370: From the Eye to the Brain

1. From the Eye to the Brain
1. Basic Concepts
1. Neural Circuits
1. Combining convergence and lateral inhibition helps solve perceptual problems
2. Neural pathways from retina to primary visual cortex (aka, VI, striate cortex, area 17...)
2. Convergence
1. Rods are good for low-light conditions because they have more convergence
3. What does Lateral Inhibition get us?
1. Lines, an enhancement of lines
2. We have receptive fields that are sensitive to edges
3. Simultaneous Contrast
1. Receptors stimulated by bright surrounding area send a large amount of inhibition to cells in center
2. Resulting perception is of a darker area than when this stimulus is viewed alone
3. Receptors stimulated by dark surrounding area send a small amount of inhibition to cells in the center
4. Resulting perception is of a lighter arean than when this stimulus is viewed alone
4. Helps us define edges more clearly.  Why is it important to see edges?
1. Helps us know simple things like where to put our hands to grab a cup
5. White's Illusion
1. Backwards from what would normally occur with lateral inhibition
2. How you visually organize things changes the illusion
3. This is NOT a good example of lateral inhibition
2. Receptive Field
1. The area on the receptor surface that when stimulated changes the firing rate of a given neuron
2. Determined by monitoring single cell response
3. Stimulus is presented to retina and response of cell is measured by an electrode
4. Center-surround Receptive Field
1. Excitatory and inhibitory effects are found in receptive fields
2. Center and surround areas of receptive fields result in:
1. Excitatory-center/inhibitory -surround
2. Inhibitory-center/excitatory surround
3. Highest response when only the excitatory area is stimulated
4. Lowest response when only the inhibitory area is stimulated
5. Intermediate responses when both areas are stimulated
3. Lateral Geniculate Nucleus (LGN)
1. Retina TO Optic nerve (ganglia cells) TO optic chiasma TO optic tract TO Lateral geniculate cortex TO primary visual cortex (VI)
2. Right half of space goes from both eyes to the left LGN
3. Left half of space goes from both eyes to the right LGN
4. The thalamus has various nuclei for early sensory processing of different sensory modalities
5. LGN
1. knee shaped part of the thalamus
2. six layered structure
3. Does early visual processing
4. LGN has 2 halves
5. LGN Layers (each half receives signal from right and left eye)
1. Layers 2, 3, and 5 receive input from the ipsilateral eye
2. Layers 1, 4, and 6 receive input from the contralateral eye
3. C I I C I C (see I.  I see.  I see)
6. Layers 1&2 are magnocellular w/ large neurons responsible for motion
7. Layers 3-6 are parvocellular with small neurons responsible for color and detail
1. These magno and parvo cells feed into different pathways in the brain