The LGN neurons are segregated into three major groups: The LGN neurons are monocular (i.e., respond to stimulation of one eye only) and have concentric (center-surround) receptive fields. The functional properties of LGN neurons are similar to those of retinal ganglion cells. The magnocellular layers (1 and 2) appear darker as the cells in these layers are larger and contain more Nissl substance than the cells in the parvocellular layers (3 through 6). The neurons of the lateral geniculate nucleus form 6 layers that are visible when stained for Nissl substance (B). Consequently, each LGN neuron responds to stimulation of one eye only. The optic tract fibers (3° visual afferents) from each eye synapse in different layers of the LGN. Thin layers of the smallest cells (i.e., the koniocellular neurons) are interposed between these principal layers.Smaller cells form the upper four (parvocellular) layers.The largest cells form the deepest two (magnocellular) layers.Like the retina, the lateral geniculate nucleus is a laminated structure, in this case, with six principal layers of cells (Figure 15.3B). The vast majority of optic tract fibers terminate on neurons in the lateral geniculate nucleus (LGN) of the thalamus (Figure 15.3A). They include the hypothalamus, pretectum and the superior colliculus. The termination sites of the retinal ganglion cell axons in three nuclei that are not considered a part of the visual pathway are also illustrated. The inferior surface of the brain illustrating the visual pathway. the suprachiasmatic nucleus of the hypothalamus - for control of diurnal rhythms and hormonal changes.the pretectum of the midbrain - for control of the pupillary light reflex and.the superior colliculus of the midbrain - for control of eye movements.the lateral geniculate nucleus of the thalamus - for visual perception.The axons in the optic tract terminate in four nuclei within the brain (Figure 15.2): Consequently, each optic tract has within it axons representing the contralateral half of the visual field. For example, the temporal (left) hemiretina of left eye and the nasal (left) hemiretina of right eye both have projected on them the right halves of their respective visual fields. Recall that the ipsilateral temporal hemiretina and the contralateral nasal hemiretina have projected on them the images of corresponding halves of their visual fields. Consequently, each optic tract contains retinal ganglion cell axons that originate in the nasal half of the contralateral retina and the temporal half of the ipsilateral retina. The fibers of the optic nerve that originate from ganglion cells in the nasal half of the retina (i.e., the nasal hemiretina) decussate in the optic chiasm to the opposite optic tract (Figure 15.1). At the optic disc, the 3° visual afferents exit the eye and form the optic nerve. ![]() The axons of the 3° visual afferents (the retinal ganglion cells) form the optic nerve fiber layer of the retina on their course to the optic disc. The visual pathway with the course of information flow from the right (green) and left (blue) hemifields of the two eye's visual fields. In the case of the visual system, the thalamic nucleus is the lateral geniculate nucleus and the cortex is the striate cortex of the occipital lobe. ![]() This chapter will provide more information about visual pathway organization and the visual processing that occurs within the brain.ġ5.1 The Visual Pathway from Retina to CortexĪs noted previously in the somatosensory sections, all sensory information must reach the cerebral cortex to be perceived and, with one exception, reach the cortex by way of the thalamus. The information from the eye is carried by the axons of the retinal ganglion cells (the 3° visual afferent) to the midbrain and diencephalon. In turn, the bipolar cells (the 2° visual afferent) synapse with retinal ganglion cells and amacrine cells, which enhance contrast effects that support form vision and establish the basis for movement detection. Within the retina, the receptors synapse with bipolar and horizontal cells, which establish the basis for brightness and color contrasts. The previous chapter described how the light-sensitive receptors of the eye convert the image projected onto the retina into spatially distributed neural activity in the first neurons of the visual pathway (i.e., the photoreceptors). The visual system is unique as much of visual processing occurs outside the brain within the retina of the eye.
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