Tuesday, November 22, 2005

Projection of the eye onto the brain

The sensory homunculus is the topographical representation of body parts on the brain. In the figure one can see that the hands have a disproportionately large representation compare to the eyes with respect to sensation but the perception of vision of course is in area 17 of the calcarine cortex.

The overview of the visual pathway is shown in the figure below. Axons from the ganglion cells in the retina)( number 1) pass through the optic chiasm (arrow 2). The temporal fibers from these axons are uncrossed and project to the lateral geniculate body in the thalamus (3). The fibers sweep downward and laterally (shown in fainter blue so that you don't think the optic tract enters the pons) into the temporal lobe (4) and eventually ascend and travel more medially to synapse in area 17 or the calcarine cortex. Macular fibers are represented more posteriorly in the calcarine cortex whereas nasal fibers are projected more anteriorly. The individual fibers can be related to the visual field and its representation on the calcarine cortex. In the figure it is evident that the projection from nasal retinal fibers shown in yellow project to the contralateral anterior calcarine cortex with synapses in the lateral geniculate body. The left side of the brain views the right visual field and the images are projected as upright compared to the visual field.

Interruption of the visual pathway results in defects in vision that are location specific. Ophthalmologists will need to know the anatomic correlate of each pattern in the visual field (see link).


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