Transcript PPT

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.
The Human Brain: Overall View
Visual information goes from the eyes via the LGN in the thalamus to the
back of the cortex
Each half of the visual field goes to a different hemisphere of
the brain
Visual pathway
Hemifield neglect
Eye
Recording
Recording from a Neuron
Neurons
Purkinje cells from cerebellum, dendrites showing
calcium concentration
Synaptic connection
Synapse
Receptors Density - Fovea
Photoreceptors
Photo response
• Retinal (the chromophore) +
opsin (rodopsin, cone-opsin)
• 11-cis retinal +
photon → all trans retinal
• Activates transducin (a Gprotein)
• Activates phosphodiesterase
• Breaks cGMP
• cGMP keep ionic channels
open, now some close
• (one retinal → 500 cGMP)
• Cell hyperpolarization
Image Capture
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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
Dynamic Range
Dynamic Range
Visual receptor types
Retina Mosaic
Color Mixing
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
Edge Detection by Z.C.
Z.C. at Two Scales
Wheel Edges 4 Resolutions
END