Sensory Neural Systems 5 February 2008 Rachel L. León Visual Processing Sensory Neural Systems 5 February 2008 Rachel L. León

Review Basic eye structure Sensory cells of the eye Retinal processing Eye Structure – conductive structures: cornea, lens; retina with fovea centralis (all cones); optic disc where there are no receptors projecting fibers from ganglion cells thru optic nerve Sensory cells – rods and cones – have “dark current” where cGMP gated channels along continual influx of Na+ when there is no light; photopigment that undergoes conformational change upon absorption of light energy; this causes the cell to hyperpolarize as Na+ channels close because cGMP gets depleted http://scienceblogs.com/clock/2006/06/bio101_lecture_6_physiology_re.php http://www.chemistry.wustl.edu/~edudev/LabTutorials/Vision/images/Rodcell.jpg http://www.matossianeye.com/art2/ANATONY2.jpg

Neural Coding in the Retina Ganglion cells influenced by many receptors Pattern of illumination that maximally excites ganglion cell is doughnut shaped Center-surround receptive field Lateral inhibition of receptive fields enhances boundaries X, Y, W ganglion cells Over 100 million receptor cells, only 1 million ganglion cells; receptors in center of illuminated field excite the ganglion cell, receptors in a fixed area outside the illuminated point inhibit; other ganglion cells have the opposite pattern – receptors within the illuminated area inhibit, outside the illuminated area, excite (on-center and off-center ganglion cells) 3 classes of ganglion cells based on their temporal responses to stimuli X – cells synapse onto parvocellular cells in LGN (small receptive fields), Y – synapse onto magnocellular cells (large receptive fields), W – not well characterized, mostly synapse at superior colliculus and pretectal area http://www.neurobio.arizona.edu/282/Lectures2005/Visual%20pathways/Blk3Lec2.htm

Outline Classical Visual Pathway Other Pathways Organization of optic nerves and tracts Lateral geniculate nuclei Primary visual cortex Other Pathways http://www.nature.com/nrn/journal/v6/n3/fig_tab/nrn1630_F4.html

Optic Nerve and Tract Temporal retinal fibers maintain ipsilateral position Nasal retinal fibers cross at optic chiasm Optic tract contains all information from contralateral visual field http://library.thinkquest.org/26313/eye_wo1.jpg

Lateral Geniculate Nucleus LGN receives most input from optic tracts 6 layer structure Magnocellular layers Parvocellular layers 80% of optic tract axons synapse in LGN, has 6 layers; ipsilateral fibers of optic nerve go to layers 2, 3, 5; contralateral fibers terminate in layers 1, 4, 6 Axons leaving the LGN remain divided according to magnocellular and parvocellular origins and synapse in different layers of the visual cortex http://www.physics.utoledo.edu/~lsa/_color/17_eye.htm

Gross Dissection – Human Visual Pathway LGN at #4; fibers from LGN go thru the optic radiation to reach primary visual cortex in occipital lobe (AKA striate cortex, V1, shown in #6) 1.Optic nerve 2.Optic chiasma 3.Optic tract 4.Lateral geniculate body 5.Optic radiation 6.Visual cortex 7.Superior colliculus of the midbrain 8.Putamen 9.Long association bundle - inferior occipitofrontal fasciculus 10.Pulvinar of the thalamus 11.Calcarine fissure 12.Posteroinferior horn of the lateral ventricle http://anatomy.uams.edu/AnatomyHTML/atlas_html/eye_38.html

Primary Visual Cortex Simple cells vs. Complex cells Cortical orientation columns Ocular dominance columns Blobs – receive parvocellular input Simple cells respond best to moving bar of light oriented at a particular angle and must be presented within a rectangular shaped receptive field – so many LGN neurons with overlapping receptive fields likely synapse onto a single simple cell Complex cells respond to the same stimulus but it can be located anywhere within the receptive field It seems that multiple simple cells with a given preferred bar orientation but different receptive fields synapse onto a single complex cell Each ocular dominance column contains all of the orientation columns assoc with one eye; have a regular pattern of excitation that alternates from left eye to right eye and back Blobs are dispersed among ocular dominance columns; exclusively devoted to processing color information, no orientation requirements http://www.neurobio.arizona.edu/282/Lectures2005/Visual%20pathways/10_022.jpg

Other Visual Pathways Most of the axons not going to LGN terminate in superior colliculus Some efferents from SC go to pulvinar Pulvinar efferents terminate in visual cortex other than V1 Some optic tract axons terminate in pretectal area