15. Visual Processing: Cortical Pathways
Part 5 of 7
Valentin Draogoi, Ph.D.
(Content adapted from material by Chiyeko Tsuchitani, Ph.D.).
Clinical Examples
Figure 15.17 The perimetry test results for Example 3 indicate a bitemporal hemianopia (i.e., a bilateral visual defect involving the temporal hemifields of both eyes). Notice that only the temporal halves of the two central areas exhibit a visual loss (i.e., appear as dark hemicircles). |
Symptoms: At his annual physical exam, the patient complains of a general malaise and changes in his vision that he noticed while playing soccer. He said he was often "blindsided" on the playing field because he "couldn't see players approaching him from the side". Ophthalmoscope examination does not reveal abnormalities in either eye2. Confrontation field testing indicates a constriction of the temporal hemifields of both eyes. The patient is referred for neuroradiographic tests and perimetry testing. |
| Perimetry Test Results: The results indicate a bitemporal hemianopia, i.e., loss of vision in the temporal hemifields of both eyes (Figure 15.17). | |
Side & Retinotopicity of damage: The visual loss
You conclude that the visual field defect is related to damage that
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Neural imaging results (Figure 15.18) indicate a pituitary adenoma that is compressing the optic chiasm. Compression of the decussating nerve fibers prevents action potentials from the nasal hemiretina to reach the contralateral lateral geniculate nucleus of the thalamus. As the tumor grows larger it will crush the optic chiasm, destroying it and eventually compromising the remaining optic nerve fibers.
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A view of the inferior surface of the brain illustrating a pituitary tumor, which is compressing the optic chiasm. |
Optic Chiasm Damage: The fibers of the optic nerve that originate from ganglion cells in the nasal half of the retina decussate in the optic chiasm to the opposite optic tract (Figure 15.1). The crossing fibers of the optic chiasm may be crushed by a pituitary tumor. Damage to the optic chiasm produces a unique form of visual field deficit, a bitemporal hemianopia (Figure 15.17). Recall that the fibers of the optic chiasm carry information about objects in the temporal hemifields of both eyes (i.e., the right hemifield of the right eye and the left hemifield of the left eye). Consequently section of the optic chiasm produces a visual loss in only the temporal half of the visual field of each eye. When the patient views the world out of both eyes, the boundary of his binocular visual field is narrower than normal.
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