Physics 1230: Light and Color
Ivan I. Smalyukh, Instructor
Office: Gamow Tower, F-521
Email:
ivan.smalyukh@colorado.edu
Phone: 303-492-7277
Lectures:
Tuesdays & Thursdays,
3:30 PM - 4:45 PM
Office hours:
Mondays & Fridays,
3:30 PM – 4:30 PM
TA: Jhih-An Yang
jhihan.yang@colorado.edu
Class # 18
The remaining lectures:
We
are
here
• Ch. 7 (Retina and visual perception),
• Ch. 9 & 10 (color & color perception).
2
Ch. 7 – Visual Perception
We
are
here
•
•
•
•
•
•
•
Parts of the visual processing system
Lightness and brightness
Retinal processing: Lateral inhibition
Hermann grid
Receptive field
Motion illusion
Craik O’Brien illusion &
simultaneous lightness contrast
• Other optical illusions
3
The Retina: Detecting the light and
processing the images
The retina and optic nerves are recognized as actually
parts of the brain (like your olifactory bulb in the nose).
They start development IN the brain and migrate…
Has 108 nerve endings to detect image
rods, for high sensitivity (night vision)
cones, for color and detail, 7 million
optic nerve = 106 transmission lines
fovea, region of best vision (cones)
More nerves in your retina than some creatures
have in their entire brains. Processing Power.
4
Rods and cones
• Rhodopsin, a photochemical, responds to light
It is destroyed and reformed.
Signal goes to a synapse, a gap between nerve
cells
• There are 3 kinds of cones for 3 colors
red, green, blue (more later).
A great deal is understood about how the
individual cells of the retina receive light, respond
to light, and transmit signals.
6
Rods and cones
Example: Rhodopsin and photosensitivity
Photo-responsive membrane
protein is known in atomic detail
Light drives a change in
molecular shape.
Opens/closes membrane
We will skip Most of cellular detail BECAUSE…
7
Does our understanding of the individual rods, cones,
and other cells of the retina do much to explain this?:
(A) Creitanly
(B) Myaby Not Mcuh
We need to understand how NETWORKS of
cells WORK TOGETHER to let us perceive.
8
Layers of the retina
Light
9
Layers of the retina
10
See text fig. 7.2
Layers of the
retina are
CROSS
Connected
From the following article:
Neurobiology: Bright blue times
Russell G. Foster
Nature 433, 698-699(17 February 2005)
doi:10.1038/433698a
a, The rods (R) and cones (C) convey visual information to the ganglion cells (G) through the bipolar cells (B). Horizontal cells (H)
allow lateral connections between rods and cones. Amacrine cells (A) allow lateral connections between bipolar and ganglion cells.
The optic nerve is formed from the axons of all the ganglion cells. A subset of ganglion cells (MG cells) also detects light directly; for
this, they require the photopigment melanopsin, as now confirmed1, 2, 3. b, Light, via melanopsin, causes changes in Ca2+ levels in
MG cells9 (a fluorescent Ca2+ indicator was used here). Counterintuitively, light passes through the transparent ganglion layer to
reach the rods and cones.
11
Connections and cross connections are
MOST important.
Photoreceptors: rods and cones
connected to the
bipolar cells
connected to the
ganglion cells, funnel “data” through axons into the
optic nerve
sideways connectors (these help with analysis)
horizontal cells, next to the photoreceptors
amacrine cells
12
Clicker question
The arrow points to:
A. Photoreceptors
B. Horizontal cells
C. Bipolar cells
D. Amacrine cells
E. Ganglion cells
13
Clicker question
The arrow points to:
A. Photoreceptors
B. Horizontal cells
C. Bipolar cells
D. Amacrine cells
E. Ganglion cells
14
Optic chiasma and brain structure
Brain damage on the left side
hurts vision on the right side.
15
See text fig. 7.3
Brain anatomy
Optic chiasma
Left field of view goes to right brain
Right field of view goes to left brain
from both eyes
Visual cortex is where you “see”
Brain damage at this location hurts vision.
16
Clicker question
If the left side of your brain is injured, you
might lose vision in your
A. left eye
B. right eye
C. left field of view
D. right field of view
E. some loss in left and right field of view
17
All this ‘hardware’ allows us to
perceive the world and
function in it.
Many complicated sub-systems have
developed. Let’s study a few to get
some insight into how vision works.
18
Interesting collective behavior 1: We detect
RELATIVE Lightness, not total Brightness
Brightness: amount of light
Lightness: property of a surface
newspaper = 0.65 (reflectance)
printer paper = 0.84
photo quality paper = 0.90-0.99
Total amount of light is far less important than
the relative amount of light, particularly as
compared with nearby objects.
Demo with room lights.
19
Lightness and brightness
Lightness constancy: brain and eye correct for amount of
light so that white, gray, and black look the same
independent of brightness.
Weber’s law: we think lightness is equally spaced when
the ratios are equally spaced
Example: lightness 0.5, 0.25, 0.125 look equally spaced.
These numbers are ½, ¼, 1/8 etc.
The spacing that looks equal is not 0.9, 0.8, 0.7, etc.
20
Demo: Lights on or lights off
Retinal processing that allows
Relative Lightness sensitivity:
Amacrine and horizontal cells “turn down” the signals from areas
adjacent to bright areas.
“Lateral
Inhibition”
21
See text fig. 7.5
“Receptive field”
The rods/cones and local
cells are connected in a
group:
Center of group causes
nerves to fire if illuminated.
Surrounding group causes
nerves to STOP firing if they
are illuminated.
Nerve cell
fires rapidly
Nerve cell
doesn’t fire
Nerve cell
doesn’t fire
Nerve cell
fires only a bit
22
See text fig. 7.12