the human visual system vision class 2006-7 the main topics covered here were: photoreceptors and...
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The Human Visual System
Vision Class 2006-7
The main topics covered here were: photoreceptors and ganglion cells in the retina. The photoreceptors are the
input, the ganglion are the output to the brain. We also discussed the role of the ganglion cells in edge detection.
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The Human Brain: Overall View Visual information goes from the eyes via the LGN in the thalamus to the
back of the cortex
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Each half of the visual field goes to a different hemisphere of the brain
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Visual pathway
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Hemifield neglect
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Eye
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Recording
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Recording from a Neuron
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Neurons
Purkinje cells from cerebellum, dendrites showing calcium concentration
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Synaptic connection
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Synapse
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Receptors Density - Fovea
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Photoreceptors
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Photo response
• Retinal (the chromophore) + opsin (rodopsin, cone-opsin)• 11-cis retinal + photon → all trans retinal • Activates transducin (a G-
protein) • Activates phosphodiesterase • Breaks cGMP • cGMP keep ionic channels
open, now some close • (one retinal → 500 cGMP) • Cell hyperpolarization
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Image Capture
• Huge dynamic range 10-8 – 10+6 μW/cm2
• Photons: poisson process.
• Noisy at low levels
• For low light: large receptors, slow integration
• Rods/cones, local adaptation,change of amplitude and time constant, motion deblur
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Dynamic Range
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Dynamic Range
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Visual receptor types
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Retina Mosaic
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Color Mixing
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Edge Detection by Z.C.
Peak in f’(x) z.c. in f’’(x) Noise: Asin(wx) > -w2cos(wx) Convolve with a Gaussian (optimal) d2/dx2(G*f) = d2/dx2(G) * f
In 2-D: d2/dx2 + d2/dy2
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Edge Detection by Z.C.
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Z.C. at Two Scales
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Wheel Edges 4 Resolutions
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