Introduction to Physiological Psychology Vision ksweeney@cogsci.ucsd.edu 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
n When light passes from one medium to another it changes speed- it becomes refracted. n That is, the angle changes. Light! Light! n White light is made up of lights of all the different colors of the rainboweach of which gets refracted differently n 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 information (temperature, touch, position) from environment or joints and muscles to the CNS Communicate sensory information (visual, auditory) from environment to the CNS Interneurons communicate from neuron to neuron; Motor neurons communicate information to muscles and 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: bring info from photoreceptors to the n Ganglion 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 n Only cones are found at the fovea!! Distribution of rods and cones 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