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Introduction to Physiological Psychology Vision [email protected] cogsci.ucsd.edu/~ksweeney/psy260.html This class… n Sensation vs. Perception n How light is translated into what we see n Structure and anatomy of the eye n Photoreceptors: rods and cones 1 Imagine… Lighta part of the spectrum of Electromagnetic Energy (the part that’s visible to us!) 2 Different ways for eyes and brains to do vision Different ways for eyes and brains to do vision 3 Take home point: n What you ‘see’ is NOT what is “out there”, but just the impression created for you by your brain “in here”!! n How does your brain do the creating? n For a human to 'see' stimuli in their external environment, several processes must occur: – Light rays are collected and focused on the retina – Visual signals are transduced (converted to neural signals) – The brain integrates the visual information and provides a perception 4 Sensation vs. perception n Sensation – The process in which the sense organs receptor cells are stimulated and bring information to the brain. n Perception – The process in which an organism selects and interprets sensory input so that it acquires meaning n Sensation – Detection of stimuli n Perception – Comprehension of stimuli Sensory Transduction n To transduce is to convert energy from one form to another. – Sensory transduction is the process by which sensory stimuli are “translated” into changes in the cell membrane potential. n In vision: what sensory stimuli? 5 Light! n When light passes from one medium to another it changes speed- it becomes ‘refracted’. n That is, the angle changes. Light! n n White light is made up of lights of all the different colors of the rainboweach of which gets refracted differently So, using a prism, white light can be separated into the spectral colors that make it up… the colors of the rainbow! 6 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. What is color ? n Hue - this is the quality that distinguishes red from blue (i.e. the hues of a rainbow.) n Brightness - describes the perceived intensity of light. n Saturation – refers to ‘purity’ of light- all one wavelength? 7 What is color ? n Hue – determined by wave length (red= longer, violet = shorter) n Brightness – determined by amplitude n Saturation – determined by purity Anatomy of an eye 8 Anatomy of an eye n The image projected on the retina is upside down and backwards! n Fovea § The region of the retina that mediates the most acute vision of birds and higher mammals. Only color-sensitive cones are found in the fovea. n Optic disk § The location of the exit point from the retina of the fibers of the ganglion cells that form the optic nerve; responsible for the blind spot. 9 n An octopus doesn’t have a blind spot! n Visual receptors have been selected for in a way that facilitates survival. – Dogs and cats see better in low-light than we do thanks to the tapetum lucidum, but their color vision isn t as rich as ours. – Hawks have greater density of receptors on top half of retina than on the bottom half – Rats have greater density on the bottom half Anatomy of an eye n The image projected on the retina is upside down and backwards! 10 Finding your blind spot Image courtesy of Michael Bach 11 Eye Movement n We continually scan the world with small and quick eye movements – saccades n The various bits of information are then integrated in higher visual areas. n If we stabilize retinal image… n Visual system (and the brain in general) is sensitive to change! n When performing a pursuit movement you can suppress saccades. Why have two of them? n Binocular disparity – What does it get us? 12 Why have two of them? n Convergence : – eyes must turn slightly inward to focus when objects are close n Binocular disparity: – difference between the images on the two retinas n Both are greater when objects are close – provides brain with a 3-D image and distance information Accomodation n A change in thickness of the lens, accomplished by the ciliary muscles. 13 The case of farsightedness Images: 1999 Joy Wagon 14 n To see something in fine detail, you need to get the image of it on the fovea! 15 We talked about bipolar sensory neurons…. (Somato)sensory Neurons Sensory Neurons Interneurons/ Motor Neurons Communicate sensory Communicate sensory Interneurons communicate information (temperature, information (visual, from neuron to neuron; Motor touch, position) from auditory) from neurons communicate environment or joints and environment to the information to muscles and muscles to the CNS CNS glands in the body 16 – Sensory neurons § detect changes in the external or internal environment and sends information about these changes to the central nervous system. – Motor neurons § control the contraction of a muscle or the secretion of a gland. – Interneurons § located entirely within the central nervous system, between sensory and motor neurons The Retina n The retina is in a sense inside-out : – Light passes through several cell layers before reaching the photoreceptors 17 The Vertical and Lateral Pathways n Vertical pathway: photoreceptors > bipolar cells > retinal ganglion cells n Lateral communication – Horizontal cells – Amacrine cells Photoreceptors: Rods and Cones n RODS: – ~120 million rods – Scotopic Vision (skotos=darkness) – Sensitive to brightness, but not color (shades of gray) – Many rods converge onto one retinal ganglion cell – Responsible for low-light vision – Not present at all in fovea 18 Photoreceptors: Rods and Cones n CONES: – – – – ~6 million cones Photopic Vision (photos=light) Sensitive to color A single retinal ganglion cell receives signals from one (or few) cones. – Responsible for high acuity vision (fine detail) – Fovea contains only cones The Vertical Pathway n Rods: Photoreceptor cells of the retina, sensitive to light of low intensity. n Cones: Photoreceptor cells of the retina; maximally sensitive to one of three different wavelengths of light and hence encodes color vision. n Bipolar cells: n Ganglion bring info from photoreceptors to the … cells: Receive information from bipolar cells, their axons form the optic nerve. 19 Lateral communication n Horizontal cells: neurons in the retina that interconnect adjacent photoreceptors and the outer processes of the bipolar cells. n Amacrine cells: neurons in the retina that interconnect adjacent ganglion cells and the inner processes of the bipolar cells. Back of eye Front of eye The Retina 20 Rods and Cones n The outer segment of a photoreceptor contains hundreds of lamellae. n Within the lamellae you find photopigments- molecules that contain an opsin and a retinal. E.g. rhodopsin n Rhodopsin is a receptor that responds to light rather than to neurotransmitters (photons bind to it) n When rhodopsin is exposed to light, it breaks down and the opsin bleaches. The effect of a bleached photopigment… n … is that a the photoreceptor s membrane potential changes. n Receptor s membrane potential affects release of NT onto bipolar cells. n Bipolar cells speak to ganglion cells, which bring information to the brain. 21 Transduction: how light becomes neural signals n 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) Cone and Rod Vision Distribution of rods and cones n Only cones are found at the fovea!! 22 Cone and Rod Vision n Less convergence in cones, increasing acuity while decreasing sensitivity n More convergence in rod system, increasing sensitivity while decreasing acuity So we have a response from a ganglion cell… now what? n Bundle of ganglion cell axons exiting the eye: blind spot n No receptors where information exits the eye: – Visual system uses information from cells around the blind spot for completion, filling in the blind spot 23 From the Eyes to the Visual Cortex Lateral Geniculate Nucleus n A nucleus within the thalamus ( relay center ) – receives information from the retina and projects to primary visual cortex. n Contains six layers of neurons – each layer receives information from only 1 eye. n First two layers: magnocellular n Next four layers: parvocellular 24 M and P channels n Magnocellular – Larger cell bodies – Responsive to movement – Input primarily from rods n Parvocellular – Small cell bodies – Responsive to color, fine details – Input primarily from cones M and P channels n Layers 1, 4, 6- contra n Layers 2, 3, 5- ipsi n In between magno and parvo layers are koniocellular sublayers 25 From the Eyes to the Visual Cortex n 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 Primary Visual Cortex (V1, Striate Cortex) ~140 million neurons just in V1! 26