Peripheral Vision and Visual Pathways

Objective:

To explain in short essays or diagrams how changes in peripheral vision can be associated with damage to the retina or central neural pathways, or reflect tumors in the brain, at the level of 85% proficiency for each student.

In order to achieve this objective, you will need to be able to:

Measure peripheral vision in two axes.
Explain the anatomy of the visual pathways

Materials:

Group Supplies:

colored pencils or pens

visual field diagrams

cutout of cross section of brain

four colors of string to represent axons

four colors of beads to represent cell bodies

Descriptions of the dorsal column parthways and spinothalamic tracts are available as a supplement: Somatosensory and Pain Pathways

Lab Supplies:

disc perimetry device and target wand

Methods and Results:

Peripheral Vision

Occlude the left eye.
Hold the large plastic half disc horizontally against nose.
Have the subject look at the center dot.
Hold wand 1 cm to the temple side of the center dot and move outward until the peripheral target disappears. (A temporary disappearance at about 15^o is normal.) As a check repeat the test starting at 90^o and move toward the center.
The nasal field is tested in the same manner.
To test superior and inferior fields hold the disc vertically.
Repeat with the right eye occluded.

Visual Field Diagram – Right Eye

Visual Field Diagram – Left Eye

Visual Pathways

Construct each pathway using a different color. Include the cell bodies, axons, and synaptic sites for each pathway. Label each nucleus and tract as they are relevant.

Use the following description and table as a guide.

Visual pathways

Axons from the retinal ganglion cells travel through the optic nerves, optic chiasm, and optic tracts to reach the Lateral Geniculate Nucleus of the Thalamus
Axons from the retinal ganglion cells of the left half of each retina synapse on neurons in the Left Lateral Geniculate Nucleus of the Thalamus.
Axons from the retinal ganglion cells of the right half of each retina synapse on neurons in the Right Lateral Geniculate Nucleus of the Thalamus
Axons from the retinal ganglion cells also synapse on neurons in the Superior Colliculus which contain a sensory map and coordinates orientation of the eyes, head and neck toward visual stimuli.
Neurons in the Left Lateral Geniculate Nucleus of the Thalamus synapse on neurons in the Left Primary Visual Cortex.
Neurons in the Right Lateral Geniculate Nucleus of the Thalamus synapse on neurons in the Right Primary Visual Cortex.
Both the Lateral Geniculate Nuclei and the Primary Visual Cortex contain a sensory map of the entire visual field and process (filter) visual signals.

Left Eye		Right Eye
Left Half	Right Half	Left Half	Right Half
Left Optic Nerve		Right Optic Nerve
Optic Chiasm
Left Optic Tract	Right Optic Tract	Left Optic Tract	Right Optic Tract
Left Superior Colliculus Left LGN	Right Superior Colliculus RightLGN	Left Superior Colliculus Left LGN	Right Superior Colliculus RightLGN
Left Visual Cortex	Right Visual Cortex	Left Visual Cortex	Right Visual Cortex

Pathways to Primary Visual Cortex

Pathways to Superior Colliculus

Discussion:

The retina extends farther forward on the medial (inside) of the eye than the lateral (outside). Did this affect the findings?
Why is the superior field less than the inferior field?
What accounts for the temporary target disappearance sometimes noted when testing the temporal fields?
Could retinal damage be localized based on the results of this test?
Think of the situation where reduced field "tunnel vision" could be dangerous.
Reduced field of vision is often a symptom of glaucoma. Can you offer an explanation for this occurrence?
Diagram the pathway and neurons involved from the retina to the visual cortex.
Describe how the tectal system (superior colliculus) pathways are involved in the control of the extrinsic eye muscles, circular muscle fibers of the iris, and ciliary muscle.
Describe the visual disturbance that often accompanies a pituitary tumor. Explain why this particular pattern is observed.
If a patient reported that he could not see with his left eye, how could you tell if the problem was the optic nerve or the optic radiation? Explain.