THE PARAOPTOMETRIC: UNDERSTANDING THE VISUAL PATHWAY

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The visual pathway starts at the retina. Light arriving at the retina is intercepted by specialized photoreceptor cells in the retina.
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Everything that the eye sees is processed and understood from the occipital lobe of the brain.

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Images and light entering the eye travel a complex network to terminate at the visual cortex. The visual information that our eyes receive allow us to move and work with amazing precision. Everything that our eyes see without moving is known as our visual field. The overlap of visual fields of both eyes is called the binocular visual field, and allows us to have depth perception and is related to hand-eye coordination.

DRAWING A LINE
The visual field of the eye is divided into hemifields. If you were to close one eye and draw an imaginary line vertically down the center of your vision, everything located temporally to that vertical line would be the temporal hemifield, and everything nasal to the line would be the nasal hemifield.

What we see in the right temporal half of our visual field in the right eye is perceived by the nasal retinal fibers, and the right nasal half of the visual field of the right eye is received by the temporal retinal fibers, and vice versa. Therefore, if damage occurs to the right side of the visual pathway in the brain, it will cause a loss of vision in the left field of vision in both eyes. Each eye gets information from both hemifields, and is processed by both halves of the brain.

Each eye only sees a portion of the visual information regarding color, motion and brightness that travels from the retina and reassembled at the visual cortex. The images received by both eyes are inverted as they pass through the anatomical lens of each eye.

RODS AND CONES  
The visual pathway starts at the retina. Light arriving at the retina is intercepted by specialized photoreceptor cells in the retina. There are two types of photoreceptor cells, the rods which respond to light and dark, and cones, which respond to color. The rods are located predominantly in the peripheral retina, while the cones are more prevalent in the fovea. Rods are used for scotopic vision, which is also known as night vision. The cones are used for photopic vision (day vision).

Light entering the eye must travel through almost the entire thickness of the retina before it encounters the photoreceptor cell layer. The photoreceptor cells convert the light energy into an electrical signal which transmits the visual impulses to the bipolar cells which then synapse to the ganglion cells of the retina. These ganglion cells converge to form the optic nerve, which in turn transmit the visual information.

Conditions such as elevated intraocular pressure, compression of the optic nerve/ optic chiasm, or tumor can disrupt the pathway of the electric impulse containing visual information being transmitted from the eye to the visual cortex.  Therefore the development of tumors and lesion at different areas of the pathway can produce gaps or blindspots in the visual field. Damage anterior to the optic chiasm will only affect the vision in one eye, while damage posterior to the optic chiasm will damage parts of the vision of both eyes.

OPTIC NERVE
The electrical impulses exit the eye via the optic nerve, which connects the retina to the brain.  It is also known as the second cranial nerve and transfers visual information from the retina to the occipital lobe. It is made up of the extensions of the ganglion cells and axons which come together to form the optic nerve bundle.

Since there are no photoreceptors at this location, the optic nerve is also the eye’s anatomical blind spot. It also functions as the gateway of oxygen and nutrients to the retina through the central retinal artery. The central retinal vein carries waste products such as carbon dioxide away from the eye. The nerve fibers from each eye travel independently and come together at the optic chiasm. Damage to the nerve from elevated intraocular pressure, compression, or poor blood flow will cause visual distortion, loss, or reduced color vision in the affected eye only. A lesion affecting the right optic nerve can cause an afferent pupillary defect and also total loss of vision in the right eye.

OPTIC CHIASM
The optic chiasm lies directly above the pituitary gland. The name “chiasm” comes from the Greek word “Chi” which is an “X”-shaped crossing of the optic nerve fibers. At this juncture, the nasal fibers from each eye cross and join with the temporal fibers of the other eye. In other words, the nasal fibers from the right eye cross over at the chiasm and join the temporal fibers of the left eye and culminate at the visual cortex on the left side of the brain, and the nasal fibers from the left eye cross and join with the temporal fibers of the right eye and culminate at the visual cortex on the right side of the brain.

Because of the crossing of the fibers, visual information from each eye is received by both the right and left visual cortex. If there is a lesion or tumor affecting the optic chiasm, the temporal visual field in both eyes is affected, and is known as a bitemporal hemianopsia and does not cross the vertical mid-line.

OPTIC TRACT
From the optic chiasm the nerve fibers from each eye travel together. The two tracts extend backwards toward the lateral geniculate body. If there is damage to this area, such as from a stroke, or a tumor, there could be a partial loss of vision in both eyes. This is known as a homonymous hemianopsia.

LATERAL GENICULATE NUCLEUS
The lateral geniculate nucleus (LGN) is also called the lateral geniculate body. It is located in the thalamus and acts as a relay station from the retina to the brain. It is composed of axons of the neurons comprised of the axons of the ganglion cells which carry visual information. Once the visual information is relayed to the LGN, it is separated, organized, and then relayed to different areas of the visual cortex, via the optic radiations.

OPTIC RADIATIONS
The optic radiations are comprised of the axons from the neurons of the lateral geniculate nucleus that  are divided into two bundles. One bundle, the temporal lobe, corresponds to the upper quadrant of the opposite eye’s visual field, and the other bundle, the parietal lobe, to the lower quadrant of the opposite eye’s visual field. Damage done to the optic radiation can result in the same type of damage as those to the optic tract.

VISUAL CORTEX
The main function of the visual cortex is to process the information received. It is located in the occipital lobe of the brain. Everything that the eye sees is processed and understood in this location. Once received by the visual cortex, the image which is received upside-down is flipped right side up. Damage affecting the visual cortex will cause a complete loss of vision in the opposite eye’s hemifield, but will spare the vision at the macula. For example, a lesion affecting the left visual cortex will affect the right hemifield but will the vision at the macula.

Janet Hunter is president of Eye Source LLC, and specializes in ophthalmic technician training.

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