Neuroscience Clerkship at UH/VA
 

NEUROLOGY OF VISION


The perception of vision is a complex phenomenon derived from processing basic visual stimuli into a vivid visual image. The pathway of vision can be understood only by reviewing the step-by-step journey of visual stimuli from the external world through the eye and into the brain. The diagram below shows the basic pathway taking by visual information from the external world to our brain.

 

In order to understand the neuroanatomy of vision, is important review step-by-step each how visual information travels.

1) First, light reflected from the visual field passes through the eye, specifically the cornea, pupil and lens and is focused on the retina. Both the cornea and lens have refractive power, which help to sharply focus visual information on the retina.

 

The diagram above shows the basic anatomy of the eye, and how a beam of light travels through the cornea, pupil, lens, and stimulates the fovea of the retina.

2) The left nasal and right temporal retinae receive visual information from the left visual field, whereas the left temporal and right nasal retinae receives visual information from the right visual field. Note, that there is considerable overlap of the left and right visual fields thus allowing us to have depth perception. This was especially important for our ancestors who needed to know exactly when it was time to run for cover from a predator.

 
3) Visual information first passes superficially to deep through all the layers of the retina to stimulate either the rods or cones (1st order receptors). The fovea, contains a high proportion of cones, which are specialized for high visual acuity and color vision, whereas the periphery of the retina contains mainly rods, which are highly sensitive at detecting light. Information is propagated from the rods or cones to bipolar neurons (2nd order neurons), and finally to ganglion cells (3rd order neurons). The axons of the ganglion cells form the optic nerve. The diagram below shows the layers of the retina and formation of the optic nerve.

 

4) The optic nerve (cranial nerve II) is a myelinated tract of the central nervous system. Each optic nerve conveys information from the ipsilateral temporal retina (contralateral nasal visual field) and ipsilateral nasal retina (ipsilateral temporal visual field).

5) The optic nerves then converge posteriorly at the optic chiasm. The optic chiasm is located dorsal to the hypophysis (i.e., pitituary stalk). The optic chiasm contains uncrossed fibers from the two temporal hemiretinae, and crossing fibers from the two nasal hemiretinae.

6) The optic chiasm splits into two optic tracts. Each optic tract contains uncrossed fibers from the ipsilateral temporal hemiretinae, and crossed fibers from the contralateral nasal hemiretinae.

Please review again the pathway that visual stimuli take through the eye to the retina, optic nerves, optic chiasm and optic tracts in the diagram below. Pay special attention to what part of the visual field is represented in each of these structures.


7) Each optic tract synapses in the ipsilateral lateral geniculate body (LGB), which is a thalamic relay nucleus. Each LGB contains six layers. Layers 1, 4 and 6 contain the information from the contralateral nasal hemiretinae. Layers 2, 3 and 5 contain the information from the ipsilateral temporal hemiretinae.

 

8) From each lateral geniculate body, information splits into two ipsilateral pathways via the geniculocalcarine tract (optic radiations).

The upper divisions contain visual information from the superior retinae (contralateral inferior visual field) and projects through the parietal lobe to the cuneate gyrus of the occipital lobe, above the calcarine sulcus.

The lower divisions contain visual information from the inferior retina (contralateral superior visual field) and project through the temporal lobe via Meyer's loop to the lingual gyrus of the occipital lobe, below the calcarine sulcus.

The diagram below shows the superior and inferior divisions of the optic radiations as they project to the cuneus and lingula respectively in the primary visual cortex of the occipital lobe.

 

9) Finally, the visual information from each ipsilateral LGB arrives in the ipsilateral visual cortex. The most posterior portion (i.e., medial occipital pole) contains macular (central) vision, whereas the peripheral vision is contained more anterior in the visual cortex of the occipital lobe.

Note, that this is the end of basic visual processing, but only the beginning of the formation of a visual perception. In reality, the occipital lobe connects to many other areas of the brain for further processing, such as association areas in the parietal lobe, etc., to give meaning to visual stimuli, and allow us to respond to the visual environment appropriately.