Visual Pathways

Imagine leaving your house and finding yourself on an unfamiliar road. Because the toad is not on any map, the only way to find out where it goes is to follow it. You soon discover that the road divides in two, and so you must follow each branch sequentially to figure out its end point. Suppose you learn that one branch goes to a city, whereas the other goes to a national park. By knowing the end point of each branch,, ^ou can conclude something about their respective functions—that one branch jcarries people to work, whereas the other carries them to play, for Example. !

The same strategy can be used to follow the paths of the visual system. The retinal ganglion cells form the optic nerve, which is the road into the brain. This road travels to several places, each with a different function. By finding out where the branches go, we can begin to guess what the brain is doing with the visual input and how the brain creates our visual world.

Let us begin with the optic nerves, one exiting from each eye. As you know, they are formed by the axons of ganglion cells leaving the retina. Just before entering the brain, the optic nerves partly cross, forming the optic chiasm (from the Greek letter v).

About half the fibers from each eye cross in such a way that the left half of each optic nerve goes to the left side of the brain, whereas the right halves go to the brain's right side, as diagrammed in Figure 8-11. The medial path of each retina, the nasal retina, crosses to the opposite side. The lateral path, the temporal retina, goes straight back on the same side. Because the light that falls on the right half of the retina actually comes from the left side of the

Optic tract Optic chiasm Optic nerve Nasal retina Temporal retina Eye

Figure 8-10

Visual Thalamus The optic nerves connect with the lateral geniculate nucleus (LGN) of the thalamus. The LGN has six layers: two magnocellular layers that receive input mainly from rods and four parvocellular layers that receive input mainly from cones.

Optic chiasm. Junction of the optic nerves from each eye at which the axons rom the nasal (inside) halves of the retinas cross to the opposite side of the brain.

Figure 8-11

Crossing the Optic Chiasm This horizontal slice through the brain shows the visual pathway from each eye to the primary visual cortex of each hemisphere. Information from the blue side of the visual field goes to the two left halves of the retinas and ends up in the left hemisphere. Information from the red side of the visual field hits the right halves of the retinas and travels to the right side of the brain.

Tectopulvinar And Geniculate System

Figure 8-12

Flow of Visual Information into the Brain The optic nerve has two principal branches: (1) the igenicujlostriate system through the LGN in the thalamus to the primary visual cortex and (2) the tectopulvinar system through the superior colliculus of the tectum to the pulvinar! region of the thalamus and thus to the temporal and parietal lobes.

Brain

TemporaJ^^ lobe

Pulvinar Thalamus

Figure 8-13

Striate Cortex The primary visual cortex is referred to as striate cortex because it appears to have striations (stripes) when stained with either a cell-body stain (left) or a myelin stain (right) in these sections from a rhesus monkey brain.

Brain

Anatomie Voies Visuelles

TemporaJ^^ lobe

Figure 8-13

Striate Cortex The primary visual cortex is referred to as striate cortex because it appears to have striations (stripes) when stained with either a cell-body stain (left) or a myelin stain (right) in these sections from a rhesus monkey brain.

Retina Projections Brain

Geniculostriate system. Projections from the retina to the lateral geniculate nucleus to the visual cortex.

Striate cortex. Primary visual cortex in the occipital lobe; its striped appearance when stained gives it this name.

Tectopulvinar system. Projections from the retina to the superior colliculus to the pulvinar (thalamus) to the parietal and temporal visual areas.

visual field, information from the left visual field goes toj the brain's right hemisphere, whereas information fromj the right visual field goes to the left hemisphere. Thus, half of each retina's visual field is represented on each side of the brain.

Having entered the brain, the axons of the ganglion! cells separate, forming two distinct pathways, charted in Figure 8-12. All the axons of the P ganglion cells and some of the M ganglion cells form a pathway called the geniculostriate system. This pathway goes from the retina to the lateral geniculate nucleus (LGN) of the thal amus and then to layer IV of the primary visual cortex, which is in the occipital lobe.

As Figure 8-13 shows, the primary visual cortex appears to have a broad stripe across it in layer IV and so is known as striate cortex. The term geniculostriate there fore means a bridge between the thalamus (geniculate) and the striate cortex. From the striate cortex, the axon pathway now splits, with one route going to vision-related re gions of the parietal lobe and another route going to vision-related regions of the tem poral lobe.

The second pathway leading from the eye is formed by the axons of the remaining M ganglion cells. These cells send their axons to the superior colliculus (located in the tectum of the midbrain; see Chapter 2). The superior colliculus sends connections toj a region of the thalamus known as the pulvinar. This pathway is therefore known as the tectopulvinar system because it goes from the eye through the tectum to the pul vinar (see Figure 8-12). The pulvinar then sends connections to the parietal and tem poral lobe.

To summarize, two principal pathways extend into the visual brain—namely, the geniculostriate and tectopulvinar systems. Each pathway eventually travels either to the parietal or the temporal lobe. Our next task is to determine the respective roles of the parietal lobe and the temporal lobe in creating our visual world.

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Responses

  • FRANCA
    Which shows the visual pathway to the brain?
    6 years ago
  • james
    What is tectopulvinar visual pathways?
    6 years ago
  • pauliina
    What is the tectopulvinar pathway?
    5 years ago

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