Exam 2 Study GuideChapter 6 Vision-Stimulus for vision is light-Visible light is a form of electromagnetic radiation-Wavelengh = color or shades of gray-Amplitude = Brightness-Sensory Receptors - receive, transduce, and transmit information about the environment-Reflection- the bending of light back towards its 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 reflects the sum of wavelengths that are selectively absorbed and reflected bythe objectThe eye-It captures light and forms detailed spatial images-Eyelid protects and cleans; Tears produced from lacrimal glandNeural functions- transduce light into neural signals, then relay & process those signalsSclera- white of your eyeCornea – extension of the scleraIris- controls the opening and closing of the eye, also determines color of the eye (melanin). Blue/green = less melanin Brown= more melaninAccommodation – allows for the adjustment of refraction so that the object falls on the center of your perceptionFovea- the area that most everything we see is occurring; lots of cones; high visual activity-Blind spot is where the optic nerve & blood vessels exit the retina. There areno photoreceptors and no visionOptic disk- Blind spot. Where all the axons leave the eyeOptic Nerve- where the axons travel back to the thalamusRetina- very thin layer of cells;-The photoreceptors are the inner most layer of the cells; light travels through several cell layers before it reaches the photoreceptors in the back ofthe retinaPigmented epithelium- provide nutrients to photoreceptors -All the information relayed back to the brain derives from the ganglion cellsRods and Cones-These light-sensing cells make up 70% of all the sensory cells in our body-There are more Rods than Cones in the eyeRods-Scoptic vision: dim light-No color, low acuity-What we use at night-Rhodopsin: composed of opsin % retinal; retinal is light aborbantCones-Photopic vision: bright light-Color, high acuity-What we use during the day-Iodopsin-Highest density of cones is at the foveaTransduction- translation of physical stimulus into electrical signals used by the nervous system (transformation of color into electrical signals so you can encode the information)Dark Current (sodium channels only open at night)-The resting potential of rod outer segment in complete darkness is about -30 mV-Sodium channels are kept open by cGMP, a second messenger-Light breaks down Rhodopsin, releasing enzymes that break down cGMP-With less cGMP, fewer sodium channels remain open, and receptor hyperpolarizes-In the dark, photoreceptors are depolarized-In the light photoreceptors are hyperpolarized-Photoreceptors produce graded potentials, not action potentials(Good chart on PP)-Low convergence of Cones in Cone-Fed Circuits yields a sharper image than High Convergence in Rod-Fed Circuits -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 lightBipolar Cells-Receives input from photoreceptors and horizontal cells-Provides output to Amacrine and Ganglion cells-Graded Potential, not action potential-Identifies light/dark contrast rather than total amount of lightBipolar Cell Receptive Field-The center of the receptive field directly inputs from a single set of photoreceptors-The surrounding parts indirectly input from horizontal cells connected to photoreceptors -When light selectively activates the middle, if it causes the bipolar cells to depolarize than its on-center. If it hyperpolarizes it than its off-center. *On-Center Cell-Light restricted to the center depolarizes bipolar cell-Light restricted to the surround (relayed through horizontal cells) hyperpolarizes bipolar cell *Off-Center Cell-Light restricted to the center hyperpolarizes bipolar cell-Light restricted to the surround (relayed through horizontal cells) deploraizes bipolar cell -Bipolar cell’s response depends on amount of light falling on its center relative to amount of light falling on its surround Ganglion Cells-Receives input from Amacrine & Bipolar cells-Generates action potentials -Provides sole output of visual information to the brain-126 million photoreceptors converge onto 1 million ganglion cellsGanglion Receptive Fields-On-center bipolar cells converge onto on-center ganglion cells-Off-center bipolar cells converge onto off-center ganglion cells-On center cell  depolarizes when light shines in the center-Off center cell  hyperpolarizes when light shines in the center-Gangion cells fire at a baseline rate (there always firing action potentials). Off and On centers can either increase or decrease that rate.-On-center cells: Light in center excites; light in surround inhibits. At the intersection there is more surround inhibition which decreases the center-mediated excitation and causes the intersection to appear darker.-Due to antagonistic center surround, ganglion cells are contrast detectors, not light detectorsSummary of Ganglion Cells•Receive input from bipolar cells•Conduct action potentials•Have receptive fields with antagonistic center-surround organization–Contrast detectors•Can further be classified :–M cells Motion: large low-contrast moving objects –P cells Color sensitive: small high-contrast color objects–K cells Color sensitive Types of Ganglion Cells-Based on appearance, connectivity, and electrophysiologyP-type (Parvo; small)-90% of ganglion cells-Small receptive fields-More receptive to high contrast -Respond to specific colors of lightM-type (Magno; large)-5% of ganglion cells-Larger receptive fields-More receptive to low contrast-Respond to light regardless of colorK-type (koniocelluar)-Similar to P cellsChapter 6-2 Light, The Eye, Flow of Visual Information-Where the optic nerves cross is the Optic chiasm-A small number of axons from each eye also project to the suprachiasmatic nucleaus  a structure that regulates the sleep/wake cycle based on incoming light information-Superior colliculus- Located in midbrain-Guides head and eye movements-10% of optic tract axonsLateral Geniculate Neculeus- Located in thalamus-Projects to primary visual cortex-Visual perception-90% of optic tract axons-About 80% of the input to LGN comes from the primary visual cortex-Allows