Slide 1Primary Visual PathwaysStructure of the EyeStructure of the RetinaColor VisionThe Visual System is BackwardBefore A Photon StrikesSlide 8Center/ Surround Receptive FieldsSlide 10Receptive Fields for Retinal Ganglion CellsPointillismDistribution of Rods and ConesAcuity vs. SensitivityPhotopic SystemScotopic SystemVisual PathwayVisual Fields Are SeparatedThalamusVisionSeptember 12, 2014Primary Visual Pathways•Neural signals from the retina travel to the lateral geniculate nucleus in the thalamus and then to primary visual cortex in the occipital lobeStructure of the Eye•Cornea and lens focus the image on the retina•Iris controls how much light enters the eye–The visual system works over 1011 orders of magnitude (only 107 for the auditory system)•Eye muscles control where gaze is directedStructure of the Retina•Receptors are rods and cones located at the back of retina•There are about 100 million rods and 4 million cones at the front of retina•The output from the retina comes from 1 million retinal ganglion cellsColor VisionCones are sensitive to different colors of light (red, green, blue), which is the basis for the trichromatic theory of color visionRods most sensitive to blue/green – primarily detect whether light is present or notThe Visual System is Backward•The image is upside down and reversed on the retina.•The receptors are at the back of the retina, behind the ganglion cells and other neurons.•Light turns off (hyperpolarizes the rods and cones).Before A Photon Strikes•In dark, cGMP keeps the Na+ channels open.•A photon spits rhodopsin into retinal + opsin.•The opsin changes cGMP to 5’-GMP, which then can not keep the Na+ channels open.•=depolarized•The Na+ channels close and the rod or cone hyperpolarizes and stops releasing neurotransmitterA Photon StrikesCenter/ Surround Receptive Fields+-+-•X retinal ganglion cells–Receive input from cones–Sensitive to color (red/green/blue)•Y retinal ganglion cells–Receive input from rods–Sensitive to black and white•Both types of retinal ganglion cells have center/surround receptive fieldsTwo Types of Retinal Ganglion CellsReceptive Fields for Retinal Ganglion Cells•Y retinal ganglion cells–respond to light/dark differences–input from rods in periphery•X retinal ganglion cells –respond to color differences–input from cones in fovea+-+-+--+-+PointillismDistribution of Rods and Cones•Cones are mainly located in the fovea•Rods are found mainly in the periphery•Problem for vision is acuity vs. sensitivity–Acuity to see details–Sensitivity to see faint stimuli at nightAcuity vs. Sensitivity•Photopic system–In the fovea--cones–Low convergence–High acuity–Good daylight vision•Scotopic system–In the periphery--rods–High convergence–Low acuity–Good night visionPhotopic SystemLight cones X retinal ganglion cell- color vision - 80% of neurons (trichromatic theory) - fovea - center/surround- high acuity receptive field- low sensitivity- daylight+--+Scotopic SystemLight rods Y retinal ganglion cell- color blind - 20% of cells - periphery - center/surround receptive field- low acuity- high sensitivity- night vision +-+-Visual Pathway•Retina projects directly to the thalamus (lateral geniculate nucleus)•From thalamus to primary visual cortex in occipital lobeVisual Fields Are Separated•Pathway from retina to thalamus crossed at optic chiasm•Only some of the fibers cross•Left thalamus and visual cortex respond to light in the right visual field, etc.Right visual fieldL RRLRight visual fieldL ROptic