The Visual System Pathways to the Brain. The neural signals initially processed by the retina travel via the axons of the ganglion cells through the optic nerves, dividing and partially crossing over into the optic chiasm and then travelling via the optic tracts to the lateral geniculate nucleus (LGN). From the LGN, the signals continue to the primary visual cortex, where further visual processing takes place.
The optic nerves and optic tracts
The optic nerve of each eye consists of a bundle of approximately 1 million retinal ganglion cell axons. The nerve connects to the posterior aspect of the eye in a position that is about 15° nasal to the macula. The connection is referred to as the optic nerve head and is visible when looking into the eye using an ophthalmoscope. The optic nerve head is approximately 1.8 mm (0.07 in) in diameter. Since there are no photoreceptors (rods or cones) overlying the optic nerve head, there is a small blind spot or “scotoma” of approximately 5° in size about 15° temporal to fixation in the visual field of each eye. When both eyes are open, the blind spot of each eye is “filled in” by the visual field of the other eye. The optic nerves of each eye continue posteriorly and then meet at the optic chiasm. It is here that axons of neurons from the nasal retina (temporal visual field) cross to the opposite or “contralateral” optic tract (e.g. axons from the right eye temporal visual field cross to the optic tract on the left side of the brain). Axons of neurons from the temporal retina (nasal visual field) continue along the same side or “ipsilateral” optic tract (same side of the brain). This means that visual signals from the right side of the visual field are traveling to the brain via the left optic tract and signals from the left visual field are traveling via the right optic tract. Each optic tract terminates at its LGN.
If a stroke, aneurism or tumor causes damage along the visual pathway, it is often possible to diagnose the exact location of the insult by measuring the visual field. For instance, a pituitary tumor would appear near the
optic chiasm and the impact on the visual field would be on the fibers that are crossing to the other side of the
brain. Since these fibers are from the nasal retina of each eye, the loss of vision would be in both temporal visual fields or a bitemporal visual field defect. Whereas an insult to one of the optic tracts would result in a loss of vision to the opposite or contralateral side of the visual field. For instance, a defect to the right optic tract would cause a loss of the left visual field of both eyes (the temporal visual field of the left eye and the nasal visual field of the right eye).
The optic nerve of each eye consists of a bundle of approximately 1 million retinal ganglion cell axons. The nerve connects to the posterior aspect of the eye in a position that is about 15° nasal to the macula. The connection is referred to as the optic nerve head and is visible when looking into the eye using an ophthalmoscope. The optic nerve head is approximately 1.8 mm (0.07 in) in diameter. Since there are no photoreceptors (rods or cones) overlying the optic nerve head, there is a small blind spot or “scotoma” of approximately 5° in size about 15° temporal to fixation in the visual field of each eye. When both eyes are open, the blind spot of each eye is “filled in” by the visual field of the other eye. The optic nerves of each eye continue posteriorly and then meet at the optic chiasm. It is here that axons of neurons from the nasal retina (temporal visual field) cross to the opposite or “contralateral” optic tract (e.g. axons from the right eye temporal visual field cross to the optic tract on the left side of the brain). Axons of neurons from the temporal retina (nasal visual field) continue along the same side or “ipsilateral” optic tract (same side of the brain). This means that visual signals from the right side of the visual field are traveling to the brain via the left optic tract and signals from the left visual field are traveling via the right optic tract. Each optic tract terminates at its LGN.
If a stroke, aneurism or tumor causes damage along the visual pathway, it is often possible to diagnose the exact location of the insult by measuring the visual field. For instance, a pituitary tumor would appear near the
optic chiasm and the impact on the visual field would be on the fibers that are crossing to the other side of the
brain. Since these fibers are from the nasal retina of each eye, the loss of vision would be in both temporal visual fields or a bitemporal visual field defect. Whereas an insult to one of the optic tracts would result in a loss of vision to the opposite or contralateral side of the visual field. For instance, a defect to the right optic tract would cause a loss of the left visual field of both eyes (the temporal visual field of the left eye and the nasal visual field of the right eye).
The lateral geniculate nucleus (LGN)
The LGN is a paired structure located at the dorsal thalamus. It is here that visual information to the brain, specifically the visual cortex, appears to be regulated and the first stage of coordinating vision from both eyes
begins. Each LGN has six layers, three receiving input from the right eye and three receiving input from the left eye. Because of the way the retinal ganglion cell axons are distributed through the chiasm and on to the optic tracts, the information processed in any one layer of the LGN represents specific areas of the visual field for one eye. Four of the layers are composed of the Parvocellular (small) ganglion cells from the retina that are primarily from the fovea. These cells are most sensitive to color and fine detail. Two of the layers are composed of the Magnocellular (large) ganglion cells from the retina. These cells are mostly from the perifoveal and more peripheral retina and are largely responsible for the processing of motion. The LGN then sends forward neurons via the optic radiations to the primary visual cortex.
The LGN is a paired structure located at the dorsal thalamus. It is here that visual information to the brain, specifically the visual cortex, appears to be regulated and the first stage of coordinating vision from both eyes
begins. Each LGN has six layers, three receiving input from the right eye and three receiving input from the left eye. Because of the way the retinal ganglion cell axons are distributed through the chiasm and on to the optic tracts, the information processed in any one layer of the LGN represents specific areas of the visual field for one eye. Four of the layers are composed of the Parvocellular (small) ganglion cells from the retina that are primarily from the fovea. These cells are most sensitive to color and fine detail. Two of the layers are composed of the Magnocellular (large) ganglion cells from the retina. These cells are mostly from the perifoveal and more peripheral retina and are largely responsible for the processing of motion. The LGN then sends forward neurons via the optic radiations to the primary visual cortex.
The Visual Cortex
The visual cortex in the occipital lobe of the brain is where the final processing of the neural signals from the retina takes place and “vision” occurs. The occipital lobe is at the most posterior portion of the brain. There are a total of six separate areas in the visual cortex, known as the V1, V2, V3, V3a, V4 and V5.
The primary visual cortex or V1 is the first structure in the visual cortex where the neurons from the LGN synapse. In V1, the neural signals are interpreted in terms of visual space, including the form, color and orientation of objects. V1 dedicates most of its area to the interpretation of information from the fovea. This mapping is known as “cortical magnification” and is typical in primates and animals that rely on information from the fovea for survival. The signals then pass through to V2 where color perception occurs and form is further interpreted.
As the neural signals continue further into other areas of the visual cortex, more associative processes take place. In the portions of the visual cortex that make up the parietal visual cortical areas, motion of objects, motion of self through the world and spatial reasoning occur. In the temporal visual cortical areas, including the middle temporal (V5) area, recognition of objects through interpretation of complex forms and patterns occurs. The final psychological and perceptual experience of vision also includes aspects of memory, expectation/prediction and interpolation subserved by other apparently non-visual areas of the brain.
The visual cortex in the occipital lobe of the brain is where the final processing of the neural signals from the retina takes place and “vision” occurs. The occipital lobe is at the most posterior portion of the brain. There are a total of six separate areas in the visual cortex, known as the V1, V2, V3, V3a, V4 and V5.
The primary visual cortex or V1 is the first structure in the visual cortex where the neurons from the LGN synapse. In V1, the neural signals are interpreted in terms of visual space, including the form, color and orientation of objects. V1 dedicates most of its area to the interpretation of information from the fovea. This mapping is known as “cortical magnification” and is typical in primates and animals that rely on information from the fovea for survival. The signals then pass through to V2 where color perception occurs and form is further interpreted.
As the neural signals continue further into other areas of the visual cortex, more associative processes take place. In the portions of the visual cortex that make up the parietal visual cortical areas, motion of objects, motion of self through the world and spatial reasoning occur. In the temporal visual cortical areas, including the middle temporal (V5) area, recognition of objects through interpretation of complex forms and patterns occurs. The final psychological and perceptual experience of vision also includes aspects of memory, expectation/prediction and interpolation subserved by other apparently non-visual areas of the brain.
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