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 G-

protein) • Activates phosphodiesterase • Breaks cGMP • cGMP keep ionic channels

open, now some close • (one retinal → 500 cGMP) • Cell hyperpolarization

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

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