Lecture 9 chapter 6 Light and Principles of Optics Structure of the Eye Phototransduction a Perceptual systems 1 Perceptual and Distal Stimuli distal stimulus from information in the proximal stimulus infer the nature of the 2 The Elecetromagnetic Spectrum a Visible light is a small part of the electromagnetic spectrum i But is a large fraction of the energy reaching the earth s surface b Blue is more energetic than red a Light exhibits wave properties as well as particle properties 3 Principles of Optics i Wave properties of light such as refraction and diffraction are important for understanding optical systems such as eyes b Properties of a Convex Lens i The principle axis PA is a line through the center of curvature rays converge at the principle focus PF 1 Focal distance distance to the principle focus ii The refractive properties of a lens allow it to form an image 1 Image shows upside down and backwards on the retina iii The image enlarges and moves farther from the focal plane as the object moves closer iv An image cannot be formed if the object is located at the focal distance 1 Rays would never come together so the image will never be seen cannot form a sharp image c Human Eye i The human eye contains a cornea and lens 1 Refract light ii Sclera 1 Outer collagenous layer 1 Filled with aqueous humor 2 Recycled via the Canal of iii Anterior Chamber a Recycles the fluid of the eye Schlemm d Lens Accommodation e Extraocular Movements i Accommodation allows close objects to be brought into focus i Eye movements are controlled by 6 pairs of extraocular muscles ii Two kinds of eye movements exist 1 Slow under control of reflexes involuntary 1 a Such as compensatory eye movements produced by head movement are involuntary i Eye moves in opposite direction of your head as you turn your head left your eyes move right ii Compensatory complemented by feedback of the visual system b Include smooth pursuit vergence and the slow phase of the optokinetic response i Smooth pursuit follow it as it moves you can keep it in focus ii Vergence iii Optokinetic back onto another Lock onto a target and stick with it as it moves if something is moving towards you the lines converge so pick target follow as far as you can then shifts iii Fast 1 Saccades only ones that are voluntary that we control are voluntary fast eye movements 4 Photopigments a Light is transduced to neural signals via the mediating actions of photopigments b These molecules change molecular structure when they absorb light energy c Example i Rhodopsin 1 Synthesized from a Retinal a chromophore i Itself doesn t absorb light but will get energy that opsin lets through b Opsin a 7TM receptor protein molecule 2 Spectral sensitivity peaks near 500 nm d Retinal changes from the cis to trans configuration and separates from opsin after photon capture by opsin i Will straighten out when absorbs energy in photon 5 Vertebrate Phototransduction a Normally many Na channels are open in the membrane of photoreceptor cells b Opsin can absorb photons starting the transduction cascade c Retinal will detach from rhodopsin after photon capture changes shape and now activates the protein activates rhodopsin d Opsin can activate transducin a G protein by collision coupling Gt e Transducin activates phosphodiesterase which converts cGMP to GMP degrades cGMP and closes a Na channel f Thus vertebrate photoreceptors hyperpolarize to light You should know What are the optical properties What is the basic structure of the human eye What are the reaction cascades involved in of a convex lens phototransduction Lecture 10 chapter 6 Structure of the retina Types of photoreceptors 2 Dark adaptation Color vision 1 The human retina a The multi layered retina contains the photoreceptors and associated neural cells ganglion cells cells whose axons form the optic nerve and go to the brain i Photoreceptors where phototransduction occurs 1 Rods a Contain rhodopsin in membranous disks in the outer segments b Function at low levels of illumination i Glutamate is released when light is detected 2 Cones a Contain photopsins in outer segments b Function at day levels of illumination 2 Spectral Sensitivity a spectral filters a Three kinds of cones exist each maximally a part of the opsin molecule acts sensitive in a different part of the wavelength spectrum blue green red short medium long 3 The Dark Current a In the dark but not the light current circulates from the outer segment to the cell body b Dark Adaptation i The sensitivity of both rods and cones increases in the dark ii Both the time course of dark adaptation and maximum Sensitivity of rods and cones differ 1 Rods require more than 30 minutes for dark adaption if exposed to high levels of light illumination 4 Outer plexiform layer a Photoreceptors synapse with bipolar and horizontal cells in the outer plexiform layer i horizontal mostly inhibitory b Cones form both depolarizing and hyperpolarizing synapses with bipolar cells i They are all releasing glutamate depends on level of glutamate 1 Glutamate can produce inhibition through metabotropic receptors form complex synpases with amacrine and ganglion cells whose axons 5 Inner plexiform Layer a Bipolar cells form the optic nerves i Amacrine ii Ganglion mainly interested in movement final pathway everything retinal knows is being transmitted by the ganglion cells 1 Some will care about color some not some intensity some brightness some not each ganglion is super specialist and doesn t know the big picture 6 Center Surround Receptive Fields a Organization emerges the in inner retina Bipol ar Cells 3 Light b Ganglion cells exhibit center surround receptive fields c Ganglion Cell Receptive Fields i Both on and off center center surround receptive fields exist 7 X and Y Ganglion Cell types a X cells i Ganglion cells with center surround receptive field organization exhibiting linear spatial tuning ii Physiology 1 Can code spatial frequency Y Cannot b Y cells i Do not exhibit center surround organization ii Respond to any change in stimulation and do not exhibit linear summation iii Respond well to moving stimuli in specific directions like magno a Helmholz trichromatic theory 8 Color vision human color vision consistent with the three cone types i Predicts that all colors can be mixed by varying proportions of these hues ii Theory cannot predict afterimage effect or manifestations of color blindness i 3 categories proposed 3 visual opponent process channels