Light and its transduction by human eye
18.6.12
Light
• Light is an electromagnetic radiation that consists
of photons each of energy
E = hν = h c / λ
• Visible light is an electromagnetic radiation that
is visible to the human eye, and is responsible for
the sense of sight
Interaction of light with matter
• Light when interacts with an
object, it can either be absorbed,
reflected or transmitted
• It can also undergo change in
direction due to change in speed
(gets refracted) when moving
from one medium to another
Visible light
• Visible light has a wavelength in the range of about 380
nanometres to about 740 nm – between the invisible
infrared, with longer wavelengths and the invisible
ultraviolet, with shorter wavelengths
Functioning of eye
• As light enters the eye, it first passes through the cornea,
the clear outer portion of the eye. Because the cornea is
curved, the light rays bend, allowing light to pass through
the pupil to the lens.
• The iris, or colored part of the eye, regulates the amount of
light that enters the eye with the ciliary muscles. These
muscles cause the pupil to contract when exposed to excess
light or to dilate when there is too little light.
• When light hits the curved surface of the lens, it is refracted
and brought into focus on the retina.
• The retina then turns the light into electrical energy. This
energy passes through the optic nerve to the brain stem and
finally into the occipital lobe, where it is converted into an
image.
The Retina
Photoreceptor of Eye: Rods and
Cones
– Rods.
• primarily for night vision & perceiving
movement.
• sensitive to broad spectrum of light but
most sensitive to blue green light
(wavelength of 500nm)
• can’t discriminate between colors.
• sense intensity or shades of gray
• Scotopic vision.
– Cones.
• used to sense color
• Photopic vision.
Photoreceptor of Eye: Rods and
Cones
• Cones: for more precise
vision, need strong light.
help to see colors.
Mostly distributed in the
center of the retina
(fovea).
• Rods: for peripheral and
night vision. Sensitive to
light. Mostly distributed
away from fovea.
Color Perception
• Perception of color begins with specialized retinal cells
containing pigments with different spectral sensitivities,
known as cone cells.
• In humans, there are three types of cones sensitive to three
different spectra, resulting in trichromatic color vision
• The three types of cone cells are sensitive to a range of
wavelengths with peak centered at 430nm, 550nm and 600nm
• The cones are conventionally labeled according to the
ordering of the wavelengths of the peaks of their spectral
sensitivities: short (S), medium (M), and long (L) cone types.
Color Perception
• A beam of light that contains mostly short
wavelength blue radiation stimulates the cone cells
that respond to 430nm light to a far greater extent
than the other two types of cones
• This beam will activate the pigment in specific Scones and that light is perceived as blue
• Light with a majority of wavelengths centered
around 550nm is seen as green and that around
600nm is seen as red
• When all the three types of cones are stimulated
equally, the light is perceived as white
Color perception
% Photons Reflected
The light that is reflected from various objects reaches our
eye. This light is absorbed by the pigments in our eyes. Some
examples of the reflectance spectra of surfaces
Red
400
Yellow
700 400
Blue
700 400
Wavelength (nm)
Purple
700 400
700
Subtractive Color Mixing
• Painting on a white surface reduces amount of
frequencies reflected
Courtesy of Peter K. Kaiser
March 16
Additive Color Mixing
• Different light frequencies are overlapped
Fun Things in Vision

Kanizsa Illusion

March 16
If you look
carefully you
will probably
see the edges
of the entire
triangle.
Fun Things in Vision

Perspective Illusion


March 16
Which colored block
looks the largest?
They’re all the same
size
Fun Things in Vision

Reversible Figures



March 16
Comes from Figure
Ground Perception
A vase or two faces
looking at each other
Depends on what is
perceived as
background
Photoreceptor Cells in the dark
• Photoreceptor cells are strange cells because they
are depolarized in the dark, meaning that light
hyperpolarizes and switches off these cells, and it
is this 'switching off' that activates the next cell
and sends an excitatory signal down the neural
pathway
• In the dark, cGMP levels are high and keep
cGMP-gated sodium channels open allowing a
steady inward current, called the dark current.
This dark current keeps the cell depolarized at
about -40 mV
• The depolarization of the cell membrane opens voltagegated calcium channels. An increased intracellular
concentration of Ca2+ causes vesicles containing
neurotransmitters, to merge with the cell membrane,
therefore releasing the neurotransmitter into the synaptic
cleft
• The neurotransmitter Glutamate that is released from the
photoreceptors in the dark binds to metabotropic glutamate
receptors (mGluR6), which, through a G-protein coupling
mechanism, causes non-specific cation channels in the
cells to close, thus hyperpolarizing the bipolar cell.
Photoreceptor cells in light
• Rhodopsin consists of cis retinal (Vitamin A) and opsin
• Light photon interacts with the retinal in a photoreceptor
cell changing its conformation from the 11-cis to all-trans
• Retinal no longer fits into the opsin binding site. yielding
opsin and all-trans retinal.
• The opsin activates the regulatory protein transducin. A
series of other reactions activates phosphodiesterase
• Phosphodiesterase breaks down cGMP to 5'-GMP. This
lowers the concentration of cGMP and therefore the
sodium channels close
Phototransduction in retina
• Closure of the sodium channels causes hyperpolarization of
the cell due to the ongoing potassium current.
• Hyperpolarization of the cell causes voltage-gated calcium
channels to close. As the calcium level in the photoreceptor
cell drops, the amount of the neurotransmitter glutamate
that is released by the cell also drops. This is because
calcium is required for the glutamate-containing vesicles to
fuse with cell membrane and release their contents.
• A decrease in the amount of glutamate released by the
photoreceptors causes depolarization of bipolar cells which
release neurotransmitters to activate ganglian cells which
fire action potentials
Dark versus light
Summary
In dark:
• High intracellular levels of Cyclic Guanosine
monophosphate (cGMP)
• •Constant Na+ influx through cGMP gated channels.
Depolarized photoreceptor cells
• Neurotransmitter released from these cells hyperpolarizes
bipolar cells
• Ganglian cells do not fire
Summary
In Light:
• Light stimulates retinal in rhodopsin to
conformation
• Released opsin activates transducin
• Transducin exchanges GDP for GTP
• Transducin activates Phosphodiesterase
• PDE breaks down cGMP to GMP
• Intracellular levels of cGMP drop
• cGMP gated Na+ channels close
• Sodium cannot pass, cell becomes hyperpolarized
change
Summary
In Light:
• Glutamate decreases
• Bipolar cells depolarizes and release
neurotransmitters
• Ganglian cells fire
Return to dark state
• Retinal converts back to cis configuration using arrestin,
rhodopsinkinase, and GTP
• Transducin reassembles, GDP remains bound
• Phosphodiesteraseis inactivated
• Guanylatecyclase makes more cGMP
• cGMP gated Na+ channels re-open
• •Sodium is able to flow into the cell, depolarizing it again!