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Lecture 2

PSYB51H3 Lecture 2: LEC 2

Course Code
Matthias Niemeier

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Friday January 17, 2014
Chapter 2: The First Steps in Vision
Today We Will Be Talking About
Optics: how light enters the eye
Anatomy of the eye: accommodation, meaning that you get something sharp and focussed, a non-
blurred image
Retina: the point where perception starts
Perception starts early in the eye in the retina, specifically the photoreceptors. The idea that the eye
is like a primitive photo-camera, it absorbs light and projects information to the brain to get
analyzed... this is wrong!
Dark and Light Adaptation
Adaptation and accommodation are two very different concepts
Lateral inhibition: you have cells ordered next to each other (e.g., photoreceptors) that interact with
each other, lateral inhibition allows you to perceive, say, edges more clearly
Retinitis pigmentosa
Receptive fields: sensory cells have receptive fields, which is the area in which they can be stimulated
in some form
Filters: cells can often be considered as filters
Contrast: change; e.g., luminescence contrast would be recognizing areas as light and dark
Light: a wave; a stream of photons, tiny particles that each consist of one quantum of energy
Light can be absorbed, transmitted, reflected, diffracted, or refracted
Absorbed: energy (e.g., light) that is taken up, transformed to other forms of energy
e.g., light is absorbed by solar panels and gets transformed into electrical energy
Filters for photo cameras absorb light, red ones absorb every colour but red
Black surfaces are warmer than white because they absorb the light
Transmitted: convey light from one place to another through a transparent medium
Reflected: energy that is redirected when it strikes a surface
Diffracted: bent, or having waves that are spread out, (e.g., waves of sound or light, as they
encounter an obstacle, e.g., pass through a narrow aperture)
Will happen to light when it passes through the pupil
Refracted: energy that is altered as it passes into another medium, e.g., light entering water from
the air
Depth of a swimming pool, lenses
You can’t see anything sharp without refraction
Rainbows are based on refraction, it has to do with spherical little drops of water
Why Do We Sense Light?
Why is sensing light a “good idea”? Why is it useful for us to have vision? Finding food, avoiding
danger, perceiving threats before they affect you, perceiving colour, interpreting the environment,
reading faces, for our circadian rhythm, sensing days, plants being able to sense the seasons
Interpreting colour: you can see signals such as a green banana not being ready (plants can put
small amount of poison in fruits that are not ready and you can get sick)
What is the role of light in the circadian rhythm? Light is what helps the circadian rhythm to gets it
rhythm right. When you are jetlagged, you train your circadian rhythm with regular daylight. There is
a set of photoreceptors in the retina that specifically inform this system in the hypothalamus
But it wasn’t always like that, mammals emerged when dinosaurs ruled the earth. Dinosaurs would eat
the mammals. Small creatures that survived at night had a keen sense of smell and hearing. Phototopic
vision was not so important. Vision became a dominant modality later. Our vision to this day is not as
good as the vision of many other animals such as birds
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Evolution of the Eye
A) Evolution probably has started with eye spots, proteins in individuals that are light sensitive (opsin),
or a cluster of photoreceptors on the surface of the animal that has neural organization, that are light
sensitive. Light from any direction goes onto this spot, so it’s location is not too specific
B) Next, this cluster of photoreceptors probably became embedded in some cavity, into a depressed/
folded area. Light becomes more selective
Advantage: the area is protected
C) Next, “pinhole” eye (a small hole allows access to light) allows finer directional sensitivity and limited
imaging, has a water-filled chamber and area of photoreceptors/retina
D) Then, transparent humor (fluid) develops in enclosed chamber, the pinhole is closed off with
something transparent in front
E) Distinct lens develops with a cornea to contribute to refraction/accommodation (much more than the
F) Iris and separate cornea develop
The Anatomy of Human Eye
Know the different parts of the eye
Cornea, aqueous humor, iris (controls pupil size for fast adaption of light), iris, lens (surrounded by
ciliary muscles), Zonules of Zinn (connect the lens with the ciliary muscle), past the lens we have the
vitreous humour (makes up the largest part of the eye; the round shape of the eye is because of the
slight pressure of the vitreous humor inside, there is higher pressure inside than outside)
Sclera, fovea, optic nerve, optic disc (where the retina has no photoreceptors)
Optic Functions of the Eye
Refraction is performed by the cornea and by the lens, and is necessary to focus light rays. This is done
by the four optical components. The cornea is much more important for this than the lens. The cornea is
a constant amount of refraction, and the lens can change its shape and change refractory power
depending on the distance of the object relative to the eye
When you look at small distant light, such as a star, the light would run parallel. The lens needs to
have very little refractory power, so it has to be very thin. How does this work?
The ciliary muscles change the shapes of the lens, and thus alter its refractive power. The ciliary
muscles relax, and as they are relaxing, the entire lens gets stretched
When you have a dot on a piece of paper in front of you, the light is not running in parallel, it is so
close by. You need a lens with a higher refractory power to bend and redirect the light so you get a
single spot on your retina. How does it work?
The ciliary muscles contract, the aperture becomes smaller, and the lens relaxes into its normal
shape. Its natural shape is a bulgy round shape
Problems with Refraction and Lens
Your vision goes downhill from 16 years of age and so on
Emmetropia—the eye has the ability to accommodate and adjust refractory power to what is expected
of a “normal” human eye
Myopia—the eyeball is longer than “normal”, lens cannot be stretched and thinned as far as needed, so
the light bundles up in front of the retina, so that at the retina it is already fanned out again and blurred
Myopia with correction—concave lens undoes some of the refractory power
Hyperopia—light gets bent at a point behind the retina, lens is less and less relaxed as you grow older;
convex lenses needed to correct this
The cornea is supposed to be spherical, but in astigmatism, it is not; it has a rugby ball shape
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