visual cortex.
From the left and right LGN, visual information is sent to the V1 region of the occipital lobe via the left and right optic radiations. V1 of the occipital lobe is also called the primary visual cortex and has six layers of neurons. The optic radiations move posteriorly and medially to the most posterior portion of the occipital lobe. P cell axons terminate in layer 4Cβ, while the M cell axons end in layer 4Cα. Finally, the K cells connect to blobs (large neurons) located in layers 2 and 3 of V1. Up to now, visual signals have been processed relatively straightforwardly. This will change once the signal enters the visual cortex.
The visual cortex is responsible for processing image information. In humans, it makes up the largest of the sensory systems that are represented in the brain. The visual cortex has a hierarchy of regions starting with V1, which is the primary visual cortex or the striate cortex. This region’s input comes directly from the LGN. The extrastriate visual cortices are regions V2, V3, V4, and V5. These secondary visual areas are necessary for processing the most basic information, such as light intensity, colors, lines and edges making “bars,” and orientation. For example, neurons in V1 and V2 are activated when specific orientations of bars or combinations of bars are seen in a specific region of the visual field. It is thought that this helps with identifying corners and edges of an image. As visual information passes through the hierarchy of the visual cortex, the processing becomes more and more complex, making the image more realistic.
Visual Association Cortex
Finally, the visual association cortex receives information from the hierarchy of the visual cortex. Here, the neurons respond to complete objects that were seen in the visual field. For example, cells in the visual association cortex are activated when a specific type of car, like a red Toyota Prius, is seen. This information is then moved into two different pathways of the brain, called the ventral and dorsal streams, to identify “what” the object is and “where” it is in space. The ventral stream moves toward the temporal lobe, while the dorsal stream moves toward the parietal lobe. The ventral stream is used in first recognizing that the object is a “vehicle,” then identifying the object as a “red car,” and then applying it to a specific category: “a red Toyota Prius.” The dorsal stream will take the surrounding environment of the red Toyota Prius to help place its location in reference to the person. This is called spatial attention.
Jennifer L. Hellier
See also: Blind Spot; Hubel, David H.; Occipital Lobe; Optic Nerve; Retina; Sensory Receptors; Visual Fields; Visual Perception; Visual Threshold; Wiesel, Torsten N.
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