Processing the ImageorCan you Believe what you see?Light and Color for NonscientistsPHYS 1230Optical Illusionshttp://www.michaelbach.de/ot/mot_mib/index.htmlVisionWe construct images unconsciously and very rapidly. Sometimesour eyes can even deceive us! These “optical illusions reveal thebrain’s assumptions about what we are seeing. Our brains are justas fertile when we use our other senses. In moments of anxiety,for instance, we sometimes "hear things" that are not reallythere. But suppose a leopard approached, half-hidden in thejungle—then our ability to make patterns out of incomplete sights,sounds, or smells could save our lives.So vision is not merely the act of forming the image of the objecton the retina. Our brain must interpret the image.The complete visual pathwayhttp://www.hhmi.org/sensesPreviously we learned howthe eye forms an imageof the object on theretina, how we cancorrect vision problems,and how we use opticalinstruments to help seeobjects.Next we will talk aboutthe following processesthat happen in the retina,the nerve pathways, andthe brain. In thefollowing we will talkabout lightness anddarkness. Then we willdiscuss depth and color.Light rays reflected by an object—for example, a pencil—enter the eye andpass through its lens. The lens projects an inverted image of the pencilonto the retina at the back of the eye. Signals produced by rod and conecells in the retina then start on their way into the brain through the opticnerve and reach a major relay station, the LGN (lateral geniculate nucleus).Signals about particular elements of the pencil then travel to selectedareas of the primary visual cortex, or V1, which curves around a deepfissure at the back of the brain. From there, signals fan out to "higher"areas of cortex that process more global aspects of the pencil such as itsshape, color, or motion.The visual pathwayhttp://www.hhmi.org/senses• The visual pathway begins in the retina,and signals travel through the followingpathway -- Photoreceptors- Horizontal cells- Bipolar cells- Amacrine cells- Ganglion cells- Optic nerve- Optic chiasma- Visual cortex of brain• A lot of pre-processing begins in the retina.• The retina is wired to quickly recognizelines and shapes• The brain is wired to quickly recognizefaces.http://ligwww.epfl.ch/~fua/vision/3/misc/exam/human/2/• The visual pathway begins in the retina,and signals travel through the followingpathway -- Photoreceptors- Horizontal cells- Bipolar cells- Amacrine cells- Ganglion cells- Optic nerve- Optic chiasma- Visual cortex of brain• A lot of pre-processing begins in theretina.• The retina is wired to quickly recognizelines and shapes• The brain is wired to quickly recognizefaces.http://ligwww.epfl.ch/~fua/vision/3/misc/exam/human/2/RECEPTIVE FIELDSThe response of each pointon the retina is influencedby neighboring regionsEach ganglion cell receivessignals from rods andcones in a certain area ofthe retina - this area iscalled the RECEPTIVEFIELDLightness and BrightnessBRIGHTNESS: Amount of illumination. The opposite is Dim.LIGHTNESS: Property of a surface e.g. the paper your notesare copied on is lighter than newspaper. The opposite isDarkness.In everyday life these words are used interchangably, but in thiscourse, brightness is used to describe the light source andlightness to describe the shade of gray. e.g. a surface thatreflects 10% of light is a shade of gray, and that shade isINDEPENDENT of the amount of light illumination (orbrightness).LIGHTNESS CONSTANCY: Our eyes and brain correct forthe amount of light available so that a newspaper looks thesame under bright or dim light.WEBER’S LAWWe perceive equal steps of lightness(equally spaced shades of gray) when theratios of lightness are equally spaced.The percent of light reflected from asurface is a quantitative measure of it’slightness. A reflectance of 50% means that1/2 of the light incident on the surface isreflected, while 1/2 is absorbed.According to Weber’s Law, reflectances of1/2, 1/4, 1/8, 1/16, will look equally spaced,but reflectances of 0.9, 0.8, 0.7 etc. will notlook equally spaced.RETINAL PROCESSING OF LIGHTNESS AND DARKNESSWhen light falls on a photoreceptor, it responds by firing more frequently. It alsoinhibits adjacent cells from firing. This is called lateral inhibition.LATERAL INHIBITIONWhen light falls on thisphotoreceptor, adjacent cellsare inhibited from firing.The Retina and Laterial InhibitionIn the figure, the green rectangles represent photoreceptors, eachgenerating a signal appropriate for the amount of light falling on it. Thered circles represent output neurons of the retina, whose signals will goto the brain through the optic nerve. Each output neuron is shown asreceiving input from an overlying photoreceptor (vertical black lines) aswell as inhibitory input from adjacent photoreceptors (angled blue lines).It is this laterally spread inhibition that gives "lateral inhibition"networks their name.lightdarkhttp://serendip.brynmawr.edu/bb/latinhib.htmlThe Retina and Laterial InhibitionAt the bottom of the schematic is a representation of the signals in theoutput neurons (purple lines). Output neurons well to the right of thedark/light border are excited by an overlying photoreceptor but alsoinhibited by adjacent, similarly illuminated photoreceptors. The same is truefar to the left of the dark/light border. Hence, assuming that the networkis organized so that equal illumination of exciting and inhibitingphotoreceptors balances out, output neurons far from the edge in eitherdirection will have the same output signals. Only output neurons near thedark/light border will have different output signals.lightdarkhttp://serendip.brynmawr.edu/bb/latinhib.htmlThe Retina and Laterial InhibitionAs one approaches the dark/light border from the left, the signals willdecrease, because inhibition from more brightly lit photoreceptors to theright will outweigh the excitation from the overlying dimly litphotoreceptors. As one approaches the dark/light border from the right,the signals will increase because excitation from brightly lit photoreceptorsis not completely offset by inhibition from the dimly lit photoreceptors tothe left.lightdarkhttp://serendip.brynmawr.edu/bb/latinhib.htmlEXAMPLES OF LATERAL INHIBITIONUniform white background: all cells are inhibited by adjacent cells, so that all of thewhite is not as bright.EXAMPLES OF LATERAL INHIBITIONGray surrounded by white: this seems darker than gray