Nad

nasal

FIGURE 7 Receptive field properties of bipolar and ganglion cells. Responses in both OFF (left panel) and ON (right panel) channels are shown. Figures (A) to (E) in each panel illustrate the connections among cones, horizontal cells (HCs), bipolar cells (BCs), and ganglion cells (GCs). The specific pattern of light/dark stimulation being experimentally applied to the receptive fields of these cells is indicated by the shading (darkness = dark gray; light = white). To the right of each figure is a set of recordings showing the response of the cells to the light/dark stimulation: the graded electrical response of the bipolar cells (a) and the action potentials generated in ganglion cells (b). The duration of the stimulation is shown in (c). OFF bipolar cells (left panel) are stimulated by direct input from cone photoreceptors located in the center of its receptive field but are indirectly inhibited by cones in the surrounding area through stimulation of inhibitory horizontal cells. Exposure to uniform dark (A) or light (B) produces no change in BC membrane potentials or GC action potentials, because the center effects are exactly counteracted by the surround effects. A very small centralized dark spot (C) or light spot (D) restricted to the center of the receptive filed produces maximal excitatory and inhibitory responses, respectively. Any nonuniform illumination of the receptive field (E) may produce a detectable response because center excitation will no longer be completely counterbalanced by the inhibitory surround. Similar mechanisms underlie electrical responses and receptive field properties of all cells in the ON channel (right panel).

FIGURE 9 Visual pathways to the cerebral cortex. The left visual hemifield is shown in blue; the right visual hemifield, in black. Left eye projections are solid lines; right eye projections are dashed lines. Each retina receives information from both visual hemifields (except for the far periphery (not shown), because the nose blocks this input); however, outgoing information from each retina is split. Fibers from the nasal portions of each retina cross to the contralateral side at the optic chiasm, whereas fibers from the temporal regions remain ipsilateral. As a result, information from a single visual hemifield (transmitted from both eyes) is combined and sent to the contralateral lateral geniculate body and cerebral cortex. Hence, the image of an airplane seen on the left is received in the right visual cortex, and the image of a pelican seen on the right is received by the left cortex.

FIGURE 9 Visual pathways to the cerebral cortex. The left visual hemifield is shown in blue; the right visual hemifield, in black. Left eye projections are solid lines; right eye projections are dashed lines. Each retina receives information from both visual hemifields (except for the far periphery (not shown), because the nose blocks this input); however, outgoing information from each retina is split. Fibers from the nasal portions of each retina cross to the contralateral side at the optic chiasm, whereas fibers from the temporal regions remain ipsilateral. As a result, information from a single visual hemifield (transmitted from both eyes) is combined and sent to the contralateral lateral geniculate body and cerebral cortex. Hence, the image of an airplane seen on the left is received in the right visual cortex, and the image of a pelican seen on the right is received by the left cortex.

to this point, designated as the optic tract, project to the lateral geniculate body and from there to the primary visual cortex. As a result of the partial decussation, each optic tract carries information solely about the contralateral visual field. There are two views of the contralateral visual hemifield: one provided by temporal fibers from the ipsilateral retina and the other provided by nasal fibers from the contralateral retina. Thus, all information from the right visual field is transferred to the left brain, regardless of which eye it enters.

Binocularly responsive neurons in the cortex receive retinotopic information from both eyes. By comparing the slight disparities in the positional information from the two eyes, these binocular cells provide the major

0 Normal vision

C. Visual field loss Name left eye right eye deficit

0 Normal vision eye optic nerve optic chiasma optic tract lateral geniculate body - thalamus optic radiations cerebral cortex occipital lobe

B. Right brain: lateral and medial views

Location of lesion no lesion

1 Anopsia of right eye optic nerve optic chiasma

2 Heteronymous hemianopsia

3 Homonymous hemianopsia

4 Upper quadrant anopsia optic tract

Meyer's loop

5 Lower quadratic anopsia upper bank with macular sparing calcarine fissure

6 Upper quadratic anopsia lower bank with macular sparing calcarine fissure

6 lower bank 5 upper bank

4 Meyer's loop

3 optic tract

2 optic chiasma 1 optic nerve

4 Meyer's loop

1 optic nerve 2 optic chiasm 3 optic tract

5 upper bank of calcarine fissure calcarine fissure 6 lower bank of calcarine fissure

FIGURE 10 Visual field deficits. Discrete lesions in the primary visual pathway lead to at least six characteristic visual deficits. The left visual field is shown in blue; the right visual field, in gray. Central visual field (macula) projections are shown as solid lines; peripheral visual field projections are shown as dashed lines. (A) View of the visual fields, visual pathways, and locations of six possible lesions. (B) Lateral view (left) and medial view (right) of the human brain showing the locations of the same lesions. Cerebellum is removed in the medial view. (C) Visual field deficits (black) that result from each injury. Note that information from both eyes is interrupted in every case, except for the optic nerve injury, and that the deficit is in the left visual field when the right brain is injured. Also note the unique pattern of loss when the crossing fibers of the optic chiasm are destroyed. This can happen when a pituitary tumor expands.

mechanism (called stereopsis) for creating a three-dimensional representation of objects in space and a means to achieve depth perception.

There is no conscious perception of the split of the visual field between the left and right visual occipital cortex. The re-merging of visual information into a single precept about an object with both left and right parts does not occur until the signals reach visual association areas in the parietal cortex. Decussating fibers in the corpus callosum provide cross-talk between left and right cortical areas to reestablish the complete visual field.

The projection of retinal and lateral geniculate fibers occurs in a highly ordered, retinotopic manner. Foveal representation of the contralateral visual hemifield covers a large area of the posterior pole of the calcarine fissure, whereas peripheral vision has a more modest representation in the anterior areas.

Retinal Projections to the Lateral Geniculate Nucleus

The lateral geniculate is a C-shaped structure consisting of six cellular layers, each receiving input from distinct sets of ganglion cells (Fig. 11). Ganglion cells with large receptive field sizes project to large neurons in layers 1 and 2, thus constituting the magnocellular (M) pathway. As described earlier, this pathway carries information about the spatial position of objects in the visual field. Ganglion cells with small receptive field sizes project to the small neurons found in layers 3 to 6, thus forming the parvocellular (P) pathway. This pathway carries information about the detailed shape of visual objects. The separation of left/right ocular input is retained so that layers 1, 4, and 6 receive contralateral projections, and layers 2, 3, and 5 receive ipsilateral projections. Some ganglion cells bypass the lateral geniculate, project directly to the brain stem, and participate in visual reflexes.

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