Visual Pathway
[Image courtesy National Library of Medicine, Bethesda, Maryland.] | If you can understand the lower figure on this page, then you have learned the major pathway that visual information takes on its way from the eye to the primary visual cortex. The figure is really not too difficult to understand....really. Imagine that the colored bar [half red, half blue] is in front of your eyes. The red part of the bar will project to the nasal part of your left retina and the temporal [lateral] part of your right retina. The blue part of the bar will project to the nasal part of your right retina and the temportal [lateral] part of your left retina. Follow the blue and red lines from the eyes to see the flow of information. From the retina, the first synapse is in the lateral geniculate nucleus of the thalamus. The next synapse is made in primary visual cortex in the occiptal lobe. |
The Visual Pathway: From Eye to Primary Visual Cortex
What happens if there is damage to the visual pathway? Different visual problems will occur depending on where the damage is. The black bars [labeled 1 through 5] indicate where damage may occur and the chart to the right of the pathway indicates the resulting "blind" area [gray shading] of the visual field.
Damage at site #1: this would be like losing sight in the left eye. The entire left optic nerve would be cut and there would be a total loss of vision from the left eye.
Damage at site #2: partial damage to the left optic nerve. Here, information from the nasal visual field of the left eye [temporal part of the left retina] is lost.
Damage at site #3: the optic chiasm would be damaged. In this case, the temporal [lateral] portions of the visual field would be lost. The crossing fibers are cut in this example.
Damage at site #4 and #5: damage to the optic tract [#4] or the fiber tract from the lateral geniculate to the cortex [#5] can cause identical visual loss. In this case, loss of vision of the right side.
Partial damage to these fiber tracts can cause other predictable visual problems.
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The spatial relationships among the ganglion cells in the retina are maintained in their central targets as orderly representations or “maps” of visual space. Importantly, information from the left half of the visual world is represented in the right half of the brain, and vice versa.
Understanding the neural basis for this arrangement requires considering how images are projected onto the two retinas, and which parts of the two retinas cross at the optic chiasm. Each eye sees a part of visual space that defines its visual field [Figure 12.4A]. For descriptive purposes, each retina and its corresponding visual field are divided into quadrants. In this scheme, the surface of the retina is subdivided by vertical and horizontal lines that intersect at the center of the fovea [Figure 12.4B]. The vertical line divides the retina into nasal and temporal divisions and the horizontal line divides the retina into superior and inferior divisions. Corresponding vertical and horizontal lines in visual space [also called meridians] intersect at the point of fixation [the point in visual space that the fovea is aligned with] and define the quadrants of the visual field. The crossing of light rays diverging from different points on an object at the pupil causes the images of objects in the visual field to be inverted and left-right reversed on the retinal surface as the rays are focused. As a result, objects in the temporal part of the visual field are seen by the nasal part of the retina, and objects in the superior part of the visual field are seen by the inferior part of the retina. [It may help in understanding Figure 12.4B to imagine that you are looking at the back surfaces of the retinas, with the corresponding visual fields projected onto them.]
Figure 12.4
Projection of the visual fields onto the left and right retinas. [A] Projection of an image onto the surface of the retina. The passage of light rays through the optical elements of the eye results in images that are inverted and left-right reversed on [more...]
With both eyes open, the two foveas are normally aligned on a single target in visual space, causing the visual fields of both eyes to overlap extensively [see Figure 12.4B and Figure 12.5]. This binocular field of view consists of two symmetrical visual hemifields [left and right]. The left binocular hemifield includes the nasal visual field of the right eye and the temporal visual field of the left eye; the right hemifield includes the temporal visual field of the right eye and the nasal visual field of the left eye. The temporal visual fields are more extensive than the nasal visual fields, reflecting the size of the nasal and temporal retinas respectively. As a result, vision in the periphery of the field of view is strictly monocular, mediated by the most medial portion of the nasal retina. Most of the rest of the field of view can be seen by both eyes; i.e., individual points in visual space lie in the nasal visual field of the other. It is worth noting, however, that the shape of the face and nose impact the extent of this region of binocular vision. In particular, the inferior nasal visual fields are less extensive than the superior nasal fields, and consequently the binocular field of view is smaller in the lower visual field than in the upper [see Figure 12.4B].
Figure 12.5
Projection of the binocular field of view onto the two retinas and its relation to the crossing of fibers in the optic chiasm. Points in the binocular portion of the left visual field [B] fall on the nasal retina of the left eye and the temporal retina [more...]
Ganglion cells that lie in the nasal division of each retina give rise to axons that cross in the chiasm, while those that lie in the temporal retina give rise to axons that remain on the same side [see Figure 12.5]. The boundary [or line of decussation] between contralaterally and ipsilaterally projecting ganglion cells runs through the center of the fovea and defines the border between the nasal and temporal hemiretinas. Images of objects in the left visual hemifield [such as point B in Figure 12.5] fall on the nasal retina of the left eye and the temporal retina of the right eye, and the axons from ganglion cells in these regions of the two retinas project through the right optic tract. Objects in the right visual hemifield [such as point C in Figure 12.5] fall on the nasal retina of the right eye and the temporal retina of the left eye; the axons from ganglion cells in these regions project through the left optic tract. As mentioned previously, objects in the monocular portions of the visual hemifields [points A and D in Figure 12.5] are seen only by the most peripheral nasal retina of each eye; the axons of ganglion cells in these regions [like the rest of the nasal retina] run in the contralateral optic tract. When the axons in the optic tract reach the lateral geniculate nucleus, they terminate in an orderly map of the contralateral hemifield [albeit in separate right and left eye layers; see Figure 12.14].
Lateral geniculate neurons, in turn, maintain this topographic order in their projection to the striate cortex [Figure 12.6]. The fovea is represented in the posterior part of the striate cortex, whereas the more peripheral regions of the retina are represented in progressively more anterior parts of the striate cortex. The upper visual field is mapped below the calcarine sulcus, and the lower visual field above it. As in the somatic sensory system, the amount of cortical area devoted to each unit area of the sensory surface is not uniform, but reflects the density of receptors and sensory axons that supply the peripheral region. Thus, like the representation of the hand region in the somatic sensory cortex, the representation of the macula is disproportionately large, occupying most of the caudal pole of the occipital lobe.
Figure 12.6
Visuotopic organization of the striate cortex in the right occipital lobe, as seen in mid-sagittal view. [A] The primary visual cortex occupies a large part of the occipital lobe. The area of central vision [the fovea] is represented over a disproportionately [more...]