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Chapter 6

PSYC 2410 DE S12 Textbook Notes Chapter 6.pdf

16 Pages
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Department
Psychology
Course Code
PSYC 2410
Professor
Elena Choleris

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Chapter 6 The Visual SystemThe visual system does not create an exactly accurate recreation of the external world for us to interpretThe information projected onto the visual receptors is distorted upsidedown and twodimensionalFrom this information the brain creates a representation of what is out thereTwo types of research inform us about our visual systemResearch that explores the visual system with neuroanatomical neurochemical and neurophysiological techniquesResearch that focuses on the assessment of what we see61 Light Enters the Eye and Reaches the RetinaSome animals have adaptations that allow them to see in very dim light but no visual system is able to work in total darknessLight Often defined as waves of electromagnetic energy that are between 380 and 760 nanometers in lengthLight has some properties of waves and some properties of particlesThis range is the spectrum we are able to perceive with our eyes however some animals are capable of perceiving wavelengths we cannotTwo properties of light are of particular interestWavelength Used interchangeably with colourIntensite Used interchangeably with brightnessThe Pupil and the LenseHow much light reaches the retinas is regulated by a donutshaped band of contractile tissue called the irisLight enters the eye through the pupil the hole in the center of the irisSensitivity The ability to detect the presence of dimly lit objectsAcuity The ability to see details of objectsPupil size is adjusted in response to light in order to balance sensitivity and acuityHigh illumination ie Sensitivity is not important leads to constricting of the pupils to allows for the greatest acuityLow illumination ie Sensitivity is important leads to dilation of the pupils to allow for greater sensitivity at the expense of acuityConstriction of the pupils allows a greater depth of focus meaning a greater range of depth differences are able to be kept in focus simultaneouslyBehind each pupil is a lens which focuses incoming light onto the retinaAccomodation The process by which the configuration of the lenses of the eyes bring images into focusCiliary Muscles Muscles which hold the lens of the eye in place and adjust its tension to allow us to focus either nearby or far awayWhen we observe something near the lens assumes its natural cylindrical shape allowing the lens to refract the light and bring close objects into focusWhen we observe something far away the lens is flattened which reduces refraction and brings distant objects into focusEye Position and Binocular DisparityOne reasons why vertebrates have two eyes is because they have two sidesBy having an eye on each side of the headthe most common vertebrate occular arrangementanimals can see in almost every direction without moving their headsHaving both eyes on the front side of the head sacrifices being able to see what is going on behind us for being able to see what is going on in front of us much more clearlyIn general predators tend to have frontfacing eyes because it enables them to accurately perceive how far away their prey arePrey animals tend to have side facing eyes because this arrangement gives them a larger field of vision and the ability to perceive oncoming predators from any directionThe movements of your eyes are coordinated so that each point in your visual world is projected onto corresponding points on each of the two retinasBecause both eyes must see corresponding images your eyes must turn in slightlyBecause your eyes are in two different places you cannot actually have the same image on both eyes at the same timeBinocular Disparity Difference in the position of the same image on the two retinasBinocular disparity is larger for close objects than for faraway objectsThe brain uses binocular disparity to create a three dimensional representation of our world based on two dimensional images captured by the retinas62 The Retina and Translation of Light into Neural SignalsThe retina converts light to neural signals conducts them towards the CNS and participates in the processing of the signalsReceptors Cells that are specialized to receive chemical mechanical or rediant signals from the environment Also proteins that contain binding sites for particular neurotransmittersoFifth and final layer of ocular cellsHorizontal Cells Type of retinal neurons whose specialized function is lateral communicationoSpecialized for lateral communication Communication across the major channels of sensory inputoFourth layer of ocular cellsBipolar Cells Bipolar neurons that form he middle layer of the retinaoThird layar of ocular cellsAmacrine Cells A type of retinal neuron whose specialized function is lateral communicationoSpecialized for lateral communication Communication across the major channels of sensory inputoSecond layar of ocular cellsRetinal Ganglion Cells Retinal neurons whose axons leave the eyeball and form the optic nerveoFirst layar of ocular cellsoProject across the inside of the retina before gathering together in a bundle and exiting the eyeballEach of these cell types comes in a variety of subtypes50 unique cell types have been identified in the retinaRetinal neurons communicate both chemically via synapses and electrically via gap junctionsLight reaches the receptor layer only after passing through the other four layersOnce the receptors have been activated the signal is transmitted back out through the retinal layers to the retinal ganglion cellsThis arrangement of signal reception and transfer results in two problems
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