Prechiasmal Segment

    • Visual pathway segment that conveys neural signals from the retina to the optic chiasm
    • Retina converts optical information into neural signals (“visual transduction”) that travel to the optic nerve
      • Retinal cones, concentrated mostly in the fovea, mediate high resolution and color vision
      • Retinal rods, concentrated outside the fovea, gather dim light across broad receptive fields and provide low resolution vision in dim light
      • Bipolar, horizontal, and amacrine cells receive signals from rods and cones, refine and convey them to retinal ganglion cells
      • Retinal ganglion cells receive signals from bipolar, horizontal, and amacrine cells and send their axons through the retinal nerve fiber layer to the optic nerve
    • Retinal nerve fiber layer has three main axon bundles
      • Maculopapillar bundle carries retinal ganglion cell axons from the fovea and retinal region between the fovea and optic disc; mediates high resolution and color signals
      • Arcuate bundles carry retinal ganglion cell axons from above and below maculopapillar bundle; mediate lower resolution and mostly achromatic signals
      • Nasal radial bundles carry signals from the nasal retina
    • Lesions typically cause focal visual field defects called “scotomas”
    • Outer retinal lesions cause defects whose shape corresponds to the extent of the lesioned retinal area
    • Retinal ganglion cell and nerve fiber layer lesions cause nerve fiber bundle defects
      • Maculopapillar bundle lesions produce central scotomas or cecocentral scotomas, mostly from toxic, nutritional, and hereditary conditions
      • Arcuate bundle lesions produce nasal steps if the lesion is small, arcuate (scimitar-shaped) defects if the lesion is medium-sized and altitudinal scotomas if the lesion is large; these 3 kinds of arcuate bundle defects, mostly arise from inflammation, compression, ischemia, increased intracranial pressure, or glaucoma
      • Nasal radial bundle lesions produce temporal wedge defects, mostly from dysplastic optic neuropathies

    Vision Pathways

    Overview Videos All Segments Considered Optical Segment Prechiasmal Segment Chiasmal Segment Retrochiasmal Segment Occipito-Parietal Segment Occipito-Temporal Segment