More Vision September 15 2014 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 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 Parallel Processing in LGN L On center R Off L center R Top four layers have medium size cells parvocellular and get input from X retinal ganglion cells color fovea photopic system Bottom two layers have big cells magnocellular and get input from Y retinal ganglion cells b w periphery scotopic system Thalamus Only 20 of input to LGN from retina 80 is from the rest of the brain Thalamic theory of attention Crick Thalamus switches on and off different cortical areas Executive control of the switch in cortex Executive Control of Spatial Attention A network involving multiple cortical areas may mediate excitability in striate and extrastriate cortex as a function of attention 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 How 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 Prosopagnosia Damage to this area near the ventral surface of the temporal lobe produces visual agnosias prosopagnosia inability to recognize faces More Visual Processing Where or spatial stream How 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 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 Center of the Brain Longitudinal Fissure 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 focus attention There is a map of visual space in the superior colliculus and a map of auditory space Smooth Pursuit Eye Movements Smooth pursuit eye movements are made by a different system Cortex Pons Cerebellum Eye muscles Alcohol can easily affect the cerebellum which eliminates the smooth pursuit and only saccadic eye movements are possible Regions of the Brain Plan Broca Where Wernicke What Output Input