The Visual Projection Pathway

The first-order neurons in the visual pathway are the bipolar cells of the retina. They synapse with the second-order neurons, the retinal ganglion cells, whose axons are the fibers of the optic nerve. The optic nerves leave each orbit through the optic foramen and then converge on each other to form an X, the optic chiasm57 (ky-AZ-um), immediately inferior to the hypothalamus and anterior to the pituitary. Beyond this, the fibers continue as a pair of optic tracts (see p. 548). Within the chiasm, half the fibers of each optic nerve cross over to the opposite side of the brain (fig. 16.41). This is called hemidecussation,58 since

57 chiasm = cross, X

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Hemidecussation

Figure 16.40 The Retinal Basis of Stereoscopic Vision (depth perception). When the eyes are fixated on the fixation point (F), more distant objects (D) are focused on the retinas medial to the fovea and the brain interprets them as being farther away than the fixation point. Nearby objects (N) are focused lateral to the fovea and interpreted as being closer.

Figure 16.40 The Retinal Basis of Stereoscopic Vision (depth perception). When the eyes are fixated on the fixation point (F), more distant objects (D) are focused on the retinas medial to the fovea and the brain interprets them as being farther away than the fixation point. Nearby objects (N) are focused lateral to the fovea and interpreted as being closer.

only half of the fibers decussate. As a result, objects in the left visual field, whose images fall on the right half of each retina (the medial half of the left eye and lateral half of the right eye), are perceived by the right cerebral hemisphere. Objects in the right visual field are perceived by the left hemisphere. Since the right brain controls motor responses on the left side of the body and vice versa, each side of the brain needs to see what is on the side of the body where it exerts motor control. In animals with panoramic vision, nearly 100% of the optic nerve fibers of the right eye decussate to the left brain and vice versa.

The optic tracts pass laterally around the hypothalamus, and most of their axons end in the lateral geniculate59 (jeh-NIC-you-late) nucleus of the thalamus. Third-order neurons arise here and form the optic radiation of fibers in the white matter of the cerebrum. These project to the primary visual cortex of the occipital lobe, where the con-

59 geniculate = bent like a knee

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scious visual sensation occurs. A lesion in the occipital lobe can cause blindness even if the eyes are fully functional.

A few optic nerve fibers take a different route in which they project to the midbrain and terminate in the superior colliculi and pretectal nuclei. The superior colli-culi control the visual reflexes of the extrinsic eye muscles, and the pretectal nuclei are involved in the photo-pupillary and accommodation reflexes.

Space does not allow us to consider much about the very complex processes of visual information processing in the brain. Some processing, such as contrast, brightness, motion, and stereopsis, begins in the retina. The primary visual cortex in the occipital lobe is connected by association tracts to nearby visual association areas in the posterior part of the parietal lobe and inferior part of the temporal lobe. These association areas process retinal data in ways beyond our present consideration to extract information about the location, motion, color, shape, boundaries, and other qualities of the objects we look at. They also store visual memories and enable the brain to identify what we are seeing—for example, to recognize printed words or name the objects we see. What is yet to be learned about visual processing promises to have important implications for biology, medicine, psychology, and even philosophy.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

20. Why can't we see wavelengths below 350 nm or above 750 nm?

21. Why are light rays bent (refracted) more by the cornea than by the lens?

22. List as many structural and functional differences between rods and cones as you can.

23. Explain how the absorption of a photon of light leads to depolarization of a bipolar retinal cell.

24. Discuss the duplicity theory of vision, summarizing the advantage of having separate types of retinal photoreceptor cells for photopic and scotopic vision.

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Responses

  • Haben
    Why can't we see wavelengths below 350nm or above 750nm?
    8 years ago
  • maria
    What is visual projection pathways?
    2 months ago

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