Introduction to Physiological Psychology Vision [email protected] cogsci.ucsd.educogsci.ucsd.edu/~/~ksweeneyksweeney//psy260.htmlpsy260.html Light- a part of the spectrum of Electromagnetic Energy (the part that’s visible to us!) 1 In a vacuum, light travels at a constant speed of ~186,000 miles/sec. So if the frequency of the oscillation varies, the distance between peaks (or wavelength ) also varies. (vertical) Route within the retina Rods and Cones Bipolar Cells Ganglion Cells The axons of the ganglion cells form the optic nerve 2 Eye movement Although each fixation generates a different sensation at the level of the retina, the brain creates a single perception Yarbus, 1967 Why have two of them? Convergence :: –– eyes must turn slightly inward to focus when objects are close Binocular disparity :: –– difference between the images on the two retinas Both are greater when objects are close –– provides brain with a 33--DD image and distance information 3 Sensory neurons for vision RODS and CONES: –– Specialized neurons that respond to light with changes in their membrane potential Photoreceptors: Rods and Cones RODS :: –– ~120 million rods –– Scotopic Vision (skotos=darkness) –– Sensitive to brightness, but not color (shades of gray) 4 Photoreceptors: Rods and Cones RODS :: –– Many rods converge onto one retinal ganglion cell –– Responsible for lowlow--lightlight vision –– Not present at all in fovea Photoreceptors: Rods and Cones CONES: –– ~6 million cones –– Photopic Vision (photos=light) –– Sensitive to color 5 Photoreceptors: Rods and Cones CONES: ––A single retinal ganglion cell receives signals from one (or few) cones. –– Responsible for high acuity vision (fine detail) –– Fovea contains only cones Rods and Cones The outer segment of a photoreceptor contains hundreds of lamellae. Within the lamellae you find photopigmentsphotopigments--moleculesmolecules that contain an opsin and a retinal. (E.g(E.g.. rhodopsin )) 6 Rhodopsin Rhodopsin is a receptor that responds to light instead of to neurotransmitters (photons bind to it) When rhodopsin is exposed to light, it breaks down and the opsin bleachesbleaches.. The effect of the bleaching is a change in the release of NT –– Not the way you might think! Strange but true… The effect of light is to turn receptor cells ““OFFOFF””…… darkness turns them ““ONON””!! Remember that receptors have a spontaneous firing rate: –– They do NOT fire action potentials, but graded potentials –– The effect of receptors firing is inhibition of the bipolar cells 7 Transduction: how light becomes neural signals A cone or rod actually releases LESS neurotransmitter when stimulated by light! –– Rhodopsin molecules are bleached by light, causing hyperpolarization of rods. –– Thus, inhibition: less release of neurotransmitter (glutamate) –– Result is: depolarization of bipolar cell (= more release of neurotransmitter) –– Ganglion cell is more likely to fire (generally) The effect of a bleached photopigment… … is that a the photoreceptor ’’s membrane potential changes. ReceptorReceptor’’ss membrane potential affects release of NT onto bipolar cells. Bipolar cells ‘‘speak ’’ to ganglion cells, which bring information to the brain. 8 So in the dark …… Photoreceptors release enough NT to prevent bipolar cells from triggering ganglion cells. –– Ganglion cells, by NOT firing, report to the brain: ““nono lightlight”” And in the light? Cone and Rod Vision Distribution of rods and cones Only cones are found at the fovea!! 9 Cone and Rod Vision Less convergence in cones, increasing acuity while decreasing sensitivity More convergence in rod system, increasing sensitivity while decreasing acuity So we have a response from a ganglion cell… now what? Bundle of ganglion cell axons exiting the eye: blind spot No receptors where information exits the eye: –– Visual system uses information from cells around the blind spot for ““completion, ”” filling in the blind spot 10 From the Eyes to the Visual Cortex Lateral Geniculate Nucleus A nucleus within the thalamus ((““relay center ””) –– receives information from the retina and projects to primary visual cortex. Contains six ““layers ”” of neurons –– each layer receives information from only 1 eye. First two layers: magnocellular Next four layers: parvocellular 11 M and P channels Magnocellular –– Larger cell bodies –– Responsive to movement –– Input primarily from rods Parvocellular –– Small cell bodies –– Responsive to color, fine details –– Input primarily from cones M and P channels Layers 1, 4, 66--contracontra Layers 2, 3, 55--ipsiipsi 12 From the Eyes to the Visual Cortex The visual system is organized retinotopically: –– The left hemiretina of each eye (right visual field) connects to the right lateral geniculate nucleus (LGN) –– the right hemiretina (left visual field) connects to the left LGN Coding of Visual Information 13 Coding of information in the retina For any sensory neuron, a receptive field is the ‘‘place ’’ in which a stimulus will cause the neuron to fire. The receptive fields in the fovea are smaller than in the rest of the retina. Receptive Fields Many ganglion cells have receptive fields with a centercenter--surroundsurround organization: excitatory and inhibitory regions separated by a circular boundary Some cells are ““onon-- center ”” and some are ““offoff--centercenter ”” 14 What does color get us? What does color get us? 15 Why can you visualize red (imagine a firefire-- truck)… and you can imagine a reddish yellow… but it is difficult (impossible?) to imagine a reddish green… … or a bluishbluish--yellow?!yellow?! Color Mixing vs. Pigment Mixing 16 Color vision theories Trichromatic theory: there are 3 different receptors (types of cones) in the eye, each sensitive to a single hue (red, green, blue) Color vision theories Trichromatic theory: there are 3 different receptors (types of cones) in the eye, each sensitive to a single hue (red, green, blue) –– Because Young noted that any color could be accounaccountetedd for by mixing just 3 lights in various proportions 17 Trichromatic Theory At the level of the retina, cones code for three wavelengths of light (different opsins): Short (S), Medium (M), Long (L): blue, green, red 18 19 The precise distribution of cones varies from person to person, but generally speaking ““blueblue”” sensitive cones are less common than ““redred””andand ““greengreen”” cones Image from David Williams, U of Rochester Color Blindness Protanopia: no red cones – see yellow and blue, red and green hues confused Deuteranopia: no green cones – red and green hues confused Tritanopia: blue cones lacking or faulty – world seen in reds and greens, no blue 20 Those with normal color vision should read the number 8. Those with redred--greengreen color vision deficiencies (protanopia, deuteranopia) should read the number 3. Total color blindness should not be able to read any numeral. The trichromatic theory doesn ’’t tell the whole story… 21 The trichromatic theory doesn ’’t tell the whole story… The retinal ganglion cells code for complementary colors. This is known as opponentopponent--processprocess coding Another type of ganglion cell only encodes brightness: ‘‘blackblack--whitewhite ’’ 22 Opponent Process Theory Ganglion cells –– Three types Red/green, yellow/blue, black/white –– Each cell represents an opponent process system Resting behavior in red/green cells is midmid--levellevel rate of response For R+GR+G--,, rate increases when red is present, decreases when green is present (opposite for RR--G+)G+) Yellow/blue (Y+B(Y+B--)) increases when both red and green are present, decreases when blue is present Opponent Process Theory Opposing retinal processes enable color vision ““ONON ”” ““OFF ”” red green green red blue yellow yellow blue black white white black 23 ReddishReddish--green?green? BluishBluish--Yellow?Yellow? You can ’’t imagine them because ganglion cells that signal red or green (or yellow or blue) can only increase or decrease rate of firing, they can ’’t do both at once!! 24 The complementary afterafter--effecteffect is caused by the fact that after adaptation, locations stimulated by green light will be less sensitive to green than to red, and vice versa. Since white light contains all colors and stimulates all photoreceptors equally, those that have been ““green adapted ”” will fire ‘‘redred ’’ to white light (and vice versa)versa)--aa larger ““redred ”” than ““green ”” signal will be generated. It ’’s the local imbalance between the red and green inputs to the opponent mechanism that generates the (relatively weak) color afterafter--effecteffects.s. We haven ’’t even reached the cortex yet! Primary visual cortex (Striate Cortex, V1) Visual Association cortex (extrastriate) 25 Primary Visual Cortex (V1, Striate Cortex) ~140 million neurons just in V1! Retinotopic Organization Information received at adjacent portions of the retina remains adjacent in V1. More cortex is devoted to areas of high acuity. (Just like the disproportionate representation of sensitive body parts in somatosensory cortex!) About 25% of primary visual cortex is dedicated to processing input from the fovea. 26 Striate Cortex Six principal layers of striate cortex Processing in Striate Cortex Layers 2 and 3 receive information from the parvocellular layers and koniocellular layers of the LGN. Cells are grouped together in ““blobs ”” –– Cells within blobs are sensitive to color –– Cells outside blobs are sensitive to orientation, movement, binocular