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Sensation and Perception Lectures Midterm 2.doc

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PSYC 2390
Lana Trick

II. Parts of the eye: Descriptions/functions/ associated disorders or problems for each part. Rods and cones are photoreceptors because they receive the light they contain photo pigments (colors) • Rods: have rhodopsin (bad at absorbing red because it is reflected back into the eyes) can use red light and not cause yourself to bleach, bleaching retinal and Opsin. • Cones: 3 different kinds with 3 different photo pigments  they absorb different kinds of light better (1 absorbs long wavelengths [red light] best; 1 absorbs medium wavelengths best [green]; 1 absorbs short wavelengths best [blue]) A. The retina 1. Disorders/problems of the Retina a. Detached retina • Get hit hard in the head – retina actually rips (lifts off back of the eye) so you end up being blind for that portion. There’s no way that the light coming in can focus and no cells can register it. Put in a cold/heat probe causing the cells to seal up (fuse together again) but there will be scar tissue and you can’t see as well as you used to. b. Diabetic retinopathy: • Diabetes  type one (can’t make insulin – early in life); type two (insensitive to insulin – later in life – lifestyle [eating junk food, not taking care of yourself]) insulin helps metabolize sugar people who have it, have poor blood circulation (might result in amputation) and have problems with high blood pressure, can develop cataracts retina is full of blood vessels that nourish the retina you have some blood vessels and the retina is hungry for blood (has lots of blood) and circulation problems in retina are serious – the blood vessels (capillaries) begin to swell up and leak and when they leak they can put drops of blood in the vitreous humour (jelly in eye) floaters are little chunks of tissue and you can see their shadows through your eye and with a leaky vessel this happens more often (mild cases) as case becomes more severe, the blood vessels aren’t giving the retina enough blood so the retina tries to grow new blood vessels (neovascularization) but they are abnormal blood vessels which contribute to the problem which can bleed into the eye and cause detached ⇒ Retina: can result in blindness laser Photocoagulation: shine laser into your eyes to kill abnormal blood vessels pan retinal photocoagulation: have a wide scattering of laser shots (2000+ tiny burns created in the retina) to deliberately destroy parts of the retina so it doesn’t need so much blood (need for blood causing abnormal blood vessels) c. Macular degeneration • Irritating disorder because it causes loss of cones in the fovea. The macula is a 5mm area in the center of the fovea which causes objects to be blurry. o Dry form: in the area around the fovea, you have the development of drusen and these are white-ish/yellowish clumps that grow in the macula which causes this disorder – basically function to destroy the cones o Wet form: drusen + neovascularization – cones are dying and developing abnormal leaky blood vessels – can use lasers to seal off abnormal vessels, use medications • Risk factors: related to diet (make sure you have lots of fruits and veggies that produce antioxidants and protect eyes from excessive light sunglasses), There is a genetic component as well (Amsler Grid) d. Retinitis Pigmentosa (Hereditary retinal disorder) • Pigment epithelium (part that contains the enzymes to put photo pigments back together after they bleach) starts showing its effect in adolescence attacks the rods and it does so from the outside in (lose periphery until you only have tunnel vision) e. Glaucoma • Retinal ganglion is being damaged there is fluid being created that goes into the aqueous humour (watery fluid) which comes in to eye and nourishes the cornea and the lens when you have this disorder, this fluid gets under higher and higher pressure because it’s coming in but can’t get out (the vent that lets in the fluid gets plugged up) – the fluid pressure begins to push on the rest of the eye and ultimately pushes of the axons on the retinal ganglion cell (goes out into the optic nerve) and this crushes the cells o Open angle glaucoma: vent gets plugged up, associated with older people and high blood pressure. The problem develops gradually which gives you blind spots (wherever there are dead ganglion cells) and this is so slow that the person may not notice that they’re losing their vision o Closed angle: iris flops over and is right on the lens (pupillary block) so the fluid pressure builds up very fast (tiny area to fill) which causes blindness quickly, blurred vision, nausea, headaches, severe eye pain *in the morning– occurs suddenly cut a hole in the iris so fluid can get out (iridectamy)  Risk factors: farsighted (hyperopia axial) – foreshortened eyeball, oriental – eyeball shape Close up on the retina I. How does transduction work?: on the retina there are specific cells that have action potentials that deal with the image focused on the retina II. After-images and photoreceptors: *stare at an image for a long period of time, when it is removed you still see the image* A.What are after-images? (Demonstration) *something floating in front of your eyes – part of “after effects” which is a case where you look at one thing and it effects the way you see another thing (caused you to see something after image is removed) B. Why does this particular after-image occur? When you see something that is white, the photoreceptors use up a lot of photochemical (a lot of action potentials) but for the parts of the eye that were on black there isn’t a lot of light in black so at these points, the cells aren’t having much photopigment used up (lack of light) so not a lot of action potentials Two bunches of cells: going crazy, just relaxing when stimulus is removed, the parts that looked at black seem bright, the parts that looked white seem dark because the parts of the retina responsible for white are fatigued whereas the black parts are not. We compare the activity of cells side by side – white areas have an after image can’t produce as many action potentials because they’re tired, the black ones can produce a lot when you see something is comparing things side by side (one side can’t react as much) III. Two ways that collector cells summarize information ← ← The retina summarizes information through the collector cells, because there are not a lot of ganglion cells. • Horizontal Cells • Bipolar Cells • Amacrine Cells ← The rods and cones just see the absence or presence of light, and pick up the color and can have action potentials. ← Inhibitory neurotranthitter reduces rate of firing by 1/10 the number of AP/ sec that isolated neuron receives. A. Spatial summation (Lateral summation) 126 million photoreceptors (rods and cones)  120 million rods; 6 million cones, 1 million retinal ganglion retina has to summarize information so we have enough that we can see and coordinate our bodies so we don’t crash into things 1. Effects of spatial summation Summing up activity of cells that are side by side Convergence: a collector cell with several other excitatory cells feeding into the collector which has an effect of adding up the charge rods are sensitive because they add up the activity of lots of individual rods whereas the foveal cones which have a linear circuit (each cone has its own ganglion) a lot of summary is side by side in space (lateral = side) so you’re adding together activity of cells side by side – makes you more sensitive but if you’re adding things together you can’t tell about the fine detail (all added together) – know there’s light but can’t see edges B. Lateral inhibition 1. Side by side cells inhibit one another and this has the effect of highlighting edges of objects (from white to dark – changes in brightness) a. Example: |  white to dark they want to find when there are differences in brightness (tells you about objects in the world) – accomplished by the horizontal, bipolar, and amacrine cells – hooking up cells that are side by side Example: Horseshoe crab (limulus) has large axons (invertebrate) so it’s easy to see what’s going on. It has ommatidium (compound lens combination of a lens and retinal cell) which are held together by the lateral plexus (connects and causes neurons that are side by side to inhibit each other) 2. Effects of lateral inhibition Cells that are side by side inhibit each other horizontal, bipolar, amacrine cells summarizing information can’t directly see what’s out there so we interpret rods and cones signal presence absence of light (black doesn’t give you a lot of light so they don’t absorb much light and they don’t have many action potentials; light gives you lots of light so lots of action potentials) sometimes part of what you see is based on how fatigued your cells are (after image) you tire out your cells looking at something on white . C. Illusions that are the consequence of lateral inhibition a. Mach bands: Perceptual illusion produced when you have a black area beside a white area side by side the white seems brighter than bright and black seems darker than dark because white received more AP/sec and black receives less b. Hermann Grid: An illusion that causes the intersections between the lines to have darker space when in reality there isn’t any dark spots floating in the intersections. This is caused by lateral inhibition - photoreceptors and collector cells responsible for specific intersections. All 4 sides are inhibiting them by lot because there is white all around the space. A spot half way between the black boxes get inhibition from top and bottom but not from the sides (not getting as much inhibitory transmitters) so these two spots are being compared causing the spot to look darker c. Simultaneous Contrast: Makes something (grey box) look darker on a white background than the same image on a black background. Trying to highlight edges to see images (differences in brightness/discontinuities in brightness highlight edges) IV. Introduction to receptive fields A. On response/Off response • On Response (+): The cell has more AP/ sec than spontaneous activity o First you have darkness (few AP), then light (a lot of action potentials), than darkness again (significant decrease in AP). • Off Response (-): The cells have less AP/ sec than at spontaneous activity. o First darkness (moderate amount of AP/sec), then light (there is fewer AP), and finishes with darkness (burst of activity in terms of AP/sec. • Spontaneous activity is important because you are always comparing AP activity to the baseline. Because we have inhibitory neurotransmitters and allows us to send more information. o Example: 20 AP/sec  On response (light)= 25 ap/sec  Off Response (light)= 15 ap/sec B. Receptive fields: are in the retinal ganglia that the retinal cells produce a response to. Higher levels cells are collector cells that have receptive fields. The cell will either have an on/off response. In the retinal ganglion cells there are complicated receptor fields are all related to how the retina is summarizing light. 1. Characteristics a. Circular: they will respond to little circular areas. Responds to little circular areas of the visual field responds to opposite ways in the center than in the surrounding area (light in the center will produce more AP/second than spontaneous activity with dark on outside; dark on inside and light on outside = less AP than spontaneous) b. Centre surround antagonism: the cell responds in opposite ways as it does in the middle as in the surroundings.  On center/Off surround: when you have a white dot in a dark surrounding it creates the most AP/ sec  Off center/On surround: it creates the most AP/sec when the surroundings are white and the center is dark o This allows you to figure out how active a cell is ← *What does this serve to highlight in the image? It is looking for little white spots of light or the donuts. Changes in brightness, it is only the contrast. ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← ← 2. Two types of retinal ganglion cells: • Magno (large, less common): it branches everywhere it has large axons and sends information very quickly. They have a transit responds, it will respond initially and then it will stop responding. It responds very well change it is motion sensitive. Respond very well to change in brightness, they are located on the visual periphery. o Large cell that branches all over the place with big and thick axons (sends information very fast), less common than parvo. o Transient response: put something in front of it, it will respond initially and then stop (gets bored easily) responds well to change because it is motion sensitive responds very well even to small differences in brightness – found in the periphery • Parvo(most common): A single charge does not go pass the pass quickly. Mostly located in the fovia there is a lot of cones, it is colour sensitive and responds to fine detail o Small cells that don’t branch very far (dendrites) and has a little thin axon (single charge doesn’t fire very fast) – most common kind of retinal ganglion cell they have a o Sustained response: put something in front of it, it will keep responding for a while (doesn’t get bored easily) are colour sensitive and give fine detail  lots in the fovea From the eye to the brain (brain and retinotopic maps) I. Important background concepts: A.Maps of visual space 1. A fixation point is where you’re looking at and then you have left visual field and right visual field 2. Information from right visual field goes to the left-brain and vice versa = there is a crossing over Retinoptic map: things that are side by side in the retina are side by side in the brain B. Feed forward and feedback (top down and bottom up) Feed forward: continue straight through from retina to higher centers of brain (bottom-up) Feedback: Higher structures communicate back to lower structures 1. Bottom- Up Processing (data based processing): is processing that is based on incoming data. Incoming data always provide the starting point for perception because without it there is no perception. i. Example: When perceiving the moth, the incoming data is the patterns of light and dark placed on the retina created by light reflected from the moth and tree. 2. Top- Down Processing (knowledge based processing): refers to processing that is based on knowledge. More involved in processing when the stimulus is complex. • Cognitive influences on perception: how the knowledge, memories and expectations that a person brings to a situation influence their perception. a. Example: When the pharmacist reads your prescription, although it might look like scribbles to you; the pharmacist uses knowledge of drugs and past experiences of this doctors writing to perceive a clear representation of the prescription. C. Distributed coding vs. Specificity coding 1. Coding information: sending information about something to another center i. Example: Want to communicate something so you use language – can’t literally put images inside the brain so we use electrical signals to say what’s there a. Specificity coding: One cell is specialized to respond to an attribute“ / “ a group of cells that if there was a diagonal line, would respond due to a high amount of Action Potential. However, it would not respond to horizontal or vertical lines. To determine whether or not “/” line is in the visual field consult specific group of cells. But if there’s damage to that cell you lose the ability to see that attribute Distributed coding: Not only one kind of cell to find out what’s going on but also the pattern of activity across all cells stimulated by the visual field are used - Different levels of stimulation. Might have one cell that responds best to “/” but it also responds a little to | So higher action potentials for \, medium for horizontal and vertical  doesn’t ignore, doesn’t respond as strongly but it has least amount of action potentials to the opposite “\” temporal lobe – cells that are specific to specific stimuli (grandmother cells) most cells in visual use distributed coding at lower levels Tilt after effect: After image created, which is special kind of after effect where you see something in front of your eyes. Stare at diagonal lines for a long time and you adapt to them and then show vertical lines, but the person sees lines tilted in opposite direction Adapt to one line orientation and then what you saw before makes you misperceive vertical as tilted in opposite way Shows that you can’t have specificity coding for line orientation because you if you did, adapting to that line would have no effect on seeing vertical lines (you’d just be tiring out specificity cells, not ones for vertical) Must be distributed coding: tire out receptor cells, one will be the least tired. V. Pathways out of the eye to the brain Each eye gets info from both right and left visual field; the left part of visual field ends up on right side of the brain, right visual field ends up on left side of the brain. A. Optic nerve • Retinal ganglion has axons and those axons pass out the back of the eye at the optic nerve • Optic nerve is where you get myelin (inside is crowded so not enough room for myelin) B. Optic chiasm • Left and right visual field comes into both eyes (right visual field goes outside corner of left eye and inside corner of right eye)The two optic nerves cross at the optic chiasm (X) • Information gets sorted out – all left cells go on right side of visual field and this involves the temporal side towards ears and nasal side toward nose • Temporal fibers go straight back (left go to left, right go to right) • Nasal fibers cross – sorts out the fact that both eyes receive both
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