Lecture 2 – Chapter 2 Reading – First steps in vision
A little light physics
Light is a form of electromagnetic radiation – energy produced by vibrations of electrically
Two ways to conceptualize light: as a wave – an oscillation that travels through a medium by
transferring energy from one particle or point to another without causing any permanent
displacement of the medium – or as a stream of photons – a quantum of visible light or other form
of electromagnetic radiation demonstrating both particle or wave properties – both tiny particles
that each consist of one quantum of energy.
In empty space the electromagnetic radiation from a star travels in a straight line at the speed of
light, and once it reaches the atmosphere, some of the starlight’s photons will be absorbed by
encounters with dust, vaporized water, etc.; and some of the light will be scattered by these
o Most of the photons will make it through the atmosphere and will eventually hit the
surface of the object.
If the ray of starlight were to strike a lightcolored surface, most of the light would be reflected.
The fact that most of the light bounces off the surface accounts for that surface’s “light”
o Most of the light striking a dark surface is absorbed.
Light that is neither reflected nor absorbed by the surface is transmitted through the surface.
If we are gazing at our star through a window, as the light travels from air into the glass some of
the light rays will be bent, or refracted, as it is transmitted.
o Refraction also occurs when light passes from air into water or into the eyeball.
Eyes That See Light
To see, we need some type of physiological mechanism for sensing light. Eyes go beyond mere
First tissue that the light will encounter is the cornea – provides a window to the world because it
is transparent (most light photons are transmitted through it, and not being reflected or absorbed)
made of highly ordered arrangement of fibers and because it contains no blood vessels or blood,
which would absorb light.
Aqueous humor a fluid fills the space right behind the cornea, supplying oxygen and nutrients to
and removing waste from the cornea and the crystalline lens.
To get to the lens, the light must pass through the pupil, which is the hole in the muscular
structure, iris – gives the eye its distinctive color.
o The pupil controls the amount of light that reaches the retina, via the pupillary light
reflex. When the level of light increases or decreases, the iris expands and contracts to
allow more or less light into the eye.
After the lens it enters the vitreous chamber (space between lens and retina), where it will be
refracted by the vitreous humor.
o The chamber takes 80% of the internal volume of the eye.
After the vitreous humor, the light emitted is brought into focus by the retina, only some of the
light will actually reach the retina.
o Much of the light energy will have been lost in the atmosphere
o The role of the retina is to detect light and “tell the brain” about aspects of light that are
related to objects in the world.
o It is where seeing really begins.
Focusing Light onto the Retina
Refractive power of the cornea, aqueous, and the vitreous humors are fixed, so they can’t be used
to bring close objects into focus. o This is done by the lens, which can alter the refractive power by changing its shape – a
process called accommodation.
This is done by the contraction of the ciliary muscles. When they are relaxed,
the zonules (tiny fibers) are stretched and the lens is relatively flat. In this state,
the eye will be focused on very distant objects, but to focus on something closer,
the ciliary muscle must contract.
Accommodation enables the power of the lens to vary by as much as 15 diopters.
Our ability to accommodate declines with age, and between 40 – 50 years they can no longer
easily accommodate the 2.5 diopters needed to see clearly at 40cm.
o This is called presbyopia – meaning “old sight”. The loss of near vision because of
Happens because the lens becomes sclerotic (harder) and the capsule that
encircles the lens loses its elasticity.
Anything that interferes with the regularity of the crystalline will result in loss of transparency.
Opacities of the lens are known as cataracts and are caused by irregularity of the crystalline.
o They absorb and scatter more light than the normal lens does.
To focus on a distant star, the refractive power of the four optical components of the eye must be
perfectly matched to the length of the eyeball. This perfect match is called emmetropia.
o Refractive errors happen when the eyeball is too long or short relative to the power of the
o If the eye it is too long for the optics, the image of our star will be focused in front of the
retina and the star will be seen as a blur –myopia – nearsightedness.
o If it is too short for the optics, the image of our star will be focused behind the retina –
hyperopia – farsightedness; accommodation needed for near objects.
o The eye can still be longer/shorter and still be emmetropic because eyes generally grow
to match the power of the optical components we’re born with.
The most powerful refracting surface in the eye is the cornea, 2/3 of the eyes focusing power,
when the cornea is not spherical, the result is astigmatism.
o Vertical lines might be focused slightly in front of the retina, while horizontal lines are
focused slightly behind it (vice versa).
o Lenses that have two focal points – provide different amounts of focusing power in the
horizontal and vertical planes can be correct astigmatism.
The optics involved are similar to those in most cameras which also include a mechanism for
regulating the amount of light and a lens for adjusting focal length so that both near and far objects
can be focused on the film spread across the of the camera.
o The purpose of the human visual system is to interpret this image.
In the retina, where the light energy from our star is transduced into neural energy that can be
interpreted by the brain.
Doctors use an ophthalmoscope to look at the back surface of their patients eye which is called the
o There are no photoreceptors, and therefore it is blind.
o You don’t notice this large blind spot because the visual system fills it in with information
from the surrounding area.
o Only place in the body where one can see the arteries and veins directly so it provides an
important window on the wellbeing of the body’s vascular system.
o Doesn’t give a detailed view of the retina, for that you need a photomicrograph
The transduction of light energy into neural energy begins in the backmost layer of the retina,
which is made up of cells called photoreceptors. o When photoreceptors sense light, they can stimulate neurons in the intermediate layers,
including bipolar cells, horizontal cells, and amacrine cells.
o These neurons then connect with the front most layer of the retina, made up of ganglion
cells, whose axons pass through the optic nerve to the brain.
Retinal Information Processing
Retinas contain roughly 100 m photoreceptors; they capture light and initiate the act of seeing by
producing chemical signals.
The human retina contains two types: rods – specialized for night vision – and cones – specialized
in daylight vision, fine visual acuity, and color.
o They are different in shape, and have different distributions across the retina and serve
o Since we have retinas with both rods and cones, they are considered to be duplex; some
animals have mostly rods or mostly cones.
Both photoreceptors consist of an outer segment – contains photo pigment molecules – an inner
segment – lies between the outer and the cell nucleus – and a synaptic terminal – the location
where axons terminate at the synapse for transmission of information by the release of a chemical
Visual pigments are molecules that are made in the inner segment and stored in the outer segment;
they are incorporated into the membrane.
o They consist of a protein (an opsin), the structure of which determines which
wavelengths of light they absorb; and a chromophore, which captures light photons.
Each photoreceptor has only one of the four types of visual pigments found in the human retina.
o The pigment rhodopsin is found in rods, concentrated mainly in the stack of
membranous discs in the outer segment.
o Each cone has one of the other three pigments – which respond to long, medium, and
short wavelengths respectively.
Recent evidence suggests there may be a third type of photoreceptor, that are sensitive to ambient
light level and contain the photo pigment melanopsin (sensitive to ambient light), and they send
their signals to the SCN, the home of the brains circadian clock.
When a photon of light makes it way into the outer segment of a rod and is absorbed by a
molecule of rhodopsin, it transfers its energery to the chromophore portion of the visual pigment
molecule. This process is known as photoactivation.
Then it undergoes hyperpolarization – an increase in membrane potential such that the inner
membrane surface becomes more negative than the outer membrane surface.
o It closes Ca channels at the synaptic terminal, which than reduces the concentration of
neurotransmitter molecules, which signals the bipolar cells that the rod has captured a
Photoreceptors don’t respond in an allornothing fashion. They pass their information on to
bipolar cells via graded potentials instead of action potentials.
Humans have more rods than cones
o Rods are completely absent from the center of the fovea, and their density increases at a
peak of 20 degrees and then declines again.
o The cones are most concentrated in the center of the fovea, and their density drops off
dramatically with retinal eccentricity – the distance between the retinal image and the
The fovea has a high acuity and we use it to identify objects, to read, and to
inspect fine detail.
Rods and cones differ
o Rods are better under conditions of dim illumination and cones require brighter
illumination. o Since all rods have the same photopigment they cant signal differences in color, but each
cone has one of 3 differences in that differ in the wavelengths at which they absorb light
3 cone pigment