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Lecture 8

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Health, Aging and Society
Joe Kim

. Lecture 8/9: Vision 
 February 27, 2011   Nearly 1/3 of the brain is devoted to processing visual information   In a showdown between our senses, if our visual sense is giving us information that is in conflict with information from another sense, we tend to bias our trust towards our 
 sense of vision   Ex. Watching a movie 
 o The speaker could be mounted on the wall, but you will tend to perceive the sound as coming from the actors’ mouths 
 The Stimulus: Light Three physical characteristics of light o Amplitude Light waves can vary in two respects  The height of each wave, called the amplitude   The distance between the peaks of successive waves, called the wavelength    Variations in amplitude affect the perception of brightness   Generally, the greater the amplitude of the light wave, the more light is being reflected or emitted by that object and so that object appears 
 brighter or more intense to us o Wavelength 
 Variations in wavelength affect the perception of colour   Wavelength is measured in nanometers   Smaller wavelengths refer to light waves with a higher frequency 
 because there is less distance between successive peaks   Larger wavelengths refer to light waves with a lower frequency The Eye   Humans are only sensitive to a tiny portion of the total range of wavelengths of electromagnetic radiation, the portion called the visible spectrum   The shortest wavelength we can see is around 360 nanometres, which looks violet to us and the longest wavelength we can see is 750 
 nanometres, which is red o Purity   Affects the perception of the saturation or richness of colours   A light that is made up of a single wavelength is said to be pure light and 
 the perceived colour would be described as completely saturated   At the other extreme, we could have a light that is a combination of all 
 the wavelengths   This light would be perceived as white light and would be described as 
 completely desaturated   Light first passes through the curved cornea, which begins the focusing process   The cornea is a transparent window at the front of the eye   The rest of the eye is covered by the white part of the eye called the sclera, a tougher 
 membrane   After the cornea, light passes through the pupil of the eye, which is the round window 
 that you see as a black dot in the middle of your eye   The iris, or the coloured part of your eye, controls the size of the pupil   The iris is basically a band of muscles that is controlled by the brain so that if not enough 
 light is reaching the retina, these muscles cause the pupil to dilate into a larger opening   If too much light is entering the eye then these muscles cause the pupil to constrict into 
 a tiny opening   After going through the pupil, light passes through the lens, a transparent structure that 
 does the final focusing of light on the retina   The Lens o The curvature of the lens causes images to land on the retina upside down and reversed from left to right o However, the final perceived image is a product of brain activity
o Thus, rather than seeing everything upside-­­down and reversed, there is a correction that allows us to see a properly oriented image
o The lens is a flexible piece of tissue, the shape of which can be altered by surrounding muscles, allowing it to focus on objects that are close or far away
o If the object is close, the lens of your eye gets fatter or rounder to produce a clearer image, but if the object is far away, the lens of your eye gets elongated to focus the image on the back of your eye
o This change in shape of the lens to focus on objects that vary in distance is called accommodation The Retina o After travelling through the lens, light passes through the vitreous humour, which is the clear, jelly-­­ like substance that comprises the main chamber inside the eyeball, until it finally lands on the retina, which is the neural tissue that lines the back of the eye The Retina   Where the physical stimulus of light is first translated into neural impulses   Retinal Layer 1: Photoreceptors o The retina is a paper-­­thin sheet that covers the back of the eye and is made up of a complex network of neural cells arranged in three different layers o The layer at the very back of the eye, farthest away from the light, is where the photoreceptors are located o Photoreceptors are cells in the retina that are responsible for translating the physical stimulus of light into a neural signal that the brain can understand o To reach the photoreceptors, light must pass through the other two layers of retinal tissue, which are transparent o The reason for this inside-­­out arrangement in the retina has to do with where the photoreceptors get their nutrients from, which is a layer of cells at the very back of the eye called the retinal pigment epithelium (RPE) o The photoreceptors would die without access to the RPE cells and if the photoreceptors were located at the front of the retina, facing the light, then they would not have access to the RPE that they need to live Photoreceptors: Rods and Cones
o Humans have about 125 M rods, but only 6 M cones o Cones Cones are designed to operate at high light intensities and are used for day vision   The cones provide us with the sensation of colour and provide good visual acuity, or sharpness of detail   Cones become more concentrated toward the fovea, a tiny spot in the middle of the retina that contains exclusively cones   When you want to see something in detail, we move our eyes so that the 
 image falls directly onto the fovea o Rods   Designed to operate at low light intensities and so are used for night vision   Provide no information from which colour can be determined and offer poor visual acuity   No rods in the fovea itself, with increasing concentration in the region just surrounding the fovea   This arrangement makes rods useful for peripheral vision This explains why, when you’re trying to see an object in an environment that is dimly lit, you’re better off looking slightly to one side of the object as opposed to trying to stare right at it When you stare right at it, you’re using your cones, which don’t work in a dimly lit environment Photoreceptors: Response to Light
o Photoreceptors contain a photo pigment, which is a complex molecule that is sensitive to light
o The human eye has four different kinds of photo pigments, one for rods and three for cones, but they all work the same
o When a photon of light is absorbed, it changes the chemical state of the photo pigment and splits into its two components of molecules, which sets off a biochemical chain reaction leading to an electrical current flowing across the membrane o The original light stimulus is now in a currency that can be understood and processed by the brain o Once light has caused a photopigment to split, high energy molecules within the photoreceptor cause the two molecules to recombine so that the photopigment is ready to react to light again o However, there is a brief period of time during which the photopigment will be unable to react to light o Each photoreceptor has thousands of photopigments and the number that are ready to react to light depend on the relative rate at which they are being split and recombined Photoreceptors: Dark Adaptations
o When exposed to very bright light, the rate of splitting of photopigments exceeds the rate at which they are being recombined
o This explains why it takes a few minutes for your eyes to get used to the dark, a phenomenon called dark adaptation
o When you enter a dark place, most of your photopigments, particularly those in the rods, haven’t had time to recombine yet, leaving few that are ready to react to light, causing you to see very little
o But after a few minutes, most of your photopigments will have recombined and be ready to respond to light
o That is the eye becomes adapted to the dark Bipolar and Ganglion Cells
o After the first layer of the retina translates the physical stimulus of light into an electrochemical signal in the photoreceptors, the photoreceptors then send their information to the next layer of cells in the retina, called the bipolar cells, by means of a transmitter substance o In turn, the bipolar cells send their information on to the next layer of cells in the retina, called the ganglion cells Retinal Layers 2 and 3 o The ganglion cells collect information from a larger segment of the retina and the axons of these cells all converge on one point in the eye, called the optic disc and then leave the eye to join the optic nerve, which travels all the way to the brain o Because the optic disc is basically an exit hole in the eye for ganglion axons, this small area contains no photoreceptors at all and so it constitutes our blind spot o We are not normally aware of our blind spot because our brains fill in the gap by blending the surrounding image most of the time   To recap, light enters the eye and must pass through the ganglion cells, bipolar cells and strike the photoreceptors on the retina at the very back of the eye   At that point the light is converted into a neural signal that is sent from the photoreceptors to the bipolar cells and then on to the ganglion cells, whose axons make up the optic nerve   Processing in the Retina o There are also cells in the retina that allow areas within a retinal layer to communicate with each other, called horizontal cells and amacrine cells
o These cells allow information from adjacent photoreceptors to combine
o This means that some amount of visual processing is done in the retina, before the signal is sent on to the brain Receptive Field in Retina o Think of the photoreceptors in the retina has being divided up into specific groups and the information from each group getting assimilated into one signal that affects the ganglion cell down the line o In the fovea, the photoreceptor group for a particular ganglion cell may only contain one cone, which means the ganglion cell is representing a very small area of the image o Since each cone in the fovea has a direct link with the brain, a lot of the detail is preserved and more visual acuity occurs in the fovea o But more often, the input from many rods and cones is combined into one neural signal for one retinal ganglion cell o These groups get larger as we move toward the periphery of the eye, which is one reason why our visual acuity is so low for peripheral vision o The collection of rods and cones in the retina that, when stimulated, affects the firing of a particular ganglion cell is called the receptive field of that retinal ganglion cell o These receptive fields in the retina come in a variety of shapes and sizes, but most are basically doughnut shaped, such that the light falling in the centre of the doughnut will either excite or inhibit the cell ad light falling in the surrounding part of the doughnut will have the opposite effect on the cell o Excitation and inhibition of the cell is determined by the rate at which that cell fires compared to baseline or the rate at which the cell would fire normally without any light signals o A cell would be excited if the rate of firing of that cell increased compared to baseline and it would be inhibited if that rate of firing decreased compared to baseline o Ex. A receptive field on the retinal surface that has a doughnut shape with the center being excitatory and the surrounding being inhibitory This means that if light struck the center of the receptive field, this would cause an increase in the firing rate of the ganglion cell   If the light struck the surrounding of the receptive field, this would cause a decrease in the firing rate of the ganglion cell   If light covered both the center and surrounding of the receptive field, these two effects would basically cancel each other out and the cell would fire at the same rate that it does at baseline, when no light is available   Either way, when a receptive field of a ganglion cell is stimulated, that ganglion cell sends signals towards the brain Lateral
o Retinal cells can also affect signalling of adjacent retinal cells through lateral antagonism or lateral inhibition
o This is done through the horizontal cells, which are activated by the photoreceptors and also through the amacrine cells which are activated by the bipolar cells
o Whenever a retinal cell is stimulated by light falling on its receptive field, that cell sends signals onto the brain, but it also sends messages sideways to neighbouring cells that inhibit their activation
o The perceptual result of this kind of mechanism is that edges of objects are easier to detect
o Ex.Imaginewehave4cells,A,B,CandD Cell A, B and C are receiving intense stimulation from the same patch of bright light, whereas Cell D is receiving moderate stimulation from a dark grey patch of light   Thus, cell C is on the edge of the bright and grey light and with lateral inhibition, this cell ends up sending more stimulation to the brain than cell B, even though both cells receive the exact same input   So cells A, B and C are strongly stimulated and since they are also neighbours, they are inhibiting each other as well   Cell D is only moderately stimulated by the dark grey patch and it sends less inhibition to its neighbour cell C than cells A, B and C do to each other   As a result, cell B receives a lot of inhibition from the intense stimulation of both its neighbours, whereas cell C only receives strong inhibition from one of its neighbours   Because of this, cell C sends out a stronger signal to the brain than cell B does, even though cell C and B are receiving the same input from the world Inhibition Visual Pathways The perceptual result for us is that edges look more distinct Visual Fields and Hemispheres
o Recall that the right and left halves of our visual field are processed by the contralateral side of our brain
o Visual input from our right visual field travels along the optic nerve to the left hemisphere and visual input from the left visual field travels along the optic nerve to the right hemisphere
o A visual field receives information from both eyes and so each hemisphere receives information from both eyes
o Before reaching their respective hemispheres, the axons from the inner region of each retina (region of retina closest to nose) have to cross over to the opposite hemisphere
o The point at which the optic nerves from the inside half of each eye cross over to the opposite hemisphere is called the optic chiasm Two Visual Pathways o After the optic chiasm, the information from each visual field arrives in the opposite hemisphere, at which point the optic nerve fibres split and travel along two pathways o Most of the retinal or ganglion cell axons travel along the main pathway and synapse in the lateral geniculate nucleus (LGN), which is part of the thalamus that receives visual information o After being processed here, the visual signals are sent to areas in the occipital lobe that make up the primary visual cortex o A smaller portion of the axons from the retinas takes a detour to an area in the midbrain called the superior colliculus, after which information is sent upwards to the thalamus and on to the occipital lobe or downward to structures in the brainstem o This smaller, secondary pathway seems to deal with coordinating visual input with information coming in from other senses as well as localizing objects in space through head and eye movements and helping to guide those movements   Main Pathway: Two Subdivisions
o The magnocellular pathway is specialized to process movement information o The parvocellular pathway deals specifically with colour and form information  
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