Visual pathway

The visual image exists in the outside world, and is designated the visual field; there is a visual field for each eye. This image is projected onto the retina, where it is now termed the retinal field. Because of the lens of the eye, the visual information from the upper visual field is seen in the lower retina (and likewise for the lower visual field). The visual fields are also divided into temporal (lateral) and nasal (medial) portions. The temporal visual field of one eye is projected onto the nasal part of the retina of the ipsilateral eye, and onto the temporal part of the retina of the contralateral eye. The primary purpose of the visual apparatus (e.g., muscles) is to align the visual image on corresponding points of the retina of both eyes.

Visual processing begins in the retina with the photo-receptors, the highly specialized receptor cells, the rods and cones. The central portion of the visual field projects onto the macular area of the retina, composed of only cones, which is the area required for discriminative vision (e.g., reading) and color vision. Rods are found in the peripheral areas of the retina and are used for peripheral vision and seeing under conditions of low-level illumination. These receptors synapse with the bipolar neurons located in the retina, the first actual neurons in this system (functionally equivalent to DRG neurons). These connect with the ganglion cells (still in the retina) whose axons leave the retina at the optic disc to form the optic nerve (CN II). The optic nerve is in fact a tract of the CNS, as its myelin is formed by oligodendrocytes (the glial cell that forms and maintains CNS myelin).

After exiting from the orbit, the optic nerves undergo a partial crossing (decussation) in the optic chiasm. The fibers from both nasal retinas, representing the temporal visual fields, cross and then continue in the now-named optic tract (see Figure 15A and Figure 15B). The result of this rearrangement is to bring together the visual information from the visual field of one eye to the opposite side of the brain.

The visual fibers terminate in the lateral geniculate nucleus (LGB), a specific relay nucleus of the thalamus (see Figure 12 and Figure 63). The lateral geniculate is a layered nucleus (see Figure 41C); the fibers of the optic nerve synapse in specified layers and, after processing, project to the primary visual cortex, area 17. The pro jection consists of two portions with some of the fibers projecting directly posteriorly, while others sweep forward alongside the inferior horn of the lateral ventricle in the temporal lobe, called Meyer's loop (see also Figure 41C); both then project to the visual cortex of the occipital lobe as the geniculo-calcarine radiation. The projection from thalamus to cortex eventually becomes situated behind the lenticular nucleus and is called the retro-lenticular portion of the internal capsule, or simply the visual or optic radiation (see also Figure 27, Figure 28B, and Figure 38).

The visual information goes to area 17, the primary visual area, also called the calcarine cortex (seen in the upper diagrams and also in the next illustration), and then to adjacent association areas 18 and 19.

Clinical Aspect

The visual pathway is easily testable, even at the bedside. Lesions of the visual pathway are described as a deficit of the visual field, for example, loss of one-half of a field of vision is called hemianopia (visual loss is termed ano-pia). Loss of the visual field in both eyes is termed homonymous or heteronymous, as defined by the projection to the visual cortex on one side or both sides. Students should be able to draw the visual field defect in both eyes that would follow a lesion of the optic nerve, at the optic chiasm (i.e., bitemporal heteronymous hemianopia), and in the optic tract (i.e., homonymous hemianopia). (Note to the Learner: The best way of learning this is to do a sketch drawing of the whole visual pathway using colored pens or pencils.)

Lesions of the optic radiation are somewhat more difficult to understand:

• Loss of the fibers that project from the lower retinal field, those that sweep forward into the temporal lobe (Meyer's loop), results in a loss of vision in the upper visual field of both eyes on the side opposite the lesion, specifically the upper quadrant of both eyes, called superior (right or left) homonymous quadrantanopia.

• Loss of those fibers coming from the upper retinal field, which project directly posteriorly, passing deep within the parietal lobe, results in the loss of the lower visual field of both eyes on the side opposite the lesion, specifically the lower quadrant of both eyes, called inferior (right or left) homonymous quadrantanopia.

Association visual areas (18, 19)

Association visual areas (18, 19)

Primary visual area (17)

Lateral ventricle (body)

Lateral geniculate n.

Optic tract

Optic nerve (CN II)

Lateral ventricle (inferior horn)

Lateral ventricle (body)

Lateral geniculate n.

Optic tract

Optic nerve (CN II)

Optic Radiation

Stria terminalis

Caudate n. (tail)

Optic radiation

Temporal loop of optic radiation (Meyer's loop)

Stria terminalis

Caudate n. (tail)

Optic radiation

Temporal loop of optic radiation (Meyer's loop)

Md = Midbrain

FIGURE 41A: Visual System 1 — Visual Pathway 1

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