Exam 2 Study Guide Chapter 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 by the object The eye It captures light and forms detailed spatial images Eyelid protects and cleans Tears produced from lacrimal gland Neural functions transduce light into neural signals then relay process those signals Sclera white of your eye Cornea extension of the sclera Iris controls the opening and closing of the eye also determines color of the eye melanin Blue green less melanin Brown more melanin Accommodation allows for the adjustment of refraction so that the object falls on the center of your perception Fovea 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 are no photoreceptors and no vision Optic disk Blind spot Where all the axons leave the eye Optic Nerve where the axons travel back to the thalamus Retina 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 of the retina Pigmented epithelium provide nutrients to photoreceptors All the information relayed back to the brain derives from the ganglion cells Rods 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 eye Rods Scoptic vision dim light No color low acuity What we use at night Rhodopsin composed of opsin retinal retinal is light aborbant Cones Photopic vision bright light Color high acuity What we use during the day Iodopsin Highest density of cones is at the fovea Transduction 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 light Bipolar 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 light Bipolar 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 cells Ganglion 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 detectors Summary 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 electrophysiology P type Parvo small M type Magno large 90 of ganglion cells Small receptive fields More receptive to high contrast Respond to specific colors of light 5 of ganglion cells Larger receptive fields More receptive to low contrast Respond to light regardless of color K type koniocelluar Similar to P cells Chapter 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 axons Lateral 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 cortex to modify its own visual input based on organism s state of arousal or attentiveness Antagonistic Center surrounds receptive fields and