1. The Human Eye and Vision 2 (Processing The Image)

2. Plexiform Layer The retina is made of three layers:
Plexiform layer is a network of nerves which carry the signals from the photo receptors.
Photo receptors.
Choroid provides nourishment to the receptors, as well as absorb any light that didn’t get absorbed by the photo receptors, like a antihalation backing in film.
Fovea
Photo receptors
Light
Plexiform Layer
The retina is made of three major layers. The plexiform layer is a network of nerves which carry the signals fom the photo receptors. The eye is actually constructed kind of backwards; the light must pass through several layers of cells before it can be fetected by the photo receptors. The back of the photo receptors is the choroid, which provides nourishment to the receptors, as well as absorb any light that didn’t get absorbed by the photo receptors. This is so that the light doesn’t bounce off the back and get absorbed by the receptors, just like the anti-halation backing in film. The nocturnal animals like cats need to respond to low light levels, so rather than choroids, they had a reflective tapetum lucidum, where the light bounces off the back to be absorbed by the reflectors. This is why the cat’s eye reflect light in the dark.
Optic Nerve
Choroid

3. Rods and Cones
Synaptic endings
Cell nucleus
Inner segments
Outer segments
Rod
Cone
The rods and cones are a special type of nerve cells sensitive to light. The rod is called a rod because it looks like a rod in reality. It is highly sensitive to low light level or scotopic conditions. The rod is sensitive to all the visible wavelengths, and it is dispersed in the periphery of the retina.
The cone is sensitive to high light level or photopic conditions. There are three types of cones responsible for the color vision. More on color vision in the later sections. The cones is concentrated in the fovea.
Highly sensitive to low light level or scotopic conditions.
Black and white.
Dispersed in the periphery of the retina.
Sensitive to high light level or photopic conditions.
Three types of cones responsible for color vision.
Concentrated in the fovea.

4. Threshold of detection
Adaptation
Why can’t you see immediately after you enter a movie theater from daylight?
The threshold of detection changes with overall light level.
The switch is quite gradual, until the sensitivities of cones and rods cross over at about 7 minutes in the dark.
Photopic (cones)
Scotopic (rods)
Threshold of detection
(log scale)
This is why you can’t see immediately after you enter a dark movie theater from outside where it is light out. They photoreceptors had adapted to the light level outside, so the sensitivity of the receptors became very low. It takes several minutes for the receptors to replenish the bleached rhodopsin disks.
It generally takes about 7 minutes for the eye to switch from the photopic vision to scotopic vision. 5 10 15 20 25 30
Time in dark (minutes)

5. Distribution of Photoreceptors
Visual Axis
Temporal
Nasal
Cones are concentrated in the fovea.
Rods predominate the periphery.
There is a blind spot where there are no photoreceptors, at the point where the nerves exit the eye (optic nerve).
20 º
40 º
60 º
80 º
80º
0 º
Blind spot
160
140
Rods
120
Number of receptors per mm2
100
The receptors are not equally distributed in the retina. The cones are concentrated in the fovea, where the photopic vision is the most acute. The rods predominate the periphery. This is why the stars seem to disappear if you stare at it- the rods are much more sensitive to low level of light and they are concentrated off axis about 20 degrees.
In addition, the eye has a large blind spot where there is no photo receptors. This is where all the nerve endings from the retina bundles and exits the eye. So how come we don’t see a big hole in the scenes we see? The visual cortex which is the part of brain responsible for processing vision fills the hole. More on perception later. 80 60 40
Cones 20
60 º
40 º
20 º
0 º
20 º
40 º
60 º
80 º
Angle

6. Human Vision Human Cone Response to Color
three cone types (S,I,L) correspond to B,G,R
400
460
530
650
600
700
500
Wavelength (nm)
Relative response
Blue
Cyan
Green
Red
490 I L S
This is a graph of the individual cone sensitivity for different wavelengths.
“S” Cones mainly detect in the blue region of the spectrum. Likewise, the “I”cones that peak in the green region, and the “L” cones respond mostly to the red region.

7. Retina The retina is made of network of nerve cells.
Light
Cones
Rods
To optic nerve
Bipolar
cells
Amicrine
cells
Ganglion
cells
Every photo receptor is not necessarily connected directly to the brain. If this was the case, a person would need close to 30 pounds of gray matter! (is that true>???) In order to reduce the amount of information carried to the visual cortex, the retina is cross linked by a complicated network of nerves.
Horizontal
cells
The retina is made of network of nerve cells.
The network works together to reduce the amount of information in a process called lateral inhibition.

8. Hermann Grid Illustrates lateral inhibition.
Hermann grid best illustrates lateral inhibition. Stare at the tiny black dot at the center of the figure and notice that illusory dark spots appear at the other intersection of the vertical and horizontal white bands. The effect is greater in your periphery vision, where lateral inhibition acts over greater distances.
Illustrates lateral inhibition.

9. Hermann Grid A B
Hermann grid best illustrates lateral inhibition. Stare at the tiny black dot at the center of the figure and notice that illusory dark spots appear at the other intersection of the vertical and horizontal white bands. The effect is greater in your periphery vision, where lateral inhibition acts over greater distances.
Point A looks darker because there are 4 inhibitory inputs
Point B looks lighter because there are only 2 inhibitory inputs

10. Mach Bands Actual brightness Perceived by you
Mach bands also illustrate lateral inhibition. Even though the brightness from each gray steps, you perceive it as though the grays are lighter or darker near the borders between the grays.
Perceived by you