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Visual System Advanced article Stephen D Van Hooser, Brandeis University, Waltham, Massachusetts, USA Article Contents . Anatomy of the Visual System Sacha B Nelson, Brandeis University, Waltham, Massachusetts, USA . Concept of a Receptive Field . Retina Humans and many other animals obtain much of their information about the world . Focusing of Light on to the Retina through their eyes. Patterns of light are transformed into nerve impulses in the retina and . First Stage of Information Processing: visual information is processed by nerve cells in the primary visual cortex. In the human Hyperpolarization of Photoreceptor Cells brain, about one-half of the cerebral cortex is dedicated in some way to the processing of . Receptive Fields of Retinal Ganglion Cells visual information. Relay of Signals from the Lateral Geniculate Nucleus to the Visual Cortex . Orientation and Directional Selectivity in Cortical Cells Anatomy of the Visual System . Double-opponent Colour Cells in the Visual Cortex . Columnar Organization of the Visual Cortex Visual processing begins in the eye, where light passes . Beyond the Primary Visual Cortex through the lens and is focused on to photoreceptors in the . Summary retina. Axons of retinal ganglion cells, the output cells of the retina, leave the eye in a bundle called the optic nerve. At the optic chiasm, some axons cross over to the opposite doi: 10.1038/npg.els.0000230 hemisphere, so that axons representing the right half of visual space travel to the left hemisphere and axons rep- resenting the left half of visual space travel to the right number of cells in the LGN and cortex; almost half of V1, hemisphere. From the optic chiasm, the retinal ganglion for example, represents the fovea. Visual information is cell axons project to visual brain structures such as the gathered through active movements of the eyes to bring the lateral geniculate nucleus (LGN) of the thalamus, the su- perior colliculus in the midbrain, and the suprachiasmatic nucleus. In primates, over 90% of these projections are to the LGN, where the retinal ganglion axons segregate into layers based on eye of origin and other properties. LGN relay cells receive large synaptic contacts from these axons, and make projections to the primary visual cortex (V1), Cornea where LGN axons representing each eye ramify in an al- ternating fashion. The anatomy of the primate visual sys- Lens Retina Optic nerve Primary visual tem is shown in Figure 1. cortex (V1) There is a second major visual pathway to the neocortex Optic from the retina via the superior colliculus. The superior chiasm colliculus projects to the pulvinar in the thalamus, which in Lateral geniculate turn projects to specialized regions of the visual cortex lo- nucleus (LGN) cated beyond V1. In primates, the superior colliculus is known to be involved in eye movements, but it receives Optic many fewer ganglion cell axons than the LGN. In many tract other mammals, such as carnivores and rodents, the supe- rior colliculus receives a larger percentage of retinal affer- Optic radiations ents than in primates, and it is likely that the superior colliculus plays a larger role in vision in these animals. This Figure 1 Anatomy of the visual system. Light arrives at the eye and is article will focus on the pathway from the retina to the focused by the lens on to the retina, where photoreceptors transduce the light into electrical signals that are processed by local retinal neurons. LGN to V1, since it is much larger in primates and is better Axons of retinal ganglion cells, the output cells of the retina, leave the retina studied than the pathway via the superior colliculus. in a bundle called the optic nerve. At the optic chiasm, some axons cross Since brains are limited in size by developmental and over, so that axons representing the right half of visual space travel to the energy constraints, the visual system does not represent all left lateral geniculate nucleus (LGN) and axons representing the left half of parts of the visual field equally, but instead emphasizes the visual space travel to the right LGN (not shown). In the LGN, the axons segregate into layers according to eye of origin and other properties. LGN area in the centre of the eye. The centre of the retina, called relay cell axons form a band called the optic radiations and project to the the fovea, contains the highest density of photoreceptors, primary visual cortex, where LGN axons representing each eye ramify in an and the fovea is represented by a disproportionately large alternating fashion. ENCYCLOPEDIA OF LIFE SCIENCES © 2005, John Wiley & Sons, Ltd. www.els.net 1 Visual System most informative parts of a scene into focus on the centre of the retina. Humans make over 100 000 such eye move- PRL Rod ONL ments, or saccades, in a single day, typically one or more Cone per second. By devoting large numbers of cells to a small OPL region of visual space and moving the eyes to informative Horizontal cell places in an image, the mammalian visual system affords INL Bipolar cell higher resolution than would be possible in an animal with ON fixed eyes and equal brain size. Amacrine cell Stimulus Light IPL OFF GCL Ganglion cell +– Time Concept of a Receptive Field (a) To optic nerve (b) In each brain structure described here, an individual cell Centre light Surround light responds to images in a small part of the visual field and Inverting only responds strongly to particular image patterns. The synapse part of the visual field to which a cell responds is called Noninverting synapse the receptive field of the cell, and the relationship between image patterns in the receptive field and the activity of the cell is referred to as the cell’s receptive field properties. Figure 2b shows an example of the receptive field properties Stimulus on of one neuron in the retina that is excited by light in the centre of its receptive field but inhibited by light in the Voltage excitation Inhibition/ surrounding part. Time depolarizing Hyperpolarizing/ (c) Retina Figure 2 (a) The major cell types in the retina and their laminar organization. PRL 5 photoreceptor layer; ONL 5 outer nuclear layer; The retina is a sheet of neurons and specialized receptor OPL 5 outer plexiform layer; INL 5 inner nuclear layer; IPL 5 inner cells located in the back of the eye. As shown in Figure 2a, plexiform layer; GCL 5 ganglion cell layer. (b) A centre–surround retinal the retina consists of six layers: the photoreceptor layer ganglion cell that responds to light in the centre of its receptive field and is (PRL), the outer nuclear layer (ONL), the outer plexiform inhibited by light in the surrounding region. The stimulus is shown on the left, and action potentials in the cell relative to the onset and offset of the layer (OPL), the inner nuclear layer (INL), the inner plexi- stimulus are shown on the right. Note that the cell responds most form layer (IPL), and the ganglion cell layer (GCL). The vigorously to a light spot in the centre surrounded by a dark annulus organization of the layers is peculiar in that the photo- (second from the top), but the cell responds much less vigorously when receptors are located at the back of the retina so that light stimulated by a large white spot (third from the top) because of the passes through all of the layers before reaching them. The inhibitory surround. These properties are often denoted symbolically with the notation at bottom, with ‘+’ indicating a preference for more light photoreceptors of the retina transduce light into electrical relative to background and ‘ 2 ’ indicating less light. (c) Schematic diagram signals that are processed by the local neurons of the retina. of retinal circuitry that mediates the centre–surround cell depicted in (b). The retinal ganglion cells are the only output cells of the Light in the centre hyperpolarizes a cone, which excites a bipolar cell, which retina, so all visual information available to the brain is in turn excites the retinal ganglion cell. Horizontal cells mediate the effect of transmitted by the axons of these cells. the surround, providing inhibition to the bipolar cell in the centre when there is light in the surround. This figure is adapted from Werblin and Dowling (1969), who studied the salamander Necturus maculosus, and similar circuitry has been found in other vertebrates. Focusing of Light on to the Retina First Stage of Information Processing: Light enters the eye through a transparent portion of the external membrane of the eye (the cornea), passes through Hyperpolarization of Photoreceptor the lens and the vitreous space, and forms an image on the Cells retina. Light is bent, or refracted, as it enters compartments that possess different refractive indices. This refraction The transduction of light into electrical activity occurs in permits the formation of a focused image on the retina. The two types of photoreceptors: rods and cones. Both rods lens contributes only about 1/4 of the refractory power of and cones consist of an inner segment that contains the cell the eye (the remainder is due to the cornea), but because the body and nucleus, and an outer segment containing a stack shape of the lens can be actively adjusted, it allows objects of membranous disks specialized for phototransduction. at various distances to be brought into focus. Rods have an elongated outer segment, are specialized for 2 Visual System detection of low intensity (scotopic) light, and are homo- to operate at different light levels. If one reads a newspaper geneous in their wavelength sensitivity. Cones have a ta- outside on a bright sunny day, the absolute amount of light pering outer segment, are specialized for detection of reflected from both the black text and the white page will be higher intensity (photopic) light, and individually are more much greater than if one reads the same newspaper in- sensitive to long (L-cones), medium (M-cones), or shorter doors.
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