Chapter 6 Notes - Psych 2220

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Psychology 2220A/B
Jennifer Hoshooley

Chapter 6 – The Visual System: How We See o Which means = greater range of depths are - Convergence: eyes must turn slightly inward when objects are simultaneously kept in focus on the retinas close so they can be focused on - Most active field of study in biopsych o When level of illumination is too low to activate o Greatest when inspecting things close upa Light enters eye  retina at back of eye receptors, pupils dilate to let in more light - Binocular Disparity: difference b/w images on ea. Retina - No species can see in complete dark  Sacrifices acuity and depth of focus o Convergence & binocular disparity > when objects - Light: are close - Lens: focuses light on retina o Photons (discrete particles travelling through space o When we direct gaze at something near, tension on o Provides brain with distance info. very fast) ligaments holding ea. lens in place is adjusted by the ciliary muscle and lens assumes natural shape o Greater for objects close than far o Waves of electromagnetic radiation o Increases ability of the lens to refract (bend) ligRetina & Translation of Light into Neural Signals - Humans? Wavelengths b/w 380-760nm brings close objects into sharp focus - Retina composed of five layers of neurons - Accommodation: lens is adjusted to focus image - Infrared waves – too long for humans to see o Receptors - Light encounters objects in environment - Distant objects? Lens does not need to bend much o Horizontal cells - Some light absorbed, some reflected - Close objects? Lens fattens to bend light rays o Bipolar cells o Reflected light (distal stimulus) o Causes more strain on the lens o Amacrine cells - Reflected light enters eye o To reduce strain, eye gradually elongates o Retinal ganglion cells - Light is bent as it passes through lens o Can cause distant objects to appear blurred - Light gets to back of eye to come in contact w/ the retina - Image is inverted and projected to back of eye onto retina and o Myopia = nearsighted is the proximal stimulus o Receptors located in Retina (incl visual) function as - Prevention of Myopia transducers - Visual field: area you can see in front of you without moving your head o Read in adequate lighting  Convert one form of energy to another - Wavelength: perception of colour o Avoid small font (lol) o Retina = inside out (light must pass through several layers to reach receptors) - Intensity: perception of brightness o If glasses are worn for Myopia, avoid wearing them when reading o Retinal Pathway: - Pupil: where light enters Eye Position and Binocular Disparity  Receptors  Bipolar Cells  Retinal o Size is regulated by iris (which gives us our eye Ganglion Cells colour) - Two eyes = useful b/c of binocular cues it provides o Lateral communication b/w cells in different layers - Sensitivity: ability to see in dim light - Predators tend to have front facing eyes bc enables them to made possible through: accurately perceive how far away pry animals are - Acuity: ability to see fine details  Horizontal Cells - Prey animals tend to have side-facing eyes bc gives them a larger field of vision and ability to see predators approaching - When pupil is constricted, image falling on ea. retina is shaper from most directions  Amacrine Cells and has a greater depth of focus - 2 problems w/ inside-out o Incoming light is distorted by retinal tissue through o Tendency for peak luminance sensitivity of the - ^^ effects of dim light simultaneously stimulating many rods which it must pass bf reaching receptors can add to influence the firing of the retinal ganglion cell onto human eye to shift toward the blue end of the which the output of the stimulated rod converges colour spectrum at low illumination  Fovea fixes this (centre of retina) - ^^ effects of same dim light applied to sheet of cones cannot - How is info about light passed to Nervous System?  Specialized for high acuity addadd to the same degree and retinal ganglion cell may not respond to light at all o Photoreceptors (rods & cones) contain photopigments that absorb photons entering eye  Thinning of retinal ganglion layer at - Low level of acuity = scotopic system fovea reduces distortion of incoming light o Photopigments are embedded in free floating discs - Density of rods reaches max. @ 20% from centre of fovea in outer segment of either rod or a cone o No visual receptors where axons of retinal ganglion leave eye = blind spot is created o More rods in nasal hemiretina (half of retina near - We know most about rods (rhodopsin especially) nose) than in temporal hemiretina (half next to temples) o G protein-linked receptor that responds to light  Nervous system adapted to this situation Spectral Sensitivity o Dark? Cyclic GMP keeps Na+ channels open and  We don’t experience visual field w/ a glutamate is continuously released from the rod part missing b/c of completion - More intense light appears brigher o Light? Cyclic GMP deactivated and Na+ channels - Graph of relative brightness of lights of same intensity close causing rods to hyperpolarize to light  Visual system uses info provided by presented at different wavelengths is called spectral sensitivity receptors around blind spot to fill in gaps curve o SO: Light signaled through INHIBITION in retinal images for you - 2 types of curves in humans and other animals: - Surface interpolation: process by which we perceive surfaces - In dim light, our sensitivity to various wavelengths is a direct consequence of rhodopsin’s ability to absorb them o Visual system extracts info about edges and from it o Photopic spectral sensitivity curve - Temporal integration allows us to still have a visual image infers appearance of large surfaces  Determined by having subjects judge even while blinking relative brightness of difference Cone and Rod Vision wavelengths of light shone on fovea - Involuntary fixational eye movements - Two types of receptors in retina o Scotopic spectral sensitivity curve o Tremor o Photopic: cones mediate colour, high-acuity vision in good lighting  Determined by having subjects to judge o Drifts relative brightness of different wavelengths of light shone on periphery o Saccades o Scotopic: rods mediate vision in dim light and of retina at intensity too low to activate highly sensitive to light trading off on acuity few peripheral cones located there - How is the whole visual scene perceived as detail and - Duplexity theory: cones and rods mediate different kinds of coloured?? vision - Phototopic conditions? Visual system maximally sensitive to wavelengths of 560n; 500nm would have to be much more o Saccades: continuous scanning of the world with - Differences found in how information is fed from individual intense than one at 560nm to be seen as equally bright small & quick eye movements receptors into retinal ganglion cells - Scotopic conditions? Visual system maximally sensitive to o Info from saccades integrated wavelengths of 500nm; 560nm would have to be much more o Scotopic: output of several hundred rods converge intense to be seen as equally bright on single retinal ganglion cell o Very important!! Demonstrate that the visual system responds to change & rapidly habituates & o Photopic: only a few cones converge on each - Purkinje Effect stops responding to unchanging stimuli retinal ganglion cell to receive input from a few cones - Transduction: conversion of one form of energy to another o Visual transduction: conversion of light to neural o Colour o Three different kinds of colour receptors (cones) signals by the visual receptors each w/ a diff spectral sensitivity o Deta
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