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Chapter 2.2

PSYB57H3 Chapter Notes - Chapter 2.2: Photic Sneeze Reflex, Retinal Pigment Epithelium, Ciliary Muscle


Department
Psychology
Course Code
PSYB57H3
Professor
George Cree
Chapter
2.2

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CHAPTER 2
THE FIRST STEPS IN VISION: FROM LIGHT TO NEURAL SIGNALS
Light form of electromagnetic radiation (energy produced by vibrations of electrically charged
material)
2 ways to conceptualize light:
o Wave
o Stream of photons, tiny particles that each consist of one quantum of energy.
Gamma rays short wavelengths
Radio and television waves long wavelengths
Visible light 400-700 nanometers
As light eahes the atosphee fo spae, soe of the stalight’s photos ae absorbed,
while some light is scattered by different particles. Most photons make it through the
atmosphere and hit the surface of an object.
If the ray of starlight strikes a light-colored surface, most light is reflected. Light hitting dark
surfaces is absorbed.
Light that is neither reflected nor absorbed is transmitted through the surface.
o Example, light travelling from air into a glass window, some light rays will be refracted
as they are transmitted.
Refraction also occurs when light passes from air into water or into the eyeball.
EYES THAT SEE LIGHT
Single celled organisms such as amoebas respond to light and change their direction of motion
to aoid ight light he it’s deteted.
An eye can form an image of the outside world; it can use light to recognize objects.
The first tissue that light encounters is the cornea.
o Coea poides a ido to the old eause it’s transparent.
o It’s ade of a highl odeed aageet of fies ad it otais o lood essels o
blood, which absorb light thus most light is transmitted and not absorbed or
reflected
o Has a rich supply of transparent sensory nerve endings, which force the eyes to close
and produce tears if the cornea is scratched, preserving its transparency
Aqueous humor, a fluid derived from blood, fills the space immediately behind the cornea,
supplying oxygen and nutrients to, and removing waste from, the cornea and the crystalline
lens.
o Crystalline lens enables the changing of focus
Like the cornea, the lens has no blood supply, so that it can be completely transparent.
o The shape of the lens is controlled by the ciliary muscle.
To get to the lens, the light must pass through the pupil, which is simply a hole in a muscular
structure called the iris.
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o The 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 level of light increases or decreases, the iris automatically expands and contracts to allow
more or less light into the eye.
When you emerge from a dark room into bright light (e.g., coming out of a movie theater), not
only does your pupil constrict, but there's a good chance that you will sneeze.
Sneezing in response to being exposed to a bright light:
o the "photic sneeze reflex not yet understood
After passing through the lens, light enters the vitreous chamber (space between lens and
etia, hee it’s efated fo the th and final time by the vitreous humor.
o This chamber comprises 80% of the internal volume of the eye.
After passing through the vitreous chamber, the light is brought into focus at the retina
o Retina contains rods and cones which receive an image from the lens and send it to the
brain through the optic nerve
o Only about half the light that arrives at the cornea reaches the retina.
o Role of etia is to detet light ad tell the ai aout aspets of light that’e elated to
objects in the world (the retina is where seeing begins)
FOCUSING LIGHT ONTO THE RETINA
Cornea, aqueous and vitreous humors help refract light, but the refractive power of these 3
stutues is fied so the a’t ig lose ojets ito fous. This jo is pefoed  the les
The lens can alter the refractive power by changing its shape, a process called accommodation.
Accommodation (change in focus) is done through contraction of the ciliary muscle. The lens is
attached to the ciliary muscle through tiny fibers (zonules of Zinn).
o When ciliary muscle is relaxed zonules stretch and lens is relatively flat eye will
focus on distant objects in this state
o When ciliary muscle contracts tension on zonules is reduced enables lens to bulge
eye will focus on close objects.
o Fatter lens more power
Ability to accommodate declines with age, starting from about 8 years old, and we lose about 1
diopter of accommodation every 6 years up to age 30 (and more after age 30).
Presbyopia: old sight. The loss of ea isio eause of isuffiiet aoodatio.
o why does this occur? lens becomes sclerotic (harder) and the capsule encircling the
lens (enables it to change shape) loses elasticity.
Lens is normally transparent because the crystallins (proteins that make up lens) are packed
together and are regular. Anything interfering with regularity of crystallins will result in loss of
taspae aeas that ae opaue o opaities.
o Opacities of the lens are called cataracts.
Congenital cataracts (present at birth) are rare, most are discovered after age 50.
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Cataracts interfere with vision because they absorb and scatter more light than the normal lens
does.
To focus a distant star on the retina, the refractive power of the 4 optical components of the eye
must be perfectly matched to the length of the eyeball, known as emmetropia.
Refractive errors occur when the eyeball is too long or too short relative to the power of the
optical components
If the eyeball is too long for the optics, the image of our star will be focused in front of the
retina, and the star will thus be seen as a blur rather than a spot of light. This condition is called
myopia (or "nearsightedness").
o Myopia can be corrected with negative (minus) lenses, which diverge the rays of
starlight before they enter the eye
If the eyeball is too short for the optics, the image of our star will be focused behind the retina-a
condition called hyperopia (or "farsightedness").
o Hyperopia can be corrected with positive (plus) lenses, which converge the rays of
starlight before they enter the eye
o Most newborns are hyperopic because the optical components of their eyes are
relatively well developed at birth compared to the length of their eyeballs.
The most powerful refracting surface in the eye is the cornea, contributing 2/3 of the ee’s
focusing power.
When the cornea is not spherical, the result is astigmatism. A vertical line may be focused in
front of the retina while horizontal lines may be focused at the back of it (or vice versa). One or
more of the lines may appear to be out of focus, while other lines appear sharp.
THE RETINA
Process of seeing begins at the retina, where light energy is transduced into neural energy that
can be interpreted by the brain.
Ee dotos use a ophthalosope to look at the ak sufae of the patiets’ ees, hih is
called fundus.
The optic disc is where the arteries and veins that feed the retina enter the eye, and where the
axons of ganglion cells leave the eye via the optic nerve
This portion of the retina contains no photoreceptors, and is blind (blind spot).
Fundus is the only place in the body where one can see the arteries and veins directly. They
spread out across the retina but stop short of the fovea.
A photomicrograph reveals that the retina is a layered sheet of clear neurons, with another layer
of darker cells (the pigment epithelium) lying behind the final layer.
Transduction of light energy into neural energy begins in the backmost layer of the retina, made
of cells called photoreceptors.
When photoreceptors sense light, they can stimulate neurons in intermediate layers (bipolar
cells, horizontal cells, amacrine cells). These neurons connect with the frontmost layer of the
retina made of ganglion cells whose axons pass through the optic nerve to the brain.
RETINAL INFORMATION PROCESSING
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