THE VISUAL SYSTEM Dan Tollin, PhD

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1 Nervous System Block Visual System - September 19-20, 2016 THE VISUAL SYSTEM Dan, PhD The visual system can detect the detailed form of objects, slight movement of objects, can work effectively in very low or very high light level conditions, can detect the three-dimensional shape of objects, and can faithfully discern more than 200 different colors, even under conditions of varying illumination. It can assemble itself developmentally from scratch, employing both genetic and environmental cues that cause some synaptic connections to become hard wired, while others remain plastic throughout life. In addition to processing visual information for conscious perception, sensory information from the eyes is used in additional ways: (Note: Clinical implications will be in italics and underlined) in a negative feedback loop to regulate the amount of light entering the eye by controlling the size of the pupil (this reflex is a simple but useful diagnostic test see Appendix II); to inform cells in the hypothalamus concerned with diurnal variations about ambient light conditions; and to talk to cells in the superior colliculus that coordinate head and neck movements. All of these functions are mediated by sensory axons that course in the optic nerves. 1. Physical nature of light Light is electromagnetic radiation that is visible to our eyes. Light can be described as having a wavelength and amplitude (or intensity). Intensity corresponds to the perceptual brightness of an object while wavelength corresponds to color. Fig 1 shows the Fig 1. Wavelengths of visible light. wavelengths of visible light nm (blue) to 700 nm (red). When light strikes a surface, it can either be reflected or absorbed. The relative amounts of light at the different wavelengths that are reflected or absorbed determine the color that we perceive an object to be. If light passes from one medium to another it is refracted (or bent). Refraction is the method by which light is focused on the retina by the cornea and lens. Dept Physiol & Biophys/ Stop 8307 RC1-N, Rm P Daniel.@ucdenver.edu 2. Gross Anatomy of the eye Fig 2. Gross anatomy of the eye ball. The eyeball is composed of a few main structures. The cornea provides ~2/3 of the refractive (or focusing) power for the eye, while the lens provides only ~1/3. The refractive power of the lens is under neural control and allows for focusing of nearby objects. The pupil is the opening through which light enters the eye. The size of the pupil is controlled via the cilliary muscles (which we will review later). The back of the inner eye contains the retina, which is the receptive organ of the eye. The output neurons of the retina, the retinal ganglion cells, group together at the optic disk forming 1

2 Nervous System Block Visual System - September 19-20, 2016 the optic nerve. The optic disk contains no photoreceptors, giving rise to the blind spot. Each optic nerve contains about a million axons. 3. The retina The Retina contains five types of neurons (Fig 3). Photoreceptors (rods and cones) capture photons of light and convert (transduction) them to an electrical signal (change in membrane potential), which is passed synaptically to bipolar cells and horizontal cells, and then to the output cells of the retina, the ganglion cells. (Amacrine cells also participate in signal analysis, but we will not consider them here.) Curiously, the photoreceptors face the back, not the front of the eyeball, which means that light must pass through all the other cells before it reaches the photoreceptors (Fig 3). Fortunately, the other retinal cells are nearly transparent (and, in the fovea, the 0.2 mm-wide region where acuity is greatest, the other cells are swept aside). Cones mediate color vision, are concentrated in the fovea, and work well only in bright light. Rods are color insensitive and work best in dim light - rods are the dominant photoreceptor away from the fovea. Humans have about 100 million rods and 8 million cones in each retina. Fig 3. Cross-section of retina (a) including main neurons comprising the retina (b). Phototransduction is the process by which light is converted to a change in membrane potential by the photoreceptors. The process, which you studied earlier, involves another curiosity: light absorption causes photoreceptors to hyperpolarize (Fig 4). The mechanism is analogous to other G-protein coupled second messenger systems. The only difference is that light rather than a chemical substance is the trigger that initiates the cascade. To catch the light, a membrane protein is packed at high density in surface membrane infoldings (cones) or intracellular membranous sacks (rods); the density is so high that a photon of the appropriate color passing through the retina has a 50 percent chance of capture. The photon is absorbed not by the protein itself, but by a pigment -- vitamin A -- bound to the protein. As described in the detail in Appendix I, the membrane protein then activates a G-protein, which in turn activates an enzyme, which destroys an intracellular ion channel ligand, cyclic GMP (cgmp). The reduction in cgmp concentration causes nonselective cation channels in the surface membrane to close, thereby hyperpolarizing the cell. In other words, in the dark photoreceptors have a relatively high concentration of cyclic GMP (cgmp), which binds to and holds opens nonselective cation 2

3 Nervous System Block Visual System - September 19-20, 2016 Fig 4. Rods and cones of the retina (LEFT) are hyperpolarized in response to a light stimulus (TOP). Rods and cones give a graded response (TOP) as a function of light intensity the more intense the light, the more hyperpolarized the cell becomes. channels in the surface membrane. With many such channels opened in the dark, the membrane is relatively depolarized. Thus, light hyperpolarizes photoreceptors. 3.1 Retinal ganglion cells: ON-Center and OFF-center receptive fields Photoreceptors talk to bipolar cells and to horizontal cells (Figs 3 & 5). The bipolar cells synapse with ganglion cells. Only the ganglion cells make action potentials; all of the other retinal cells communicate by graded changes in membrane potential, which alter the rate of exocytosis of neurotransmitters in a graded fashion. While the human retina possesses nearly 100 million photoreceptors, the ganglion cells to the brain contain ~1% as many axons. Thus, there is a tremendous amount of analysis in the retina itself. In brief, the retina is wired to detect contrast - it doesn't care much at all about absolute levels of illumination, only changes in brightness. It is the first stage in building specific detectors for edges, corners, shapes, and even faces. Sensory neurons are defined in part by their 'receptive field', which basically is the best stimulus to get them to change their action potential firing rate. Ganglion cells have donutshaped receptive fields and come in two opposite types (Fig 6). Some are excited by light shining in their centers and inhibited by light in the periphery ('on' center ganglion cells). Others are the opposite ('off ' center ganglion cells). In the fovea, a ganglion cell receptive field center may be only as wide as a single cone, with an antagonistic surround not much bigger. Receptive fields are larger in the periphery of the retina. Ganglion cells fire action potentials spontaneously in the dark, and so are very sensitive to slight changes in both excitatory and inhibitory inputs. Tracing the synaptic interactions (Fig 5) that give rise to these properties is simple in principle, but complicated in practice because of multiple negatives (e.g., inhibition of inhibition makes excitation). The key determinant in the receptive field type of ganglion cells is actually the type of receptor on bipolar cells. Retinal processing that gives rise to ganglion cell receptive fields is simple if we keep in mind a few simple rules: Photoreceptors are hyperpolarized by light, resulting in less neurotransmitter release Photoreceptors release glutamate, but Bipolar cells can be either excited (OFF-center) or inhibited (ON-center) by glutamate (due to different receptor types) Bipolar cells always make excitatory synapses on ganglion cells Let s apply these simple rules to an example. If we find that glutamate from the photoreceptor excites the bipolar cell, then shining light on the photoreceptor will do what to the ganglion cell - 3

4 Nervous System Block Visual System - September 19-20, 2016 Fig 5. ON-center and OFF-center ganglion cells are determined by ON-center and OFF-center bipolar cells. ON-center bipolar cells are inhibited by glutamate while OFF-center are excited. excite (ON-center) or inhibit (OFF-center)? The answer of course is: INHIBIT! A reduction of excitation is equivalent to inhibition. This is an example of an OFF-center ganglion cell (Fig 5, left panel, OFF-center pathway). If a bipolar cell possesses inhibitory glutamate receptors, then it will be tonically inhibited in the dark, and light will relieve the inhibition, making an ON-center ganglion cell receptive field (Fig 5, left panel, ON-center pathway). The receptive field surround is mediated by horizontal cells. Horizontal cells behave as though they have excitatory receptors for glutamate released from photoreceptors, and make inhibitory synapses on the neighboring photoreceptors in the field center (Fig 5, right). Thus, if a spot of light is shone in the periphery of an ON-center ganglion cell receptive field, what will happen to the membrane potential of the cells in the pathway? Answer (Fig 5, right, ON-center pathway): The illuminated photoreceptors in the surround will hyperpolarize, reducing the secretion of transmitter, reducing the activation of excitatory receptors on the horizontal cells, which will hyperpolarize the horizontal cells. That will decrease their secretion of GABA onto photoreceptors in the field center, and so will decrease inhibition of those photoreceptors in the field center, causing them to release more transmitter onto the bipolar cells. Since this is an ONcenter cell, the receptors on the bipolar cells in the field center are inhibitory. So the inhibition of the bipolar cells will increase when light shines on the periphery, which will reduce the bipolar cell excitatory input to the ganglion cell, which will reduce the firing rate of the ganglion cell. In summary, of the 4 synapses in the pathway, two are always excitatory (i.e., transmitter depolarizes the postsynaptic cell) these are the surround photoreceptor to horizontal cell synapses, and bipolar cell to ganglion cell synapses. One synapse is always inhibitory horizontal cell to photoreceptor synapses. One synapse may be either excitatory (OFF-center bipolar cells) or inhibitory (ON-center bipolar cells) field center photoreceptor to bipolar cell. Typical responses of ganglion cells, recorded with an extracellular microelectrode, are shown in Fig 6. Each vertical line in a trace represents one action potential. The duration of the light flashes is indicated by the horizontal arrows at the bottom. Note the clear ON-center and OFF-center responses to the appropriate pattern of light stimulus (top 2 rows). Notice that one feature of the response to light in the inhibitory area (be it center or surround) is a rebound response when the light is turned off. This results from an abrupt removal of inhibition when the 4

5 Nervous System Block Visual System - September 19-20, 2016 light is turned off. With this arrangement, what will happen if diffuse light shines on the entire receptive field? The answer: not much, if the center and surround exactly cancel each other (Fig 6, bottom row). In fact, usually the center wins, slightly. A demonstration that the visual system is interested primarily in stimulus contrast (light-dark interfaces) is shown in Fig 7. Notice that the central stripe seems darker on the left and lighter on the right. In fact, the bar has the same intensity across the image! Why does it seem to change? Because your ganglion cells care only about contrast. Fig 7 (right) shows why in terms of ON-center cells. Receptive fields of Cells A and E are presented with uniform illumination, and so they don t respond much (Fig 6, bottom row). Cell C is right on the boarder of light and dark, so it also doesn t respond much. However, Cells B and D are saying a lot. The center of Cell B is in the dark, and so its Fig 6. Responses of ON-center and OFFcenter ganglion cells to different patterns of light stimuli. response is reduced indicating that the center is darker than the surround (eg, Fig 6, 2 nd row). The center of Cell D is in the light, so its response is increased indicating that the center is much lighter than the surround (Fig 6, 1 st row). Hence, only those cells at the borders of the middle stripe are firing, and those on the left are saying, 'darker in center', while those on the right are saying, 'lighter in center'. The brain (mis)assembles this information, and your perception is fooled. Fig 7. Retinal ganglion cells care only about stimulus contrast the relative difference between light and dark areas. LEFT: The central stripe seems to grow lighter moving from left to right. In fact, it is the same throughout. RIGHT: The illusion results because the ganglion cells near the boarders of the bars are fooled by the local contrast. 4. Central visual pathways: optic nerve, optic chiasm, and the lateral geniculate nucleus The retinal ganglion cells group together at the optic disk forming the optic nerve (Fig 2). The optic nerves from the two eyes merge at the optic chiasm, where about half of the axons from each eye cross to the other side, and then continue on as the optic tract to the lateral geniculate nucleus (LGN) of the thalamus (Fig 8). At the chiasm, the axons from the nasal half of each retina cross over to the opposite side (decussate). The result is that, for example, the right optic tract contains axons 5

6 Nervous System Block Visual System - September 19-20, 2016 from the right side of each retina, which see the left side of the visual world. In this way, if you look straight ahead, all of the information to the right of your center of view goes first to your left visual cortex. In other words, the LGN represents the contralateral visual field. Beyond the LGN, axons involved in visual processing fan out in the optic radiations to the visual cortex at the back of the brain (Fig 8). Within each optic tract axons project in an orderly fashion to the cortex (retinotopic projection). For example, the lower half of each retina (which sees the upper half of the visual world) projects to the lower half of each visual cortex (the lower bank of the calcarine sulcus). Clinically, it is important to understand how the retina is connected (via the lateral geniculate nucleus) to the primary visual cortex, since blindness in specific parts of visual space (visual field defects) can be useful in diagnosing the location of brain damage due to injury, strokes or tumors (see Appendix III). 4.1 Lateral geniculate nucleus (LGN) The ganglion cell axons end in the LGN. In primates and humans, the LGN is composed of 6 layers (Fig 9). Information from each eye projects to separate layers so that there is no direct interaction between the eyes at the level of the LGN (that is, LGN cells are NOT binocular). Layers 1, 4, and 6 receive inputs from the contralateral eye (these inputs desiccate at the chiasm) while layers 2, 3, and 5 receive inputs from the ipsilateral eye. There is also a secondary segregation of information in these 6 layers: Layers 1 and 2 receive inputs from the so-called magnocellular ganglion cells while layers 3-6 receive inputs from the so-called parvocellular ganglion cells. These two different kinds of ganglion cells have different kinds of response properties to light stimuli beyond their simple ON- and OFF-center receptive fields. Parvocellular System Object Vision color, form, detail 1. High acuity (fine detail) 2. Small receptive fields 3. Not responsive to motion 4. Color vision (input from cones) Fig 8. Ascending pathways in the visual system. Optic nerve fibers from the nasal and temporal parts of left and right retina, respectively, travel to the right LGN. The eye of origin is kept separated in the 6 layers of LGN (see Fig 9), but converges in primary visual cortex (V1) Magnocellular System Spatial Vision Motion and depth 1. Low acuity (crude form) 2. Large receptive fields 3. Responsive to motion 4. No color vision (input from rods) These two systems are established at the retina, remain segregated at the LGN, and travel in separate, but parallel pathways through the visual cortical areas. These two pathways mediate the perception of different attributes of vision, object vision (parvo) and spatial vision (magno). 4.2 Cortical processing: Binocularity, Orientation, and Color As shown in Fig 8, the LGN axons radiate out to the visual cortex (area V1 vee one ) at the very back of the brain, where they map the retina point-by-point, creating a retinotopic 6

7 Nervous System Block Visual System - September 19-20, 2016 Fig 9. Cortical hypercolumns. Inputs to Layer 4 of primary visual cortex (V1) from the 6 layers of the LGN remain partially segregated according to eye of origin, ipsilateral (I) or contralateral (C). The pinwheel organization of orientation tuning and ocular dominance columns are also s projection. The projection is distorted, however, for the foveal region, which occupies a tiny fraction of the retina, occupies nearly one-half of the visual cortex (this is the retinal area of greatest acuity and color vision, and it needs a lot of cortex to handle the data). Each micro-region of V1, called a hypercolumn, is about 1 mm on a side, receives about 10,000 LGN axons, and possesses the same basic structure, as shown in Fig 9. It is layered from layer 1 (the top, at the surface of the brain) to layer 6 (the bottom, at the border with the white matter). LGN axons terminate in layer 4, as shown in Fig 9. First, viewed from the surface of the cortex, each hypercolumn is divided in two parts, one half for each eye; these are called ocular dominance columns. This correspondence in itself is pretty amazing - ganglion cells in every corresponding region of retina in the two eyes send axons (via the LGN) to exactly sideby-side slabs of cortex. In other words, axons from the contralateral (C) eye, which decussate in the optic chiasm, match up with axons from exactly the corresponding region of the ipsilateral (I) retina. About half of the cortical cells - the ones in each hypercolumn near the border between the two eyes - become binocular; they receive inputs from both eyes. Second, the orientations of lines in the visual field lie in different rays of pinwheels (like pieces of a pie) that radiate from the central blobs (Fig 9). All cells in a vertical column are sensitive to the same orientation. Marching around the circle, the orientation preference for lines in the visual field changes progressively, with all angles around the clock represented. Third, color information, which is separated out from spatial information in the retina, is handled in central regions of the hypercolumns called blobs. Below, we will consider each of these attributes - binocularity, orientation, and color. 7

8 Nervous System Block Visual System - September 19-20, 2016 The experiments that revealed this cortical organization were performed with extracellular microelectrodes. The electrode is advanced blindly through the visual cortex of an anesthetized animal (cats and monkeys have been most studied). When it nears an active cell, it begins picking up action potentials from that cell. After such a cell is found, simple images of different sizes, shapes, and colors are shown on a screen in front of the animal (whose eyes are open and focused on the screen). When the stimulus with the correct attributes is presented, the firing rate of the cell changes. In this way, the receptive field of the cortical cell is mapped. This work, for which they won the Nobel Prize in Physiology or Medicine in 1981, was pioneered by David Hubel and Torsten Wiesel. In a typical experiment, hundreds of individual cells would be identified and their receptive fields mapped. Usually, 5-20 cells are encountered during a single penetration of the 2 mm-thick cortex. Then the electrode is moved to a new area Simple cells For a long time, Hubel and Wiesel used spots of lights to study cortical cells, without much success. It was only when they used more complex stimuli, such as bars or edges of light, that the cells started to give brisk responses. Even though the receptive field shape was more complex than that of ganglion cells, they called these cells simple cells. For example, a simple cell might have an ON area that is a narrow line at some preferred orientation that is flanked on each side by OFF areas (eg., Fig 10, left side). Such a cell is best stimulated by a narrow line of light covering all of the center ON areas Fig 10. Hypothetical synaptic circuit to create simple cells in the primary visual cortex from the convergence of LGN cells with slightly different ON-center or OFFcenter RFs (LEFT). This pattern of convergence creates elongated center-surround RFs that are exquisitely sensitive to the orientation (RIGHT) of a bar of light. without intruding on the flanking OFF areas. Diffuse light is entirely ineffective. The spatial position and the orientation of the line is crucial; the cell is tightly tuned to within a few degrees of its best orientation (Fig 10, right). Other cells encountered in the same penetration show the same orientation selectivity (Fig 9). A new electrode penetration, a fraction of a millimeter away from the first, reveals cells with the same properties, but slightly different orientation preference. Exhaustive mapping studies have shown the kind of organization in Fig 9, with orientation columns organized as pinwheels, with adjacent rays having similar but not identical orientation preferences. While some simple cells have ON-centers with OFF flanking lines, others are the reverse (OFF-center, ON-surround), just like retinal ganglion cells (but responsive to lines, not spots). In summary, simple cells have receptive fields with antagonistic flanking regions; the shape of the field is a straight line (the longer the line the better, up to a maximum length, beyond which no further change occurs), and the orientation of the line is crucial. 8

9 Nervous System Block Visual System - September 19-20, 2016 The synaptic circuitry that creates simple cells is illustrated in Fig 10. On the left are several overlapping ON-center LGN (and ganglion cell) receptive fields, lined up along a diagonal. Suppose three of those cells converge on one cortical cell in area V1, and that each input is excitatory. The cortical cell will then have a receptive field that is the sum of the LGN cells' receptive fields (Fig 10, left). In this example, its ON-center will be a diagonal line, with flanking OFF regions. (The overlapping centers and surrounds of the LGN receptive fields will not cancel each other, because the entire center of one cell overlaps with only a small part of the surround of another.) This is called hierarchical processing: Several cells with similar but spatially offset receptive fields converge on a higher order cell to create an altogether new type of receptive field. Once you have this basic idea in mind, the rest becomes relatively easy. For example, with this simple principle, we can create just about any kind of receptive field you can imagine - and the brain does just that, increasingly abstracting some qualities (like position) and refining others (like the lines that make up faces), until one has a cell that can essentially 'recognize' your mother's face appearing anywhere in your visual field Binocular cells In about half of the cells recorded from V1, the cell receives input from the LGN on one side (monocularly driven by contra or ipsi eye, Fig 9). The other half receive inputs from the LGN from both eyes (binocular), and the receptive fields of the two eyes are virtually identical - same orientation, same region of retina, same width of line, same on-off organization. Binocular cells tend to be found at the boarders of the occular dominance columns (Fig 9). Binocular cell are sensitive to and mediate depth perception Complex cells The so-called Complex cells have receptive fields like simple cells, with one big exception: they abstract for position. That is, while simple cells require a line or edge (of a specific orientation, with specific ON and OFF regions) at a particular position in the visual field, complex cells are not so finicky about the position. The line or edge can be anywhere within the RF, and these cells especially like to see lines or edges moving across the field. How are cells with complex receptive fields created? The answer is: by convergence of several simple cells whose positions are slightly offset (Fig 11). The converging simple cells make excitatory synapses on the complex cell, and any single simple cell can cause the complex cell to fire. Simple cells and complex cells are located together in the same hypercolumns. As shown in Fig 9, the vertical organization of columns is as follows: LGN axons terminate in layer 4, Fig 11. Hypothetical synaptic circuit to create complex cells in V1 from the convergence of simple cells with slightly different RF centers (LEFT). This pattern of convergence creates a rectangular-shaped RF that is sensitive to the orientation (RIGHT) but not the absolute position of a bar of light. That is, complex cells abstract for spatial position. 9

10 Nervous System Block Visual System - September 19-20, 2016 creating simple cells. Simple cells send axons up and down in the same hypercolumn to higher and to lower cortical layers, creating complex cells. The output of each hypercolumn, which contains about five times more axons that does the LGN input, exits from layers 3 and 6 to higher order visual areas. How does one hypercolumn relate to its neighbors? Because of the retinotopic projection, neighboring hypercolumns attend to neighboring retinal regions. The ocular dominance columns line up in stripes. The orientation column pinwheels spin out over the cortical surface, interconnected with neighboring hypercolumns (Fig 9). Summary of the properties of different cells in the visual pathway. (RF=Receptive Field) Cell Location Diffuse light? RF shape Orientation selective? Binocularly driven? Position sensitive? Photoreceptor Retina ok tiny spot no no yes Ganglion cell Retina so-so donut no no yes Simple cell Cortex no bar yes yes yes Complex Cortex no edge yes yes no Color Vision Humans have 3 different kinds of cones blue, green, and red - named after the approximate peak of their sensitivity to colors of light (Fig 12): these are also referred to based on their wavelength sensitivity: short (blue), middle (green), and long (red). The wavelength absorption curves overlap considerably, meaning that any single wavelength of light in the visible spectrum (400 to 700 nm) will be absorbed to a different extent by each cone type. A single cone type cannot encode color information because there is a confound between stimulus wavelength (color) and stimulus intensity. It is from the relative activities of the 3 cone types by single wavelengths of light that the nervous system extracts the information it needs to create cells in the cortex that respond only to particular colors. Humans can discriminate approximately 200 different hues in the visible spectrum. This means that for every shift of about 3 nm in the wavelength of light entering the eye we sense the change in color. It is likely that we code colors in different neurons, so this means that we have at least 200 different neurons somewhere in the brain, each responding only to a very narrow band of wavelengths (e.g, nm). How the visual system gets from unequal excitation of the 3 broadly-tuned types of cones to cortical cells that respond only to one narrow band of wavelengths is a magnificent achievement, whose mechanisms are not very well understood. Fig 12. Absorption spectra of the 3 cone types. Blue (or short wavelength), Green (middle) and Red (long). The absorption spectra for the rods is also shown (dotted). Color-opponent ganglion cells Cones of different color preferences converge in the retina to produce ganglion cells with receptive fields that are partial to particular colors. In the fovea, where color discrimination is best because all of the photoreceptors there are cones, most of the bipolar cells are connected directly to one kind of cone in the field center (e.g., a red cone) and indirectly (via horizontal cells) to cones with a different color 10

11 Nervous System Block Visual System - September 19-20, 2016 preference (e.g., green cones) in the field surround, thereby creating a RED ON-center and GREEN OFF-surround receptive field, which is passed along to the ganglion cells. As you might expect, all combinations of red-green, on-off opposing fields exist. These are called color opponent cells. In addition to red-green opponents, there are blue-yellow opponent cells, thereby spanning the entire spectrum (the yellow selectivity is created by converging both red and green cones). Fig 13. Parallel pathways and modular organization. Two primary pathways through visual cortical areas, the dorsal and ventral pathway, process spatial information and mediate object recognition (including color), respectively. Information encoded in V1 is separated in V2, and then passed on to visual cortical areas V4 and V5 (or middle temporal, MT). V4 is often considered the color area while V5 is considered the motion area. 5. Parallel Pathways and Modular Organization of visual cortical areas For different dimensions of the image (e.g. shape, color, motion, spatial information) we have analogous systems that use hierarchical processing to construct higher levels of representation in their dimensions. This is called parallel processing, and it is sometimes confused as an alternative to hierarchical processing. Parallel processing is simply the requirement that dissimilar dimensions (e.g. color and form) must be analyzed by separate, but parallel, neural systems. Eventually the brain unites all the dimensions of the retinal image (form, color, movement, depth), after they have been separately analyzed, into a unified perception. We understand little about how these features, processed hierarchically but in parallel streams, are knitted together. This is called the binding problem, and is perhaps one of the greatest unsolved questions in all of biology. As we ascend the visual system, each higherorder cell surveys a number of lowerorder cells, and as a result, can abstract the collective properties of those lower-order cells. Thus, complex cells are more powerful in their capacities to respond to the retinal image than are simple cells. There are even more complex formprocessing cells in higher-order visual cortices (V2, V4, V5, etc.). This is what is meant by 11

12 Nervous System Block Visual System - September 19-20, 2016 hierarchical processing--using successive synaptic integrations of highly specific synaptic inputs to construct higher and higher levels of representation of the retinal image until eventually we have cells that respond only to the complete form of an object (e.g., "face cells.). There are two primary parallel pathways, or streams, through the ascending visual systems (Fig 13). The Dorsal Pathway travels from V1 dorsally to the parietal lobe and is generally believed to be responsible for spatial vision, including motion and depth perception. The Ventral Pathway travels ventrally from V1 to the temporal lobe and is generally believed to be responsible for object vision, including color, form, and pattern vision. Although these two pathways are separated out spatially in area V2, they have their beginnings in the retina the magnocellular and parvocellular pathways project to separate layers in LGN and from LGN project to different layers in V1 (section 4.1). These pathways remain segregated in the output pathways from V1 to V2 (Fig 13). As seen in Fig 13, the Dorsal Pathway courses through the thick stripe region of V2, and then onto area V5. V5 is often called MT (middle temporal). 95% of cells in MT are selective for the direction of visual motion, and many of those neurons are also sensitive to visual depth. Lesions of area MT result in impaired motion and depth perception. Recent studies have shown that the activity of neurons in MT correlates directly with the perception of visual motion and/or depth. The Ventral Pathway courses through both the stripe and interstipe regions of V2, and then onto area V4. The stripe regions of V2 receive inputs from the blobs in V1 cells in the blob areas do not care about shape, only color. These cells do not have center-surround anatomy, but are simpler: their receptive fields comprise a uniform area of retina within which light of one color excites the cell and light of another color inhibits the cells. For example, a red spot may excite the cell, and a green spot in the same area will inhibit the cell. This is like the color-opponent cells in the retina, except that the blob color cell does not care about shape - it signals unambiguously information about color, and nothing else. However, it is not highly tuned to specific colors - it may, for example be excited by red and orange, and inhibited by green and cyan. Clearly, these cells are merely a way station on the road to conscious color perception, since we have no trouble distinguishing red from orange. How are these cells created from lower order cells? The rules are the same as for simple and complex cells, but the details vary. Coloronly blob cells receive input from many color-opponent neurons. For example, a red-on green- OFF cell gets excitatory synaptic inputs from both red ON-center green OFF-surround cells and from green OFF-center red ON-surround cells, as well as inhibitory synaptic inputs from green ON-center red OFF-surround cells and red OFF-center green ON-surround cells. The positions of all of these fields overlap entirely, so there is no spatial information in the color-only blob cell's response. The cell also cares nothing for white light. More advanced color processing continues in higher cortical areas. In area V4, anterior and inferior to the primary visual cortex (V1), cells have relatively large receptive fields in the central areas of the retina (where cones predominate) and respond only to fairly narrow bands of wavelengths over the visible spectrum, some as narrow as 10 nm. Given a little lateral inhibition at a higher level of color processing, color cells with bandwidth selectivities as narrow as our perceptions are (~3 nm) could easily exist. Lesions in V4 can result in impairments in color discrimination. 12

13 Nervous System Block Visual System - September 19-20, Plasticity in the Visual System More than four decades of intense research from hundreds of different laboratories have elucidated the first stages of visual processing described above. As presented here, the results are largely descriptive - this is the way the visual system analyzes information. Several interesting avenues of further investigation have naturally emerged from the work. One of course is clinical. It is becoming increasingly possible to diagnose strokes that produce incredibly specific lesions. For example, cases have been reported in which patients lose the ability to recognize specific faces (proposagnosia), even their own, while retaining the ability to recognize parts of a face - nose, lips, and so forth. One person lost altogether the ability to detect movement, which made crossing the street a very dangerous proposition, since cars zipping past appeared stationary. Others have lost all color discrimination (although their retinas are intact). It is not difficult to imagine the existence of higher visual centers where the responsible cells reside. Besides impacting our understanding of practical clinical matters, this work also raises interesting questions about the remarkable specificity of synaptic connections: how do these incredibly specific connections develop, and how are they maintained? Are connections determined genetically, or environmentally, or both? (answer: both) Are they subject to change given altered visual experience? (answer: depends on age). Hubel and Wiesel, after working out the basic wiring that creates simple and complex cells, addressed these questions in a remarkable series of experiments. The work was performed first on cats and kittens, and then repeated on monkeys. The results were very similar. Here the data from cats will be given. We will consider three basic experiments: monocular deprivation, binocular deprivation, and strabismus. In each case, the visual experience of kittens was altered for a period of time, and the properties of cells in the visual cortex were then examined. One property is of particular interest: ocular dominance, which is a measure of the relative synaptic input to a cell from each eye. Hubel and Wiesel defined seven categories of ocular dominance (Fig 14): category 1 cells are driven only by the eye that is contralateral to the cortical cell that is being recorded from; category 4 cells are driven equally by both eyes; category 7 cells are driven only by the eye that is ipsilateral to the cell being studied. Thus, categories 1 and 7 contain monocularly driven cells, while all other categories contain binocular cells of varying eye preference. 6.1 Monocular deprivation As shown in Fig 14, the primary visual cortex of a normal adult cat contains all types of cells, although binocular cells predominate. Recall that, when a cell receives inputs from both eyes, the receptive field positions and orientations in the two eyes are identical. How do these connections develop? If the same experiment is repeated on a newborn kitten, the results are similar: the cortex contains many binocularly driven cells (results are not shown here), proving that this aspect at least of cortical wiring is genetically determined. [Receptive fields in newborns are not as crisp (e.g., orientation selectivity is lower) and responses are not brisk (e.g., fewer action potentials) as in adults. But ocular dominance histograms look quite adult-like.] Thus, while the connections are determined genetically (i.e., arise before any normal vision begins), one can ask: are the connections immutable? As shown in the middle panel of Fig 14, if one eye of a kitten is closed for even a few days, the cortical cells lose virtually all connections to the deprived eye (cells in the column labeled NR were Non-Responsive, that is, they were spontaneously active, but couldn't be driven by any visual stimuli). Results from a control experiment are shown in the right hand panel of Fig 14: if the same experiment (close one eye for several days) is performed on an adult cat, there is virtually no effect. This means 13

14 Nervous System Block Visual System - September 19-20, 2016 that the connections, genetically determined, are not immutable, at least for a while after birth. The period of time when the connections can be altered by visual experience is called the critical period or the sensitive period. In kittens, it lasts about three months, and the synapses are maximally sensitive to deprivation at 4-6 weeks of age, when a single day of deprivation can silence the cortex from the deprived eye. In general, the longer the period of monocular deprivation during the sensitive period, the worse the outcome (in terms of loss of connections from the deprived eye) - even a few days' deprivation is long enough to produce severe effects. What about recovery? If one eye is closed for 3-6 days, and then reopened for the duration of the sensitive period, does the cortex recover? No, it does not recover. The connections, once lost, are gone for good. Finally, none of these results is due to lack of light falling on the retina. Blocking form vision with a translucent light diffuser (which let through most light but blurs any contrast) produces cortical results indistinguishable from eyelid closure. In addition, retinal ganglion cell and LGN receptive fields are intact (although LGN cells are smaller than normal), showing that the defect is in the cortex. Use it or lose it? These results suggest the explanation known as 'disuse atrophy' (colloquially, 'use it or lose it'). If one eye is deprived of vision, the synaptic connections in the cortex from that eye degenerate and disappear. Further experiments, however, showed that the situation is more complex, and far more interesting. 14

15 Nervous System Block Visual System - September 19-20, Binocular deprivation The second type of visual deprivation experiment involved binocular deprivation. According to the 'lose it or lose it' hypothesis, this should have produced a nearly silent cortex, with few synapses from either eye. However, the result was very different (not illustrated here): the primary visual cortex was for the most part normal! (There were more non-responsive cells, but most cells could be driven, and most were binocular. Altogether, about half of the cells had normal receptive fields.) The cats, however, were behaviorally blind, which means that higher order visual cells were completely disrupted. So much for the 'use it or lose it' theory. Something is happening that involves an interaction - perhaps a competition - between the two eyes. To say it another way, if you are asked what happens to the binocularity of cortical cells of a kitten whose left eye was deprived for a week during the sensitive period, you cannot answer unless you also know the status of the right eye. If the right eye received normal visual input, all cortical cells will be driven by it, and only it. But if the right eye was, like the left eye, deprived of vision, then the cortex will contain many binocularly-driven cells. This result helps to explain another, involving monocular deprivation. Recall that if one eye is deprived for a few days, no cortical cells can be driven by the deprived eye, even if the deprived eye is reopened for a long period of time. What do you think happens if, when the deprived eye is reopened, the other eye is deprived (still during the critical period)? The answer is: the reopened eye recovers, and the newly deprived eye loses its connections with the cortex. This experiment further illustrates the competition - active suppression by the active eye - that occurs between converging inputs from each eye in the cortex. 6.3 Deprivation of normal binocular experience by strabismus The competitive interactions between the two eyes during the sensitive period were tested further in the third type of experiment - strabismus (Fig15). The experimental paradigm is simple: one extraocular muscle (medial rectus) was cut in one eye of a kitten. This caused the eye to deviate laterally, producing a strabismus ('wall eyed'). Thus, the two eyes looked at different parts of the visual world, but over the course of a day each eye individually received the same amount of normal, crisp, high contrast visual input. The only difference was that, at any instant, each eye saw a different part of the visual world. The result (Fig 15) in the cortex was stunning: there were very few binocular cells! Almost all cells were driven exclusively by one eye or the other (there were about equal numbers of each). Hubel and Wiesel reasoned that the only change from normal vision in this experiment concerned the timing of synaptic inputs to the cortical cells. In normal vision, cortical cells are driven synchronously by the two eyes, because the retina of each eye sees corresponding parts of the same scene. However, with a strabismus, at any particular 15

16 Nervous System Block Visual System - September 19-20, 2016 instant the input from one eye to a cortical cell may be excited by a feature in its receptive field, while the receptive field of the other eye may be excited, or inhibited, or not affected at all, depending on what it sees at that particular instant. This gave rise to the notion, 'cells that fire together wire together'. It is the synchrony of inputs that leads to stabilized synapses. If the inputs are asynchronous, a competitive struggle ensues, and one of the eyes wins everything, and the other loses altogether. Thus, with a strabismus, nearly all cells become monocularly driven. The same effect as strabismus can be achieved by alternating (on a daily basis) monocular deprivation. 6.4 Anatomical correlates of plasticity In recent years, several aspects of these studies have been explored in greater detail, thanks to an anatomical technique that reveals how the LGN axons terminate in layer 4 of the visual cortex (Fig 16). If radiolabeled amino acids are injected into one eye, they are taken up by retinal ganglion cells, incorporated into proteins, and transported to the cortex (crossing the synapse in the LGN). At birth, LGN axon terminals commingle, and a broad white stripe marks layer 4. Over time, the LGN terminals sort themselves into discreet bands characteristic of the adult animal, and layer 4 shows a banded appearance (Fig 16). These are the ocular dominance columns of the adult animal, and the technique provides striking confirmation of the electrophysiological experiments. This anatomical technique has been used to examine the effects of various experimental manipulations on this developmental process. For example, if one eye is deprived at birth, the bands change - the bands from the deprived eye are reduced in size, and the normal eye bands are expanded, just what one would expect if LGN terminals compete with each other, and the competition is activity-dependent. 6.5 Direct experimental control of neural activity induces plasticity Tetrodotoxin, a potent local anesthetic was applied chronically to the optic nerves, blocking all action potentials during development. This blocked the emergence of the ocular dominance columns: the radioactive bands of LGN terminals failed to segregate. No firing, no wiring. Next, action potentials were blocked with tetrodotoxin (as before) and stimulating wires were implanted in each optic tract, beyond the tetrodotoxin block. Synchronous stimulation of both optic tracts produced a normal ocular dominance wiring pattern: most cells recorded from were 16

17 Nervous System Block Visual System - September 19-20, 2016 binocularly driven. Finally, the nerves were blocked with tetrodotoxin, and asynchronous stimulation (alternating trains of stimuli to each optic tract) was delivered. In this case, the majority of cortical cells were monocularly driven. These experiments directly confirm the hypothesis that synchronous activity from both eyes is necessary to insure that proper synaptic connections form during development in the visual cortex. No sync, no link. The molecular basis of the competition between neurons has been explored in more detail in recent years. The role of NMDA receptors, which you may recall act at 'smart' synapses as coincidence detectors, were an early candidate to mediate the effects on synaptic strength in the visual cortex. Indeed, blocking NMDA receptors chronically during the critical period interferes with the normal emergence of ocular dominance. The coincidence of pre- and postsynaptic firing activates NMDA receptors, and calcium ions enter the postsynaptic cell, where they trigger processes that lead to strengthening of those synapses. Evidence suggests that the postsynaptic cell releases a trophic factor that diffuses back across the synapse to the presynaptic terminal. The presynaptic terminal needs the trophic factor to survive, but evidently can take it up only if it has been recently active. Thus, terminals that do not fire in synchrony miss out on the opportunity, wither, and die. In summary, the main experiments that have led to our current understanding of mechanisms of wiring in the visual cortex are 1) monocular deprivation, which suggests disuse atrophy ('use it or lose it') as an explanation, 2) binocular deprivation, which shows that competition between converging synaptic inputs from the two eyes, not disuse atrophy, is the mechanism, and 3) artificial strabismus suggests that 'cells that fire together wire together'. Subsequent work on the cellular and molecular mechanisms of this plasticity have confirmed the conclusions from these three experiments. APPENDIX I: Phototransduction Following the numbers on the cartoon above: 1. If a photon is absorbed by a molecule of rhodopsin, it can bind to and activate a GTP-binding protein (G protein) called transducin. In fact, before being inactivated by phosphorylation after about 200 msec, it can activate about 500 transducin molecules, which are found in high 17

18 Nervous System Block Visual System - September 19-20, 2016 concentration in the disc membrane surrounding the rhodopsin molecules. Each activated transducin molecule remains active for tens of msec before its bound GTP is converted back to GDP (transducin has intrinsic GTP-ase activity) and in that time 2.. it binds to and activates (by removing an inhibitory subunit) a cgmp phosphodiesterase (PDE) molecule, which exist in large numbers attached to the cytoplasmic surfaces of disc membranes. 3. The cgmp PDE molecule remains active as long as the transducin bound to it remains active and in that time can hydrolyze about 1000 molecules of cgmp, which exists in relatively high concentration in outer segment cytoplasm. Thus, one molecule of rhodopsin activated by one photon of light can result in the hydrolysis of around 500,000 (500 * 1000) molecules of cgmp in the immediate vicinity of the disc. This is a tremendous amplification of the effect of one photon of light and largely accounts for the incredible sensitivity of photoreceptors. This mechanism for the action of light in photoreceptors is analogous to certain hormonally activated 2nd-messenger systems you have already heard about. For example, catecholamines act by binding to specific membrane receptors, which then can bind to G proteins in the membrane, which in turn activate adenylate cyclase to increase the intracellular concentration of c-amp. The increase in c-amp stimulates protein kinases to phosphorylate proteins that can affect ion channels. In photoreceptors, then, light is acting like a hormone, activating receptors (pigment molecules) to bind G proteins and setting in motion a cascade of intracellular events. However, in the case of light, it is a phosphodiesterase that is ultimately activated, lowering the intracellular concentration of a 2nd-messenger (cgmp). Indeed, the molecular structure and conformation of rhodopsin in the disc membrane has been shown to be very similar to that of the catecholamine receptor and other ligand-binding receptors that can couple via G proteins to intracellular 2nd-messenger systems. 4. How does the light-induced decrease in the cgmp concentration in the rod outer segment produce a change in the cell's membrane potential? The plasma membranes of outer segments of visual receptors contain only one kind of channel, a non-selective cation channel. It is gated by cytoplasmic cgmp molecules reversibly bound to it (cgmp is synthesized in the outer segment by the cytoplasmic enzyme guanylate cyclase.) When a single photon of light activates a single rhodopsin molecule in one of the rod discs resulting in the hydrolysis of 500,000 molecules of cytoplasmic cgmp, the cytoplasmic concentration of cgmp in the vicinity of that disc falls sufficiently to cause one or more of the cgmp molecules to unbind from all (several thousand) of the cation channels within a micrometer (um) of that disc, and all of those channels close. The total Na permeability of the rod outer segment is reduced by around 3%, producing a 1 mv hyperpolarization of the rod membrane. This change in membrane potential spreads passively to the synaptic ending of the rod (the distance is less than 50 microns) and closes some Ca channels, which, in turn, reduces the rate of transmitter release. Thus, the effect of light on a vertebrate photoreceptor is to reduce the rate of transmitter release. In the dark the maximum number of cation channels in the outer segment are open. The action of light is to close them, the brighter the light (the more absorbed photons of light), the more of these channels close and the more the cell hyperpolarizes. 18

19 Nervous System Block Visual System - September APPENDIX II: The pupillary light reflex: When bright light shines on the retina, the muscles in the iris contract, reducing the size of the pupil and therefore reducing the amount of light entering the eye. Action potentials in ganglion cells excite CNS neurons that diverge to and excite preganglionic parasympathetic motor neurons on both sides of the brain, which leads to excitation of the muscles in the iris. This reflex is an important diagnostic tool in medicine. Under normal conditions, the pupils of both eyes respond identically, regardless of which eye is stimulated; that is, light in one eye produces constriction of both the stimulated eye (the direct response) and the nonstimulated eye (the consensual response). Comparing the response in the two eyes is often helpful in localizing a lesion (see Problem Set). There are two other important targets of retinal ganglion cell axons. One is the suprachiasmatic nucleus of the hypothalamus, a small group of cell bodies at the base of the diencephalon. The retinohypothalamic pathway is the route by which variation in light levels influences the broad spectrum of visceral functions that are entrained to the day/night cycle. The other target is the superior colliculus, a prominent structure visible on the dorsal surface of the midbrain. The superior colliculus coordinates head and eye movements. 19

20 Nervous System Block Visual System - September 19-20, 2016 APPENDIX III: Visual deficits: Five lesions to the visual system are shown on the brain with the corresponding visual field deficit expected for each eye (visual fields). Gray means the field is intact; black means a deficit. Note these are visual fields (what the eye actually sees ), not retinas. For example, the lesion at B cuts the optic chiasm, so the nasal portion of each retina cannot cross the midline. Thus, each eye only sees the part of the visual field that impinges on the temporal part of the retina, which corresponds to images contralateral to the midline. In this case, for example, the left eye cannot see images that are to the left of the midline (black) but can see those to the right (grey). 20