{"id":4436,"date":"2018-10-07T00:28:42","date_gmt":"2018-10-07T04:28:42","guid":{"rendered":"https:\/\/www.amyork.ca\/academic\/zz\/?p=4436"},"modified":"2019-05-25T23:30:28","modified_gmt":"2019-05-26T03:30:28","slug":"the-first-steps-in-vision-from-light-to-neural-signals","status":"publish","type":"post","link":"https:\/\/www.amyork.ca\/academic\/zz\/sensation-and-perception\/the-first-steps-in-vision-from-light-to-neural-signals\/","title":{"rendered":"The first steps in vision, from light to neural signals"},"content":{"rendered":"<p><em>A little light physics<\/em><\/p>\n<ul>\n<li>Light: electromagnetic radiation o Wave: an oscillation that travels through a medium by transferring energy from one particle or point to another without causing any permanent displacement of the medium \u2192 light is made up of waves when it moves around the world (400-700 nm)\n<ul>\n<li>Photon: a quantum of visible light or other form of electromagnetic radiation demonstrating both particle and wave properties \u2192 light is made up of photons when it is absorbed<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p><em>\u00a0<\/em><em>Eyes that see light<\/em><\/p>\n<ul>\n<li>Eye: cornea \u2013 aqueous humor \u2013 crystalline lens \u2013 pupil \u2013 iris \u2013 vitreous humor &#8211; retina<\/li>\n<li>Light is transmitted through the cornea, due to its transparency. The cornea is transparent because it is made of a highly ordered arrangement of fibres (with a rich supply of transparent sensory nerve endings, that are there to force the eye to close and produce tears if the cornea is scratched) and does not contain blood vessels or blood<\/li>\n<li>Aqueous humor: fluid derived from blood that fills the space immediately behind the cornea, and supplies oxygen and nutrition to and removes waste from the cornea and the crystalline lens<\/li>\n<li>The shape of the crystalline lens is controlled by the ciliary muscle; when the ciliary muscle is relaxed the lens is relatively flat and the eye will be focused on very distant objects; when the ciliary muscle is contracted the lens is bulged and the eye will be focused on objects nearby<\/li>\n<li>The iris is the coloured part of the eye and consists of a muscular diaphragm surrounding the pupil and regulate the light entering the eye by expanding or contracting the pupil<\/li>\n<li>Photic sneeze reflex: sneezing in response to being exposed to the sun after being in the dark o Aristotle: due to the warmth of the sun\n<ul>\n<li>Bacon: the sun light makes the eyes water and that moisture then seeps into and irritate the nose, causing you to sneeze<\/li>\n<li>Current: due to crosses wires in the brain<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p><em>Focusing light on the retina<\/em><\/p>\n<ul>\n<li>Accommodation of the lens is needed to produce a sharp image on the retina, the lens can change the refractive power by changing its shape (whereas the cornea, aqueous and vitreous humor have a vast refractive power)<\/li>\n<li>Presbyopia: the loss of near vision because of insufficient accommodation (you lose about 1 diopter of accommodation every 5 years (100\/dioptre=cm)), the lens becomes harder and the capsule that encircles the lens loses elasticity<\/li>\n<li>Cataracts: caused by irregularity of the crystallines and lead to loss of transparency; infer with vision because they absorb and scatter more light than the normal lens does<\/li>\n<li>Emmetropia: the condition in which there is no refractive error, because the refractive power of the eye is perfectly matched to the length of the eyeball<\/li>\n<li>Myopia: nearsightedness, light entering the eye is focused in front of the retina (the eyeball is too long for the optics) and distant objects cannot be seen sharply; can be corrected with negative lenses<\/li>\n<li>Hyperopia: farsightedness, light entering the eye is focused behind the retina (the eyeball is too short for the optics) and nearby objects cannot be seen sharply; can be corrected with positive lenses<\/li>\n<li>Astigmatism: a visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea<\/li>\n<\/ul>\n<p><em>The retina<\/em><\/p>\n<ul>\n<li>Seeing begins at the retina, where light energy is transduced into neural energy that can be interpreted in the brain<\/li>\n<li>Optic disc\/blind spot: the point in the retina where the arteries and veins that feed the retina enter the eye and where the axons of the ganglion cells leave the eye via the optic nerve<\/li>\n<li>Transduction of light: photoreceptors \u2013 bipolar cells, horizontal cells, amacrine cells \u2013 ganglion cells \u2013 brain (figure 2.7 page 37)<\/li>\n<\/ul>\n<p><em>Retinal information processing<\/em><\/p>\n<ul>\n<li>Photoreceptors: rods (absent from the fovea, density increases to a peak at about 20 degrees and then declines again) and cones (most concentrated in the centre of the fovea, and density drops dramatically with the distance from the fovea increasing) o Outer segment: adjacent to the epithelium, stores visual pigments and incorporates them into the membrane\n<ul>\n<li>Inner segment: makes visual pigments Visual pigments:<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p><u>Consist of<\/u>:<\/p>\n<ul>\n<li>A protein, the structure of which determines which wavelengths of light they absorb<\/li>\n<li>A chromophore, which captures light photons <u>Four types<\/u>:<\/li>\n<li>Rhodopsin: found in rods<\/li>\n<li>Three other pigments in the cones which respond to either long, medium or short wavelengths<\/li>\n<li>Melanopsin: sensitive to the ambient light level, sends its signals to the suprachiasmatic nucleus (SCN), which regulate the 24h pattern of behaviour and physiology<\/li>\n<\/ul>\n<ul>\n<li>Synaptic terminal<\/li>\n<\/ul>\n<ul>\n<li>When a photon makes its way into the outer segment of a rod and is absorbed by a molecule of rhodopsin, it transfers its energy to the chromophore portion of the visual pigment molecule (<u>photo-activation<\/u>). This initiates in the closing of channels in the cell membrane that normally allows ions to flow into the rod outer segment. Closing these channels alters the balance of electrical current between the inside and outside of the rod outer segment, making the inside of the cell more negatively charged (<u>hyperpolarization<\/u>). Hyperpolarization closes calcium channels at the synaptic terminal, thereby reducing the concentration of free calcium inside the cells. The lowering of calcium, in turn, reduces the concentration of neurotransmitter (glutamate) molecules at the synaptic terminals, and this change signals to the bipolar cells that the rod has captured a photon.<\/li>\n<li>Photoreceptors pass their info on to bipolar cells via <u>graded potentials<\/u>, the amount of glutamate present in the photoreceptor-bipolar cell synapse at any time is inversely proportional to the number of photons being absorbed by the photoreceptor<\/li>\n<li>Rods: sensitive to much lower levels of light and so play an essential role in semidarkness, or when you\u2019re trying to view a fairly dim stimulus; they are colour-blind due to the fact that they all have the same photopigment<\/li>\n<li>Cones: less sensitive, need much more incoming light to operate; are sensitive to differences in wavelength and therefor provide the basis for colour vision, about 5-10% S-cones and they are absent from the centre of the fovea, there are more L-cones than M-cones<\/li>\n<\/ul>\n<p><em>The man who could not read<\/em><\/p>\n<ul>\n<li>Age-related macular degeneration (AMD): a disease associated with aging that affects the <u>macula<\/u>, the central part of the retina that has the highest concentration of cones. AMD gradually destroys sharp central vision, making it difficult to read, drive and recognize faces.<\/li>\n<\/ul>\n<p>The result of this disease is a <u>scotoma<\/u>, a blind spot in the visual field o Wet AMD: abnormal blood vessels behind the retina start to grow under the macula, and because they are fragile, they leak blood and fluid, which causes the macula to rise and results in rapid loss of central vision<\/p>\n<ul>\n<li>Dry AMD: occurs when the macula cones degenerate<\/li>\n<\/ul>\n<p><em>Lateral inhibition through horizontal and amacrine cells<\/em><\/p>\n<ul>\n<li>Horizontal cells: run perpendicular (90 degree angle) to the photoreceptor cells; specialized retinal cell that contacts both photoreceptor and bipolar cells o Lateral inhibition: enables the signals that reach retinal ganglion cells to be based on differences in activation between nearby photoreceptors<\/li>\n<li>Amacrine cells: run perpendicular (90 degree angle) to the photoreceptor cells in the inner layers of the retina, where they receive inputs from both bipolar cells and other amacrine cells and sends signals to bipolar, amacrine, and retinal ganglion cells.<\/li>\n<\/ul>\n<p><em>Convergence and divergence of info via bipolar cells<\/em><\/p>\n<ul>\n<li>Lateral pathway: horizontal and amacrine cells<\/li>\n<li>Vertical pathway: photoreceptors, bipolar cells and ganglion cells<\/li>\n<li>Bipolar cell: a retinal cell that synapses with either rods or cones and with horizontal cells, and then passes the signals on to the ganglion cells o Diffuse bipolar cell: a bipolar retinal cell whose processes are spread out to receive input from multiple cones in the retinal periphery, or multiple rods. A diffuse bipolar cell may fire at the same rate in response to a single spot of bright light or several spots of dim light, so the ganglion cell listening to the diffuse bipolar cell will be unable to tell which pattern of light is present. The high degree of convergence in the retinal periphery ensures high sensitivity to light but poor acuity.<\/li>\n<li>Send their signals to <u>M ganglion cells<\/u>: feed the magnocellular layer of the lateral geniculate nucleus (LGN)\n<ul>\n<li>Midget bipolar cells: a small bipolar cell in the central retina that receives input from a single cone. The fact that one-to-one pathways between the cones and ganglions exist only in the fovea accounts for why images are seen most clearly when they fall on this part of the retina. The low degree of convergence in the fovea ensures high acuity but poor sensitivity to light.\n<ul>\n<li>Send their signals to <u>P ganglion cells<\/u>: feed the parvocellular layer of the LGN &#8211; Each foveal cone contacts two bipolar cells:<\/li>\n<\/ul>\n<\/li>\n<li>ON bipolar cell: a bipolar cell that responds to an increase in light captured by cones o OFF bipolar cell: a bipolar cell that responds to a decrease in light captured by cones <em>Communicating to the brain via ganglion cells<\/em><\/li>\n<\/ul>\n<\/li>\n<li>Ganglion cells: o P cells: relatively small receptive fields; provide a finer resolution, if there is enough light to operate; respond with sustained firing while light shines on their excitatory regions \u2192 provide info mainly about the contrast in the retinal image\n<ul>\n<li>M cells: relatively large receptive fields; more sensitive and therefor better able to detect visual stimuli, under low lightning conditions; responds with a brief burst of impulses when the light is turned on, and then quickly returns to its spontaneous rate even if the light remains lit \u2192 signal info about how the image changes over time<\/li>\n<li>Koniocellular cells: project to the koniocellular layers, located between the parvocellular and magnocellular layers of the LGN\n<ul>\n<li>The ones with input from S-cones, may be part of the primordial blue-yellow pathway, other ganglion cells correspond to non-blue koniocellular cells<\/li>\n<\/ul>\n<\/li>\n<li>Receptive field: the region on the retina in which visual stimuli influence a neuron\u2019s firing rate, either excitatory or inhibitory o Kuffler: ganglion cells have concentric receptive fields (figure 2.14 page 45)\n<ul>\n<li>ON-center cell: a cell that increases its firing rate in response to an increase in light intensity in its receptive-field center, and decreases its firing rate in response to an increase in light intensity in its surround<\/li>\n<li>OFF-center cell: a cell that decreases its firing rate in response to an increase in light intensity in its receptive-field center, and increases its firing rate in response to an increase in light intensity in its surround -Center-surround organization:<\/li>\n<\/ul>\n<\/li>\n<li>Ganglion cells will respond best to spots of a particular size, and by doing so they act as a filter by editing the info send to the brain<\/li>\n<li>Ganglion cells are most sensitive to differences in the intensity of the light in the center and in the surround, and they are relatively unaffected by the average intensity of light<\/li>\n<\/ul>\n<\/li>\n<li>Mach bands: the vivid perception of a dark and a light stripe come from operations performed by the visual nervous system. In fact the Mach pattern is initially constant, increases smoothly over a short distance and is steady again at a new level. Figure 2.15 p46 o Neuronal explanation: the receptive field of the ON-center ganglion cell in the left area receives less light than the receptive field of the ON-center ganglion cell in the right area. Therefor the brain will conclude that the left area is darker than the right area. However if the center of the receptive field is in the left area, and part of the surround is in the right area, the contrast between center and surround is larger and therefor the brain will interpret this info as the point being darker, creating the illusionary dark stripe. Likewise if the center of the receptive field is in the right area, and part of the surround is in the left area, the contrast between centre and surround is larger and therefor the brain will interpret this info the point as being lighter, creating the illusionary light stripe. Figure 2.16 page 47<\/li>\n<\/ul>\n<p><em>\u00a0<\/em><\/p>\n<p><em>Whistling in the dark: dark and light adaptation<\/em><\/p>\n<ul>\n<li>Pupil dilation helps to adapt to light and dark conditions, but only to a small amount<\/li>\n<li>Photopigment is highly available in dim lighting conditions, and rods and cones respond to as many photons as they can. After a photopigment molecule is bleached (used to detect a photon), the molecule must be regenerated before it can be used again to absorb another photon. As the overall light increases, the number of photons starts to overwhelm the system: photopigment molecules cannot be regenerated fast enough to detect all the photons hitting the photoreceptors.<\/li>\n<li>Rods provide exquisite sensitivity at low light levels, but they become overwhelmed when the background light becomes moderately bright, leading to a loss in info quality. Cones are much less sensitive, but their operating range is much larger<\/li>\n<li>Ganglion cells encode the pattern of relatively light and relatively dark areas in the retinal image, creating an pattern of illumination which ignores whatever variation in overall light level is left over.<\/li>\n<\/ul>\n<p><em>The man who could not see the stars<\/em><\/p>\n<ul>\n<li>Retinitis pigmentosa (RP): a progressive degeneration of the retina that affects night vision and peripheral vision. Rods are affected before the cones, therefor people with this disease first notice vision problems in their peripheral vision and under low lighting conditions.<\/li>\n<\/ul>\n<p>Eventually visual loss spreads toward the fovea, often leading to total blindness o Bone spicules: clumps of pigment in the fundus of a RP patient<\/p>\n","protected":false},"excerpt":{"rendered":"<p>A little light physics Light: electromagnetic radiation o Wave: an oscillation that travels through a medium by transferring energy from one particle or point to another without causing any&hellip; <a class=\"continue\" href=\"https:\/\/www.amyork.ca\/academic\/zz\/sensation-and-perception\/the-first-steps-in-vision-from-light-to-neural-signals\/\">Continue Reading<span> The first steps in vision, from light to neural signals<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[112],"tags":[],"_links":{"self":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts\/4436"}],"collection":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/comments?post=4436"}],"version-history":[{"count":1,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts\/4436\/revisions"}],"predecessor-version":[{"id":4843,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts\/4436\/revisions\/4843"}],"wp:attachment":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/media?parent=4436"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/categories?post=4436"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/tags?post=4436"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}