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Chapter 4a - Vision Video Lecture Psych 1XX3

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Joe Kim

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Video Lecture Psych 1XX3 Chapter 4: Vision Vision I The Stimulus: Light  Thee physical characteristics of light that translate into the three psychological perceptions of the visual world o Amplitude o Wavelength o Purity  Amplitude and Brightness o Light travels as a wave that moves at about 300 000 km/sec o Amplitude: The height of each wave o The greater the amplitude of the light wave, the more light is being reflected or emitted by that object  greater amplitude = appears brighter  Wavelength and Color o Wavelength: The distance between the peaks of successive waves o Measured in nanometers (millionths of a millimeter) o Smaller wavelengths = higher frequency  because there is less distance between successive peaks o Larger wavelengths = lower frequency o Visual Spectrum – the small portion of the total range of wavelengths of electromagnetic radiation that humans are sensitive to  Shortest wavelength that we can see is around 360 nanometers (looks violet to us)  Longest wavelength that we can see is around 750 nanometers (appears red) o Bees can see wavelengths shorter than 360 nm in the ultra violet spectrum – may perceive colors differently and see differences in colors that we see as the same o Snakes can see wavelengths longer than 750 nm in the infrared spectrum – allows them to find prey in the dark by being able to see the body heat that is emitted by the pray  Purity and Saturation o Pure Light – a light that is made up of a single wavelength; the perceived color would be considered completely saturated o Completely Desaturated – light that is a combination of all the wavelengths; perceived as white o Most of the colors we see in our everyday light are a mixture of wavelengths Video Lecture Psych 1XX3 The Eye  Sclera o White part of the eye that is a tougher membrane (white part) and covers the eye  Cornea o Light first passes through the curved cornea, which begins the focusing process o Transparent window at the front of the eye  Pupil o Light passes through the pupil after the cornea o Round window (seen as a black dot in the middle of the eye)  Iris o A band of muscles that controls the size of the pupil o Controlled by the brain  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, iris causes the pupil t constrict into a tiny opening  Lens o Light passes through lens after going through the pupil o Transparent structure that does the final focusing of light on the retina o Flexible tissue; shape can be altered by surrounding muscles allowing it to focus on objects that are close or far away  The final perceived image is a product of brain activity o Curvature of the lends causes images to land on the retina upside-down and reversed from left to right – the brain processes it allowing us to see a properly oriented image  Accommodation – change in the shape of the lens to focus on objects that vary in distance o If object is close – lens gets rounder to produce a clear image o If the object is far away – lends is elongated to focus the image on the back of the eye  Vitreous Humor o Light passed through the vitreous humor after travelling through the lens o Clear jelly-like substance that comprises the main chamber inside the eyeball  Retina o Neural tissue that lines the lack of the eye Video Lecture Psych 1XX3 The Retina  Physical stimulus of light is first translated into neural impulses in the retina  Paper-thin sheet that covers the back of the eye  Made up of a complex network of neural cells arranged in three different layers  Photoreceptors o Cells in the retina that are responsible for translating the physical stimulus of light into a neural signal that the brain can understand o Located in the layer at the back of the eye o Light must pass through the other two layers of retinal tissue which are transparent in order to reach the photoreceptors o Retinal Pigment Epithelium (RPE) – at the back of the eye and give nutrients to photoreceptors o Two kinds of photoreceptors (named for their shape)  Rods  Plentiful; humans have about 125 million  Designed to operate in low light intensities  Used for night vision  Provide no information from which color can be determined  Offer poor visual acuity  No rods in the fovea, but has increasing concentration in the region just surrounding the fovea  Useful for peripheral vision  When you’re trying to see something in dim light – better off looking slightly to one side as opposed to directly at it  Cones  Humans have about 6 million  Designed to operate at high light intensities Video Lecture Psych 1XX3  Used for day vision  Provide the sensation of color  Provide good visual acuity (sharpness of detail)  Become more concentrated toward the fovea – a tiny spot in the middle of the retina that contains exclusively cones o Photo Pigment – complex molecule that is sensitive to light, located in photoreceptors  Human eye has 4 different kinds; one for rods, three for cones  When a photon of light is absorbed, it changes the chemical state of the photo pigment and splits into its two component molecules which sets off a biochemical chain reaction leading to an electrical current flowing across the membrane  The original light stimulus is now in a currency that can be understood and processed by the brain  Once light has causes a photo pigment to split, high energy molecules within the photo receptor cause the two molecules to recombine so that the photo pigment is ready to react to light again  There is a brief period of time during which the photo pigment will not be able to react to light  The number of photo pigments that are ready to react to light depend on the relative rate at which they are being split and recombined  Dark Adaptation  When exposed to very bright light, the rate of splitting of photo pigments exceeds the rate at which they are being recombined  Reason why it takes a few minutes for eyes to get used to the dark Bipolar and Ganglion Cells  Bipolar Cells – the next layer of cells in the retina  Photo receptors send their information to bipolar cells by means of a transmitter substance  Bipolar cells send their information to the next layer of cells – ganglion cells Video Lecture Psych 1XX3  Ganglion Cells o Collect information from a larger segment of the retina, and the axons of these cell converge on one point in the eye called the optic disc the leave the eye to join the optic nerve, which travel all the way to the brain o Optic Disk – an exit hole in the eye for ganglion axon, contains no photoreceptors, constitutes our blind spot  Recap o Light enters the eye and passes through the ganglion cells, bipolar cells and photoreceptors on the retina at the back of the eye o 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 axon make up the optic nerve Processing in the Retina  Horizontal cells and Amacrine cells – cells in the retina that allow areas within a retinal layer to communicate with each other  These cells allow information from adjacent photoreceptors to combine o We know this because the information from over 130 million rods and cones in the retina converge to travel along only 1 million axons in the optic nerve Video Lecture Psych 1XX3 o Some amount of visual processing is done in the retina before the signal is sent to the brain Receptive Field In The Retina  Think of the photo receptors in the retina as 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 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 are visual acuity is so low for peripheral vision o Receptive Field (of a retinal ganglion cell)– the collection of rods and cones in the retina that when stimulated affects the firing of a particular ganglion cell  These receptive fields in the retina come in a variety of shapes and sizes,  Most of them are basically donut shaped – light falling in the center of the donut will either excite or inhibit the cell and light falling in the surround part of the donut will have the opposite effect on the cell  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 Video Lecture Psych 1XX3 o A cell would be excited if the rate of firing of that cell increased compared to baseline o A cell would be inhibited if the rate of firing decreased compared to baseline  When a receptive field of a ganglion cell is stimulated, that ganglion cell sends signals towards the brain o Eg/ Donut shaped retina field – center excitory, surrounding inhibitory  If light struck the excitory center – increase in the firing rate of ganglion cell  If light stuck the inhibitory surrounding – decrease in firing rate of ganglion cell  If light struck entire area – cell would fire at baseline; same as when no light is available Lateral Inhibition  Retinal cells can affect signaling of adjacent retinal cells through Lateral Antagonism or Lateral Inhibition o Done through the horizontal cells, which are activated by the photo receptors, 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 neighboring cells that inhibit their activation o The perceptual result of this kind of physiological mechanism is that the edges of objects are easier to detect  Mach Bands – strips of grey that range from dark grey to light grey o Within each strip, the color is constant, yet it appears as though a darker band exists right at the border of each strip o Can be explained by lateral inhibition and this is one way in which the retina enhances our ability to detect edges in visual images  Eg/ Explains Mach Bands – Imagine we have 4 cells A, B, C and D o Cell A, B and C are receiving intense stimulation from the same patch of bright light o Cell D is receiving moderate stimulation from a dark grey patch of light o Cell C is on the edge of the bright and grey light – with lateral inhibition, Cell C ends up sending more stimulation to the brain than Cell B, even though both cells receive the exact same input o Cells A, B and C are strongly stimulated and are inhibiting each other because they are neighbors Video Lecture Psych 1XX3 o Cell D is only moderately stimulated by the dark grey patch – it sends less inhibition to its Cell C, than do Cells A, B and C do to each other o Cell B receives a lot of inhibition from the intense stimulation of both its neighbors o Cell C only receives strong inhibition from one of its neighbors o 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 Vision II  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 o Visual input from the left visual field travels along the optic nerve to the right hemisphere o Visual field receives information from both eyes, each hemisphere receives information from both eyes  Optic Chiasm – the point at which optic nerves from the inside half of each eye cross over to the opposite hemisphere o Before reaching their respective hemispheres, the axons from the inner region of each retina have to cross over to the opposite hemisphere  The optic nerve fibers split and travel along two pathways when the information arrives in the opposite hemispheres after the optic chiasm Video Lecture Psych 1XX3  Most of the retinal or ganglion cell axons travel along the main pathway and synapse in the lateral geniculate nucleus (LGN), which is
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