Chapter 6 Vision OUTLINE Light The Eye Sensation Brain must process information from both within and outside the body Sensory receptors receive transduce and transmit information about the environment Light Stimulus for vision is light Visible light is a form of electromagnetic radiation Wavelength Color or shades of gray Amplitude Brightness The Electromagnetic Spectrum Light Interacts with the Environment Reflection The bending of light back towards it source accounts for most of the light we see Absorption Object retains light cannot be seen but can be felt Refraction The changing of light at a boundary such as that between air and water responsible for forming images in the eye Color Unlike size or shape color is not an intrinsic characteristic of objects Color reflects the sum of wavelengths that are selectively absorbed and reflected by the object The Eye Optical Functions capture light form detailed spatial images The Eye Protective Functions Eyelid protects and cleans Tears produced from lacrimal gland The Eye Neural functions transduce light into neural signals then relay process those signals Visual Hemifields The Eye Landmarks of the Retina Central vision Peripheral vision The Fovea Fovea high visual acuity Blind spot Where optic nerve blood vessels exit the retina No photoreceptors no vision The Retina Photoreceptors Notice how light travels through several cell layers before it reaches the photoreceptors in the back of the retina T he Retina To optic nerve Photoreceptors Amacrine Cell Retina Lens Rod Cone Bipolar Cells L ight Cornea Axons of Ganglion Cells L ight moves through four levels of transparent neurons which consist of the ganglion amacrine bipolar and horizontal cells Horizontal Cell Ganglion Cells V isual information is processed through the four layers of neurons Light moves through eye Pigmented Epithelium The brain receives this information via axons joining at the ganglion cells T owards the back of the retina the discs in the rods and cones absorb the light entering the eye Processing of visual information Rods and Cones These light sensing neurons make up 70 of all of the sensory cells in our body Vision is very important to humans Rods and Cones Rods Scotopic vision dim light No color low acuity Human 120 million Cones Photopic vision bright light Color high acuity Human 6 million Rods and Cones Highest density of cones is at the fovea Primates are only species whose fovea contains only cones Rods and Cones Transduction translation of physical stimulus into electrical signals used by the nervous system Outer segment of photoreceptors filled with light sensitive chemicals called photopigments Rods Rhodopsin composed of opsin retinal retinal is light absorbent Cones Iodopsin Retinal 11 cis retinal is bound to opsin Retinal 11 trans light transforms breaks apart retinal and opsin The Dark Current The resting potential of rod outer segment in complete darkness is about 30 mV Sodium channels are kept open by cGMP a second messenger The Dark Current Light breaks down Rhodopsin releasing enzymes that break down cGMP With less cGMP fewer sodium channels remain open and receptor hyperpolarizes Transduction in the Rod DEPOLARIZED HYPERPOLARIZED The Dark Current In the dark photoreceptors are depolarized In the light photoreceptors are hyperpolarized Bright light leads to greater hyperpolarization Dim light leads to less hyperpolarization Photoreceptors produce graded potentials not action potentials Rods Photoreceptors in the Retina Cones Rods Scotopic Vision Cones Photopic Vision Number of receptors per eye 100 million 4 million Location Periphery Fovea Shape Long cylinder Short pointy Sensitivity to Dim Light Good Poor Acuity Poor Good Convergence High Low Color No Yes Photopigment Rhodopsin Iodopsin LOW CONVERGENCE YIELDS A SHARPER IMAGE BETTER ACUITY The Responses of Rods and Cones to Different Wavelengths EXPLAINS DIFFERENCES BETWEEN RODS AND CONES IN COLOR VISION The Responses of Rods and Cones to Different Wavelengths Rhodopsin rods breaks apart after absorbing only a little light sensitive to low levels of light Iodopsin cones breaks apart only in presence of bright light Cellular Pathway Both cell types integrate information from PRs bipolar and ganglion cells Bipolar Cells Receives input from photoreceptors Horizontal cells Provides output to Amacrine Ganglion cells Bipolar cells also produce Graded potentials instead of action potentials Bipolar cells are key in identifying light dark contrast rather total amount of light basically boundaries Bipolar Cell Receptive Field Center direct input from single set of photoreceptors Surround indirect input from horizontal cells connected to photoreceptors Bipolar Cell Receptive Field Light in center of receptive field ON CENTER CELL bipolar cell depolarized OFF CENTER CELL bipolar cell hyperpolarized Bipolar cells produce graded potentials not action potentials Bipolar Cell Receptive Field ON CENTER CELL Light restricted to the center depolarizes bipolar cell Light restricted to surround relayed through horizontal cells hyperpolarizes bipolar cell X PR releases less glutamate which depolarizes bipolar X PR releases less glutamate hyperpolarizing horizontal cells which releases less GABA back onto PRs increasing PR glutamate release and hyperpolarizing bipolar Bipolar Cell Receptive Field ON CENTER CELL OFF CENTER CELL Light restricted to the center depolarizes bipolar cell Light restricted to the center hyperpolarizes bipolar cell Light restricted to surround relayed through horizontal cells hyperpolarizes bipolar cell Light restricted to surround relayed through horizontal cells depolarizes bipolar cell Bipolar Cell Receptive Field ON CENTER OFF CENTER Antagonistic centersurround organization Bipolar cell s response depends on amount of light falling on its center relative to amount of light falling on its surround Cellular Pathway Ganglion Cells Receives input from Amacrine Bipolar cells Generates action potentials more like a standard neuron Provides sole output of visual information to brain 126 million photoreceptors converge onto 1 million ganglion cells Ganglion Receptive Fields Photoreceptors Bipolar cells Ganglion cells On center bipolar cells converge onto on center ganglion cells Off center bipolar cells converge onto off center ganglion cells On and Off Center Ganglion Cells Ganglion Cell Receptive Field Antagonistic center surround organization aka Lateral Inhibition Light across whole