Slide 1Primary Visual PathwaysStructure of the EyeStructure of the RetinaColor VisionThe Visual System is BackwardBefore A Photon StrikesSlide 8Connection to Ganglion CellsCenter/ Surround Receptive FieldsSlide 11Receptive Fields for Retinal Ganglion CellsPointillismDistribution of Rods and ConesAcuity vs. SensitivityPhotopic SystemScotopic SystemVisual PathwayVisual Fields Are SeparatedThalamusLateral Geniculate Nucleus (Thalamus)Parallel Processing in LGNPrimary Visual CortexReceptive Fields in Primary Visual CortexOrganization of Primary Visual CortexComplex and Hypercomplex Receptive FieldsHeirarchical OrganizationMore Visual ProcessingRepresentation of Words in Temporal LobeProsopagnosiaHemifield NeglectBalint’s SyndromeFocusing Visual AttentionRole of the Superior ColliculusSmooth Pursuit Eye MovementsVisionFebruary 5 & 8Primary 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.•The Na+ channels close and the rod or cone hyperpolarizes and stops releasing neurotransmitterA Photon StrikesConnection to Ganglion CellsCenter/ Surround Receptive Fields+-+-•P retinal ganglion cells–Receive input from cones–Sensitive to color (red/green/blue)•M retinal ganglion cells–Receive input from rods–Sensitive to black and white•Non-P-or-M retinal ganglion cells•These three types of retinal ganglion cells have center/surround receptive fields•Photosensitive ganglion cells •Ganglion cells that are important for eye movementsFive types of Retinal Ganglion CellsReceptive Fields for Retinal Ganglion Cells•M retinal ganglion cells–respond to light/dark differences–input from rods in periphery•P 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–Low convergence–High acuity–Good daylight vision•Scotopic system–In the periphery–High convergence–Low acuity–Good night visionPhotopic SystemLight cones P retinal ganglion cell- color vision - 80% of neurons (trichromatic theory) - fovea - center/surround- high acuity receptive field- low sensitivity- daylight+--+Scotopic SystemLight rods M 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 chiasmThalamusLateral Geniculate Nucleus (Thalamus)•Multiple layer structure (6 layers in humans)•Right eye and left eye in different layers•Receptive fields are center/surround with on-centers and off-centers processed in different layers•Only 20% of input to LGN from retina; 80% is from the rest of the brainLRRLOn centerOff centerParallel Processing in LGN•Top four layers have medium size cells (parvocellular) and get input from P retinal ganglion cells (color – fovea – photopic system)•Bottom two layers have big cells (magnocellular) and get input from M retinal ganglion cells (b/w – periphery – scotopic system)•Areas between layers called koniocellular receive input from Non-P-or-M ganglion cellsLRRLOn centerOff centerPrimary Visual Cortex•From the LGN visual information is sent to primary visual cortex at the posterior tip of occipital cortex.Receptive Fields in Primary Visual Cortex•The convergence of LGN cells with adjacent off-center/on-surround receptive fields produces a primary visual cortex neuron that responds to a bar at a specific orientation at a specific location in visual space•This is a “simple” visual receptive field first described by Hubel and Wiesel+-+-+-+-+-+-+-+-+-+-+-=Organization of Primary Visual Cortex•Primary visual cortex has alternating occular dominance columns•Within a column cells have different orientation selectivity and respond to different color combinationsComplex and Hypercomplex Receptive Fields•Cells with “complex” receptive fields respond to a bar at a specific orientation anywhere within a larger receptive field (simple →complex)•“Hypercomplex” fields include inhibition in one portion of the field (complex →hypercomplex)RespondDoesn’t Respond+_+_Heirarchical Organization•Primary visual cortex (V1) has simple receptive fields•Secondary visual cortex (V2) has complex and hypercomplex receptive fields“What” or object stream“Where” or spatial streamMore Visual Processing•Visual information appears to be processed in two streams in cortex•The dorsal stream in the posterior parietal lobe is concerned with “where” things are. •The ventral stream in temporal cortex analyzes “what” things are. Primary Visual