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 surroundings slope down more steeply. 10
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 confirmed 1, 2, 3. b, Light, via melanopsin, causes changes in Ca 2+ levels in MG cells 9 (a fluorescent Ca 2+ 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. See text fig. 7.5 16
Receptive field center and surround are lit up separately See text fig. 7.12 17 Receptive field (again) The yellow is the region receiving light. See fig. 7.11 18
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 ½, ¼, 1/8 etc. The spacing that looks equal is not 0.9, 0.8, 0.7, etc. Demo: gray sheets of paper 21 Simultaneous lightness contrast Craik O Brien Illusion Contrast at the edge affects your perception of the center. Are the small gray patches below identical? See fig 7.7 22
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
Hermann grid illusion: dark areas are from lateral inhibition 27 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. http://www.michaelbach.de/ot/index.html 40
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. Victor Vasarely, artist 41 A B Previous experience effect: Here, the eye is fooled into thinking the light is from above. The panel A has lots of light, so it must be really dark. But B must be lighter because it is in the shade. 42
A B Previous experience effect: Here, the eye is fooled into thinking the light is from above. The panel A has lots of light, so it must be really dark. But B must be lighter because it is in the shade. 43 Contrast constancy effect The image at left has too much contrast, so the eye turns down contrast which makes the central gray squares more alike. On the right, the central gray squares look more different (higher contrast). 44
Size constancy: Are all the vertical lines the same height? 45 http://www.michaelbach.de/ot/index.html 46
Other illusions There are many optical illusions with varying explanations. Many are poorly understood. 47 Are the blue shades the same? Lateral inhibition cannot explain this! 48
49 Lateral inhibition alone does not explain this effect, the Munker-White illusion. 50
http://www.newworldencyclopedia.org/entry/muller-lyer_illusion Müller-Lyer illusion Which arrow is longer? 51 http://www.michaelbach.de/ot/index.html Müller-Lyer illusion This is the back corner of a room, it is further away, hence it must be larger. 52