Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityRods and conesPSY 310Greg FrancisLecture 04When we all go color blind.Purdue UniversityOphthalmoscope Able to see into an eyeProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOphthalmoscope Here is what you seePurdue UniversityBlood The back of youreye is covered inblood vessels Why don’t yousee them? They do castshadows!Prof. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityBlood To see the bloodvessels in your eyestry the following Shine a small penflashlight into youreye at a steep angle Move the lightaround in jerkymotions Try to keep your eyestill and look fordark lines in yourvision These are veinshadows!It takes practice!Purdue UniversityImage The light coming into your eye projects on to theback of your eye It is, of course, much smaller than the original objectProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityImage How do we describe the size of visual stimuli? A smaller image that is closer is exactly the same on the backof the eye!Purdue UniversityImage How do we describe the size of visual stimuli? A larger image that is further way is exactly the same on theback of the eye!Prof. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityVisual angle We describe the size of stimuli in terms of visualangle Imagine the world as a sphere around your head Light comes from the interior surface of the spherePurdue UniversityVisual angle Without further information, we cannot say how“big” a stimulus is in absolute size feet, inches, meters We can only talk about how much of the spherethey cover Visual angle Degrees, minutes, seconds, radiansProf. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversityComputing visual angle Suppose you want to compute the visualangle, ,of a line (b,c)€ θPurdue UniversityComputing visual angle You need to know the distance from youreye to the object, dProf. Greg Francis7PSY 310: Sensory and Perceptual ProcessesPurdue UniversityComputing visual angle You need to know the size of the object2s2sPurdue UniversityComputing visual angle Trigonometry works best with rightangles, so we find the middle of (b,c) andwork with one triangle (say, the top)sProf. Greg Francis8PSY 310: Sensory and Perceptual ProcessesPurdue UniversityComputing visual angle This is a side-angle-side problem fromtrigonometry You can figure out all other side lengths and anglessPurdue UniversityComputing visual angle This is a side-angle-side problem fromtrigonometry You can figure out all other side lengths and anglessd€ θ2€ tanθ2      =sdProf. Greg Francis9PSY 310: Sensory and Perceptual ProcessesPurdue UniversityComputing visual angle This is a side-angle-side problem fromtrigonometry You can figure out all other side lengths and anglessd€ θ2€ θ= 2a tansd      Be sure tokeep units fors and d thesame!Purdue UniversityComputing visual angle This is a side-angle-side problem fromtrigonometry You can figure out all other side lengths and anglessd€ θ2€ θ= 2a tansd      The atanfunction (arctangent) mightreport inradians ordegreesProf. Greg Francis10PSY 310: Sensory and Perceptual ProcessesPurdue UniversityExample Suppose the snowman is 6 feet tall (s=6 feet)and 20 feet away (d=20 feet)€ θ= 2a tan620      Purdue UniversityExample Suppose the snowman is 6 feet tall (s=6 feet)and 20 feet away (d=20 feet)€ θ= 2a tan 0.3( )Prof. Greg Francis11PSY 310: Sensory and Perceptual ProcessesPurdue UniversityExample Suppose the snowman is 6 feet tall (s=6 feet) and 20 feetaway (d=20 feet) My calculator gives an atan angle in radians, so I multiplyby 180 degrees and divide by Pi radians€ θ= 2 × 0.29 radians( )×180 deg( )πradians( )Purdue UniversityExample Suppose the snowman is 6 feet tall (s=6 feet) and 20 feetaway (d=20 feet) My calculator gives an atan angle in radians, so I multiplyby 180 degrees and divide by Pi radians€ θ= 33.4Prof. Greg Francis12PSY 310: Sensory and Perceptual ProcessesPurdue UniversityRule of thumb A pretty good estimate is found by holding yourthumb at arms length The width of your thumb is approximately 2degrees of visual angle There are 60 minutes to an angle There are 60 seconds to a minute I sit about 50 cm from my computer monitor,which is around 31 cm across Visual angle is around 64 degreesPurdue UniversityOphthalmoscope The fovea is a specialplaceProf. Greg Francis13PSY 310: Sensory and Perceptual ProcessesPurdue UniversityDetecting light The back of theeye is coveredwith cells thatare sensitive tolight PhotoreceptorsPurdue UniversityTypes of cells There are two mainclasses ofphotoreceptors Rods: respond well to lowlight levels Cones: respond well tohigher light levelsProf. Greg Francis14PSY 310: Sensory and Perceptual ProcessesPurdue UniversityPhotoreceptors They are not distributed evenly across the retina Most cones are in the foveaPurdue UniversityPhotoreceptors Here’s another way of looking at itProf. Greg Francis15PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOptic disc This is whereblood vessels andnerves enterand/or leave theeye There are nophotoreceptors Light that fallshere cannot bedetectedPurdue UniversityCogLab In theexperiment,you were tostare at asmall squareon the leftwith yourright eye It takespractice tokeep it thereProf. Greg Francis16PSY 310: Sensory and Perceptual ProcessesPurdue UniversityCogLab On each trial adot appearedsomewhere Your task wasto reportwhether yousaw it or not Again, it’sdifficult to keepyour eye frommovingPurdue UniversityCogLab If the goldcircle indicatesthe location ofthe optic disc,then any dot inhere shouldnot be seenProf. Greg Francis17PSY 310: Sensory and Perceptual ProcessesPurdue UniversityCogLab Here’s the actual data It’s a plot of the proportion of reports of seeing the dot at different positions Averaging across subjects makes it blurry Some subjects move their eyes and so report seeing the dotPurdue UniversityCones There arethree kinds ofcones thatrespond todifferentwavelengthsof light Short (blue) Medium (green) Long (red)Prof. Greg Francis18PSY 310: Sensory and Perceptual