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

2. 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

3. 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

4. Outline Classical Visual Pathway Other Pathways
Organization of optic nerves and tracts
Lateral geniculate nuclei
Primary visual cortex
Other Pathways

5. 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

6. 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

7. 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

8. 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

9. 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