Lecture 15 End Chap 6 Optical Instruments 2 slides Begin Chap 7 Visual Perception Mar 2 2010 Homework 6 on Ch 6 due March 4 Read Ch 7 skip 7 10 1 2 35 mm slide projector Field lens is used to put the most light on the slide Field lens or condenser Color slide Projection lens Mirror The condenser focuses the light from the filament through the projection lens to increase brightness 3 Viewgraph projector Mirror Projection lens Fresnel lens condenser and viewgraph location Curved mirror returns light otherwise lost 4 Ch 7 Visual Perception Parts of the visual processing system Lightness and brightness Retinal processing Lateral inhibition Hermann grid Receptive field Motion illusion Craik O Brien illusion simultaneous lightness contrast Other optical illusions 5 Components of the visual processing sytem Photoreceptors rods and cones Horizontal cells connect between photoreceptors Bipolar cells Amacrine cells connect between ganglion cells Ganglion cells Axons Optic nerve Optic chiasma Visual cortex of the brain 6 Brain anatomy Optic chiasma Left field of view goes to right brain Right field of view goes to left brain from both eyes Visual cortex is where you see Brain damage at this location hurts vision 7 Optic chiasma Brain damage on the left side hurts vision on the right side 8 See text fig 7 3 Processing without thinking The retina is wired to quickly recognize lines and shapes Motion is quickly recognized The brain is wired to quickly recognize faces 9 Channels A channel is a subsystem in the brain that responds to specific features Motion channel keeps firing after watching a moving object causing motion aftereffect Size channel size is judged relative to other objects Large people make you look small This is called simultaneous size contrast Orientation channel the surroundings a floor are used to help you determine orientation A stream can look like its flowing uphill if the 10 surroundings slope down more steeply Connections Photoreceptors rods and cones connected to the bipolar cells connected to the ganglion cells funnel data through axons into the optic nerve sideways connectors these help with analysis horizontal cells next to the photoreceptors amacrine cells 11 Layers of the retina Light 12 Layers of the retina 13 See text fig 7 2 Layers of the retina From the following article Neurobiology Bright blue times Russell G Foster Nature 433 698 699 17 February 2005 doi 10 1038 433698a a The rods R and cones C convey visual information to the ganglion cells G through the bipolar cells B Horizontal cells H allow lateral connections between rods and cones Amacrine cells A allow lateral connections between bipolar and ganglion cells The optic nerve is formed from the axons of all the ganglion cells A subset of ganglion cells MG cells also detects light directly for this they require the photopigment melanopsin as now confirmed1 2 3 b Light via melanopsin causes changes in Ca2 levels in MG cells9 a fluorescent Ca2 indicator was used here Counterintuitively light passes through the transparent ganglion layer to reach the rods and cones 14 Layers of the retina 15 Retinal processing Amacrine and horizontal cells turn down the signals from areas adjacent to bright areas 16 See text fig 7 5 Receptive field center and surround are lit up separately 17 See text fig 7 12 Receptive field again 18 The yellow is the region receiving light See fig 7 11 Edge detection is enhanced Half illumination gives bigger signal 19 Lightness and brightness Brightness amount of light Lightness property of a surface newspaper 0 65 reflectance printer paper 0 84 photo quality paper 0 90 0 99 20 Lightness and brightness Lightness constancy brain and eye correct for amount of light so that white gray and black look the same independent of brightness Weber s law we think lightness is equally spaced when the ratios are equally spaced Example lightness 0 5 0 25 0 125 look equally spaced These numbers are 1 8 etc The spacing that looks equal is not 0 9 0 8 0 7 etc 21 Demo gray sheets of paper Simultaneous lightness contrast Craik O Brien Illusion Contrast at the edge affects your perception of the center Are the small gray patches below identical 22 See fig 7 7 Here is a related illusion involving lateral inhibition Each vertical band has equal light intensity across its width However the left side of each bar appears darker than the right side due to lateral inhibition at the edges One side is next to a lighter bar while the other side is next to a darker bar less inhibition looks lighter more inhibition looks darker 23 Craik O Brien Illusion Simultaneous lightness contrast These are the patches without the surround 24 Simultaneous lightness contrast again Checker shadow illusion Which square is lighter in shade square A or square B 25 Simultaneous lightness contrast Checker shadow illusion Slide them together and compare A is surrounded by light squares and B is surrounded by dark squares in the previous slide 26 27 Hermann grid illusion dark areas are from lateral inhibition 1 3 2 The red areas show the receptive field Lateral inhibition is greater at 1 than at 2 The fovea has a smaller receptive field So the lateral inhibition is the same everywhere in the white area 28 White space is larger than receptive field 29 It is blacker away from a corner where there is more inhibition 30 31 The music A Kitaoka 32 Does the center stripe have constant lightness Or is the center stripe darker in the middle and at the ends 33 The center stripe has constant lightness 34 Victor Vasarely Zebras The black white boundaries outline the necks There are no lines in this drawing The artist has made use of the tendency of the eye to find lines The regions of color don t have edges but appear to Picasso 36 French artist George Seurat used edge enhancement by lateral inhibition to make figures stand out sharply Lighter just before edge Darker just before edge 37 El Greco 38 The white crosses are an illusion Victor Vasarely artist The edges of the squares seem lighter because of the dark surrounds 39 Which creature is larger Previous experience in tunnels tells us that the creature in back is further away and hence must be larger 40 http www michaelbach de ot index html Previous experience interprets these flat images as being from 3 dimensional boxes The shadows tell us what is a floor and what is a wall 41 Victor Vasarely artist A B Previous experience effect Here the eye is fooled into thinking the light is from above The