Retinocortical Pathways

The role of the eye as an optical apparatus is to transmit, with minimal distortion, images of the external world to the specialized photoreceptors that are distributed along the posterior part of the eye bulb (i.e., the retina). A number of structures, located in the anterior part of the eye, contribute to this task. These include the cornea, iris, cilliary muscle, and lens (see Figure 16 ). The latter is a biconvex structure capable of changing shape through the action of the cilliary muscle. The variable shape of the lens ensures that objects located at different distances from the observer form sharp images on the retina (i.e., images with minimal blur or defocus). The light passes through the cornea, the lens and the vitreous humor to reach the retina. The latter, as shown in Figure 16 , contains a layer of photoreceptor cells, and several other layers that contain first- and second-order afferent neurons (the bipolar and retinal ganglion cells, respectively), and two main types of interneurons: horizontal and amacrine cells. It is important to remember that the photoreceptor cell layer forms the outer border of the retina. Thus, the incoming light must travel through all the other retinal layers before it reaches the photoreceptors. In a small circular region of the retina, the fovea, 1 the internal cell layers are displaced laterally, allowing light to reach the photoreceptors with minimal distortion. The fovea, and especially its central region, the foveola, contains the highest density of photoreceptors than any other region of the retina. The photoreceptors are specialized organs capable of responding to light particles (photons) with a change in their membrane potential. They synapse with bipolar cells which, in turn, form contacts with retinal ganglion cells. The axons of the latter exit the eye at the optic disc and, after acquiring a myelin sheath, form the optic nerve. The optic nerves (one from each eye) decussate partially in the optic chiasm, which is located at the base of the brain, just rostral to the hypothalamus (see Figure 17 ). After leaving the optic chiasm, the ganglion cell fibers form two optic tracts. Each optic tract contains fibers from the temporal half of the ipsilateral retina and from the nasal half of the retina in the contralateral eye. The majority of these fibers project to the dorsal lateral geniculate nucleus (dLGN) on each side.