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Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityLight and the eyePSY 310Greg FrancisLecture 03Why does my daughter look like a demon?Purdue UniversityThe perceptual processAttended stimulusEnvironmental stimulusActionRecognitionPerceptionProcessingTransductionStimulus onreceptorsPurdue UniversityBrain structure Neuralcircuits arespecializedto processcertaintypes ofinformationPurdue UniversityBrain structure Blowing up a partof the imagehighlights a oddfeature about howthe brain works Some of theinformation fromthe retina crosseson its way to thebrainPurdue UniversityContralateral processing Neural fibers fromeye cross on wayto cortexPurdue UniversityBrain structure The left hemisphere of the brain takes in input from: the left part of the left eye The left part of the right eye ==> objects to the right of where you are looking The right hemisphere of the brain takes in input from: the right part of the left eye The right part of the right eye ==> objects to the left of where you are looking An fMRI brain scan demonstrates that different hemispheresrespond to contralateral stimuli Movie: moving wedgeProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityBrain structure The two hemispheres are pretty similarfor most perceptual processing But they differ for cognitive processing The right hemisphere plays a bigger rolethan the left in judging facesPurdue UniversityCogLab: Brain asymmetry On each trial you saw two oddlyconstructed faces And had to judge which face appeared youngerPurdue UniversityCogLab: Brain asymmetry If you look at the middle of an image, theleft side goes to the right brain, and theright side goes to the left brainThis face might lookyoung, because thejudgment is basedmore on the left sideof the image.Purdue UniversityCogLab: Brain asymmetry If you look at the middle of an image, theleft side goes to the right brain, and theright side goes to the left brainThis face might lookold, because thejudgment is basedmore on the left sideof the image.Purdue UniversityCogLab: Brain asymmetry The faces are actually mirror images ofeach otherPurdue UniversityCogLab: Brain asymmetry The hemispheres of left-handed people are notas different as for right-handed people Experiment measures the percentage of choiceswith younger half face on left67Right57LeftPercentageHandednessProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityLight Visual perception involves the detection of light We interpret patterns of light that vary in space and time ina way to learn something about objects in the world This wasn’t always obvious People used to think that perception involved some kind of“ray” going out of the eyes to “ touch” objects The fact that photographs are convincing demonstrates thatthis idea is not correct Light is electro-magnetic energy It can be described as a wave The distance between two neighboring peaks of a wave is thewavelength Usually given in nanometersPurdue UniversityLight It can be described as a wave The distance between two neighboring peaks of a wave is thewavelength Can also fix a distance and talk about the number of peaks in thewave frequencyPurdue UniversityLight People arevisuallysensitive toonly a smallfraction of thewavelengthsof lightPurdue UniversityLight Different species are sensitive to differentwavelengths of lightPurdue UniversityLight The wavelength of light often corresponds to a particular color E.g. blue skies are blue because they scatter low wavelength light more thanhigh wavelength light The non-scattered light reaches our eyes directly (the sun looks yellow, orange,red) The scattered light is what we see when we look to other parts of the skyPurdue UniversityColor But be careful! Color is aperceptualexperience, not aproperty of light The magenta andorange haveexactly the samewavelength!Prof. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityIlluminant and reflectance We need light tosee things, butwe are not ofteninterested insources of light. A source oflight is calledan illuminant Light energyfalling onto asurface isilluminancePurdue UniversityIlluminant and reflectance We are reallyinterested inobjects Light from anilluminantreflects off ofobjects Light energyreflected of anobject orsurface isluminancePurdue UniversityMeasuring light intensity Most illuminants produce light at lots of different wavelengths We only sense a small part of the wavelengths So how do we measure the strength of a source of light? Cannot just measure total electro-magnetic energy Who cares about theradio waves or gammawaves? They do notcontribute to visual perception. There are specialcalculations that weightsdifferent frequencies toinclude only thoseimportant for visual perception Candelas per square meter Foot-lambertsPurdue UniversityThe eye Light reflects off objects and some of it enters the eyePurdue UniversityThe eye Cross-sectionshows moredetail The eye ishollow, butfilled withclear fluid The cornea isa clear lens So isthe”lens” They helpfocus lightPurdue UniversityThe eye Cross-sectionshows moredetail Light entersthe pupil The cornea isa clear lens So isthe”lens” They helpfocus lightProf. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityThe eye The interior ofthe eye ishollow, but isfilled with aclear liquidPurdue UniversityThe eye Light passes through the cornea, lens, and fluid and projectson to the back of the eye The retina The image is upside down because the lens inverts theimagePurdue UniversityThe eye You change the shape of the lens to make certain the imageis sharp on your retina Accommodation (automatic) As you age, the lens loses flexibility and cannot be shaped sowell Then you need glassesPurdue UniversityThe eye Why is the pupil black?Purdue UniversityThe eye Much of the visible light that goes into the eye isabsorbed by the detection process (next time) What is reflected back comes out of the right back tothe source Unless your eye is right where the source of light islocated you do not see the light coming out of the eye Thus it looks blackPurdue UniversityThe eye Why is the pupil red in photographs? With a camera’s flash, the camera lens is very


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Purdue PSY 31000 - Lecture notes

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