Vision February 5 8 Primary 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 lobe Structure 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 directed Structure 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 cells Color Vision Cones are sensitive to different colors of light red green blue which is the basis for the trichromatic theory of color vision Rods most sensitive to blue green primarily detect whether light is present or not The 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 Strikes 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 neurotransmitter Connection to Ganglion Cells Center Surround Receptive Fields Five types of Retinal Ganglion Cells 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 movements Receptive 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 Pointillism Distribution 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 night Acuity 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 vision Photopic System Light cones color vision P retinal ganglion cell 80 of neurons trichromatic theory fovea center surround high acuity receptive field low sensitivity daylight Scotopic System Light rods color blind periphery M retinal ganglion cell 20 of cells 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 lobe Visual Fields Are Separated Right visual field 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 L L R R Optic chiasm L R Right visual field Thalamus Lateral Geniculate Nucleus Thalamus L On center R Off L center R Multiple layer structure 6 layers in humans Right eye and left eye in different layers Receptive fields are center surround with oncenters and off centers processed in different layers Only 20 of input to LGN from retina 80 is from the rest of the brain Parallel Processing in LGN L On center R Off L center R 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 cells Primary 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 combinations Complex 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 Respond Doesn t Respond Hypercomplex fields include inhibition in one portion of the field complex hypercomplex Heirarchical Organization Primary visual cortex V1 has simple receptive fields Secondary visual cortex V2 has complex and hypercomplex receptive fields More Visual Processing Where or spatial stream Visual information appears to be processed in two streams in cortex Primary The dorsal stream in Visual Cortex the posterior parietal lobe is concerned with where things are The ventral stream in What or object stream temporal cortex analyzes what things are Representation of Words in Temporal Lobe Activation of cortical areas from fMRI studies Prosopagnosia Damage to this area near the ventral surface of the temporal lobe produces visual agnosias prosopagnosia inability to recognize faces Hemifield Neglect A parietal lesion on one side causes to a neglect syndrome in which patients do not acknowledge anything that occurs on the side of the body opposite the lesion Balint s Syndrome Bilateral parietal lesions can cause a loss of object based attention In Balint s syndrome patients have less of an ability to attend to multiple objects perceive objects one at a time Here red or green until connected by a line and then just one object Focusing Visual Attention Distance Smooth pursuit Target Saccade Time The solution to acuity vs sensitivity in the retina produced the fovea but this gives us tunnel vision Need to move eyes to focus attention Two types of movements 1 Saccades jumps 2 Smooth pursuit Role of the Superior Colliculus Cortex LGN Superior colliculus eye The superior colliculus receives direct input from the retina and also from all areas of cortex not just visual Makes decision where to move eye to