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Basic Structure of the Human Visual System

light eye information figure

Input to the visual system occurs through the eyes. The eyes are sensory organs, and their primary role is to detect light energy, code it into fundamental “bits” of visual information, and transmit it to the rest of the brain for analysis. In the higher levels of the brain, the information bits are eventually recombined to form mental images and our conscious visual perception of the world. The visual system is perhaps the most complex of all sensory systems in humans. This is evidenced by the multiplicity of brain areas devoted to vision, as well as the complexity of cellular organization and function in each of these areas.

The human eye is a globe cushioned into place within a bony orbit by extraocular muscles, glands, and fat. The extraocular muscles move the eyes in synchrony to allow optimal capture of interesting visual information. Eye movements can be either involuntary, reflex reactions (e.g., nystagmus, convergence during accommodation), or voluntary actions. Light rays enter the eye through a circular, clear, curved cornea, which converges them into the anterior chamber of the eye (Figure 1). The number of light rays allowed to pass through the rest of the eye is controlled by the iris—a contractile, pigmented structure that changes in size as a function of light intensity, distance to the object of interest, and even pain or emotional status. The iris controls not only the amount of light entering the posterior chamber of the eye, but also the focal length of this light beam, and thus the overall quality of the image achievable. After the iris, light passes through the lens, a small, onion-like structure whose different layers vary in refractive index which provides final focusing power to precisely position the visual information on the retina. The retina is the most photosensitive component of the human central nervous system and covers most of the inner, back surface of the eye. Its highly regular neuronal structure is normally organized into seven cellular and fiber layers (Figure 2), with the light having to pass through all these layers, plus several vascular plexuses, to reach the photoreceptors. Photoreceptors are neurons, which fall into two classes—rods and cones. Although both rods and cones respond to a range of wavelengths of light, the perception of color, as well as vision at high (photopic) light levels, depends primarily upon the cones. Rods distinguish between colors only on the basis of lightness, but their specialty is detecting tiny amounts of light (even a single photon at a time) under low (scotopic) light levels.

The retinal architecture is disrupted at two locations: the optic disc and the fovea (Figure 1). The optic disc, which creates each eye’s “blind spot,” is a region devoid of neurons, in which the axons of retinal ganglion cells exit the eye and enter the optic nerve. These axons are the only means by which visual information is transmitted from the eye to the rest of the brain. Their loss or dysfunction in diseases such as glaucoma or optic neuritis causes blindness. The fovea is a retinal specialization dedicated to achieving high-resolution vision along the visual axis of the eye. In this small region, the many neural and vascular layers of the retina are displaced sideways to allow light rays to reach photoreceptors directly. Unlike photoreceptors in imaging systems, the photosensitive elements of the eye are not distributed randomly. The fovea contains the highest concentration of photoreceptors in the retina (about 200,000 cones/mm 2 ). Its view of the world is relatively unimpeded by distortions due to overlying neural tissue and blood supply. The concentration of cones decreases rapidly away from the fovea, to a density of 5000/mm 2 , at the outer edges of the retina. On the other hand, there are no rods in the fovea, but their concentration increases rapidly to a maximum about 20 degrees off the visual axis and then decreases gradually toward the outer edges of the retina.

While the capture and initial processing of visual information occurs in the eye, it is generally accepted that our conscious sense of visual perception occurs because of processing in the brain, and in particular, in the many cortical areas devoted to vision (Figure 3). On its way to the cerebral cortex, visual information that leaves the eye is first partitioned among different subcortical nuclei. These nuclei determine the ultimate use of the visual information. The great majority of visual information is sent to the lateral geniculate nucleus (LGN) of the thalamus. After some limited processing and sorting, LGN neurons transmit this information to the primary visual cortex (V1), located in the occipital lobes of the brain in most mammalian species (Figure 3). This information flow is the primary route that gives rise to conscious visual perception. The rest of the visual information originating from the eye is sent to three clusters of subcortical nuclei for processing. One cluster, the pulvinar/lateral posterior nucleus/superior colliculus group, is thought to play a role in visuomotor processing, visual attention, and other integrative functions in conjunction with the visual cortex. A second cluster (comprised of the intergeniculate leaflet, ventrolateral geniculate nucleus, and olivary pretectal nucleus) mediates responsiveness to light, especially the reflex regulation of pupil size. Finally, the suprachiasmatic nucleus and associated structures control circadian pacemaker functions for the entire body. Visual input to the suprachiasmatic nucleus provides signals for photic entrainment of the organism.

The proportion of the cerebral cortex devoted to visual processing in primates is significantly greater than that devoted to any other sensory or motor modality. Most of the visual information generated by the eyes and processed by intermediate, subcortical centers reaches the primary or occipital visual cortex first. It is then distributed to at least ten other visual cortical areas in the human brain (V2, V4, MT, inferior temporal contex; Figure 3). Each visual cortical area contains a separate map of visual space. In addition, neurons in different visual cortical areas exhibit different electrophysiological responses to visual stimulation, suggesting that individual cortical areas carry out different forms of visual processing (Figure 3). Consistent with this notion is the observation that damage to different visual cortical areas affects different aspects of vision.

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