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PS263 - Ch 6 Textbook.docx

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Wilfrid Laurier University
Todd Ferretti

PS263 – Chapter 6: The Visual System  Visual Illusions: The eye is tricked into seeing things the way they aren’t.  Fortification Illusion: Begins with gray area of blindness near center of visual field – they gray area expands into a horseshoe with a zigzag pattern of flickering lines at its advancing edges – Mrs. Richards Light Enters the Eye and Reaches the Retina  Light can be thought of as photons travelling through space at 300,000km per sec (186,000 miles) or as a wave of energy  Electromagnetic Energy: This is what light is sometimes defined as between 380 and 760 nanometers in length; humans and animals see different wavelengths; rattlesnake = infrared waves; too long for humans.  Wavelength: Plays an important role in the perception of colour, while Intensity: plays an important role in the perception of brightness. o Refer to intense light with a WV of 700 nm as being a bright red light.  Pupil & Lens: The amount of light reaching the retina is regulated by the donut shaped bands of contractile tissue; irises – gives our eye its colour. Light enters through the hole in the iris; pupil – the adjustment of the size in response to changes in light represents: o Sensitivity: Ability to detect the presence of dimly lit objects o Acuity: Ability to see fine details of objects. o When the illumination is high the visual system constricts the pupils – the image falls on each retina and there is a greater depth of focus (range of depths kept simultaneously on the retinas) when illumination is low, the pupils dilate to let in more light, sacrificing acuity and D.O.F.  When the illumination is high the visual system constricts the pupils – the image falls on each retina and there is a greater depth of focus (range of depths kept simultaneously on the retinas) When illumination is low, the pupils dilate to let in more light, sacrificing acuity and depth of focus   Behind each pupil is the lens, focusing the incoming light on the retina - tension on ligaments holding the lens in place is adjusted by ciliary muscles o Increases the ability of the lens to refract light (bend) and sharpens close images, when we focus on a distant object the lens is flattened. o Accommodation: Process of adjusting the configuration of the lenses to bring images into focus on the retina.  Eye Position: The fact that what we view can be seen simultaneously through both eyes is important for our ability to create 3D perceptions from 2D retinal images – vertebrates with eyes in front = predators vs. prey = sides. o Binocular Disparity: The difference in position of the same image on 2 retinas – greater for close objects than distant objects – 3D from 2D. The Retina & Translation of Light to Neural Signals  After light passes the pupil and lens it enters the retina where it is converted into neural signals and conducted toward the CNS, processing the signals.  The retina is composed of 5 layers of neurons: receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells (over 50 types) o Amacrine and horizontal are specialized for lateral communication; communication across the major channels of sensory input  Retinal neurons communicate chemically through synapses and electrically through gap junctions – light reaches receptor layer after passing through other 4, it then is activated, transmitting the neural message back through the layers to the retinal PS263 – Chapter 6: The Visual System ganglion cells whose axons project across the inside of the retina before bundling in the eyeball. o Problems: 1) Incoming light is distorted by the retinal tissue it passes en route to receptors, 2) The bundle of retinal ganglion cells need a gap in the receptor area (blind spot) in order to leave the eye. o The fovea minimized problem 1 – it in an indentation (0.33cm) at the center of the retina – the area that is specialized for high-acuity vision, the thinning of the R.G.L. reduces the distortion of incoming light. o The blind spot – the visual system uses info provided by the receptors around the blind spot to fill in the gaps in your retinal images (completion) o Surface Interpolation: the process by which we perceive surfaces; the visual system extracts info about edges & infers the appearance of large surfaces.  Species that are active in the day tend to have cone-only retinas, while those active at night tend to have rod-only retinas. o Duplexity Theory: Theory that cones and rods mediate different kinds of vision  Photopic Vision: cone-mediated vision that predominates in good lighting and provides high acuity colour perceptions of the world. Scotopic Vision: When there is not enough light, these are more sensitive (rod- mediated) however, it lacks detail and colour.  In the scotpoic there is an output of several hundred rods converging on a single R.G.C where in the photopic only a few cones converge = the effects of dim light stimulating many rods can add to the firing of retinal ganglion cells. (ONLY CONES IN THE FOVEA and more in the NASAL HEMIRETINA (closest to nose) vs. TEMPORAL HEMIRETINA)  Spectral Sensitivity: A graph of the relative brightness of lights of the same intensity presented at different wavelengths (curve) o Photopic Spectral Sensitivity Curve – determined by having subjects judge the relative brightness of different wavelengths of light on the fovea. o Scotopic Spectral Sensitivity Curve – determined by asking subjects to judge the relative brightness of different WV of light shone on the periphery of the retina at a intensity too low to activate the few peripheral cones located there. o ** Photopic conditions are maximally sensitive to WV of 560 nm while scoptopic is maximally sensitive to WV of 500nm **  Purkinje Effect: Noticed the flowers in his garden that were red & yellow were very bright compared to the blue, a few minutes later it was shades of gray with blue being the brighter gray shade  Eye Movement: Our eyes are constantly scanning the visual field, our visual perception at any instant Is a summation of recent visual information (temporal integration) that the world does not vanish when we blink o When we fix our gaze on an object our eyes continuously move involuntarily fixational eye movements: tremors, drifts and saccades.  Transduction: conversion of one form of energy to another – visual is when light is converted into neural signals by visual receptors – noted when a red pigment was extracted from rods rhodopsin and exposed to tense light, it was bleached and it lost its ability to observe light – when returned to the dark it regained it and redness  Rhodopsin is a G-Protein coupled receptor that responds to light rather than to NT molecules which initiate the cascade of chemical events when activated – in darkness rods Na channels are slightly open, keeping rods depolarized, but when bleached these are closed – signals are transmitted by inhibitions PS263 – Chapter 6: The Visual System From Retina to Primary Visual Cortex  Retina Geniculate Striate Pathways: conduct signals from each retina to the primary visual cortex or striate cortex via the lateral geniculate nuclei of the thalamus.  90% of the axons of the R.G.C. become part of the pathway – the signals from the left VF and the right primary visual cortex either ipsilaterally from the temporal hemi retina of the right eye or contra laterally via the optic chiasm  Retinotopic Organization: Each level of the system is organized like a map of the retina – 2 stimuli presented to adjacent areas of the retina excite adjacent neurons  Parvocellular Layers (P Layers): Composed of neurons with small cell bodies (parvo means small) – the first 4 layers; colour, fine details, patterns, stationary or slowly moving objects – receive input from Cones  Magnocellular Layers (M Layers): Composed of neurons with large cell bodies – the bottom 2 layers – responsive to movement – receive input from Rods Seeing Edges  Edges are important because they define the extent & position of various objects – a visual edge is a place where 2 areas of a visual image meet.  Mach Bands: they enhance the contrast at each edge and make the edge easier to see  Contrast Enhancement: Every edge we look at is highlighted for us by the contrast enhancing mechanisms of our nervous system – perc
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