Lecture 4: The Eye, Part the First Flashcards Preview

PSYCH 3310: Sensation & Perception > Lecture 4: The Eye, Part the First > Flashcards

Flashcards in Lecture 4: The Eye, Part the First Deck (54):
1

Light

Electromagnetic radiation that can be conceptualized as a wave or a stream of photons.

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Wave

Oscillation that travels through medium by transferring energy from one particle or point to another.

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Photon

a quantum of visible light or other electromagnetic radiation demonstrating both particle and wave properties

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Particle

Something with mass and energy

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Think of light as:

Think of light as Waves: when it moves around the world.

Think of light as Photons (particles): when absorbed.

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Absorbed Energy

Energy (e.g., light) that is taken up, and not transmitted (i.e., it is transformed into different form of potential or kinetic energy)

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Scattered Energy

Energy that is dispersed in an irregular fashion (i.e., randomly)

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When light enters the atmosphere, much of it is absorbed or scattered and never reaches our eyes

.....

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Transmitted

Energy that is passed on through a surface (when it is neither reflected nor absorbed by the surface)

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Refracted

Energy that is altered as it passes into another medium, (e.g., light entering water from the air, or a lens/system, e.g., the eye)

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Index of Refraction

Indicates how much light ‘bends’ when it goes from one medium to another.

Responsible for image formation and rainbows.

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The human eye is made up of various parts:

Cornea: The front part of the eye; a transparent “window” into the eyeball.

Aqueous Humor: The watery fluid in the anterior chamber (between cornea & lens).
Provides oxygen & nutrients.
Removes waste from cornea & lens.

Crystalline Lens: The lens inside the eye, which allows changing focus (Accommodation).

Pupil: The circular opening (aperture) at the center of the iris in the eye, where light enters the eye.

Iris: The colored part of the eye, between the sclera (white part) and the pupil, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the eye by expanding & contracting the pupil.

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The human eye is made up of various parts:

Cornea: The front part of the eye; a transparent “window” into the eyeball.

Aqueous Humor: The watery fluid in the anterior chamber (between cornea & lens).
Provides oxygen & nutrients.
Removes waste from cornea & lens.

Crystalline Lens: The lens inside the eye, which allows changing focus (Accommodation).

Pupil: The circular opening (aperture) at the center of the iris in the eye, where light enters the eye.

Iris: The colored part of the eye, between the sclera (white part) and the pupil, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the eye by expanding & contracting the pupil.

Vitreous Humor: The transparent fluid that fills the large (vitreous) chamber in the posterior part of the eye (i.e., between the lens and the retina)
Where floaters occur

Retina: A light-sensitive membrane in the back of the eye that contains rods and cones (and other neurons, supporting vessels, and structures), which receives an image from the lens and sends it to the brain through the optic nerve.

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Cornea

The front part of the eye; a transparent “window” into the eyeball

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Aqueous Humor

Aqueous Humor: The watery fluid in the anterior chamber (between cornea & lens).

Provides oxygen & nutrients.
Removes waste from cornea & lens.

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Crystalline Lens:

The lens inside the eye, which allows changing focus (Accommodation)

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Pupil

The circular opening (aperture) at the center of the iris in the eye, where light enters the eye

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Iris

The colored part of the eye, between the sclera (white part) and the pupil, consisting of a muscular diaphragm surrounding the pupil and regulating the light entering the eye by expanding and contracting the pupil.

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Vitreous Humor

The transparent fluid that fills the large (vitreous) chamber in the posterior part of the eye (i.e., between the lens and the retina)

Where floaters occur

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Retina

A light-sensitive membrane in the back of the eye that contains rods & cones (and other neurons, supporting vessels, and structures), which receives an image from the lens and sends it to the brain through the optic nerve

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Refraction is necessary to focus light rays onto the retina and this is accomplished by ____ .

the lens

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Accommodation

change in focus

The process in which the lens shape is changed, by the ciliary muscle, thus altering its refractive power

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Emmetropia

No refractive error

No need for corrective eyewear

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Some Refractive Indices:

air = 1

pure water = 1.333

cornea ~ 1.376;

crystalline lens: 1.386-1.406

diamond: ~2.419

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Diopter

unit of measurement of the optical power of a lens (or curved mirror).

Reciprocal of focal length (in meters).
D = 1/f(m)

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Problems of refraction:

Sometimes the way the lens refracts light causes the point of focus of the image to be in front of or behind the retina. Corrective lenses (spherical and/or cylindrical) are needed to allow for normal vision
Myopia: When light entering the eye is focused in front of the retina, and distant objects cannot be seen sharply; nearsightedness
Hyperopia: When light entering the eye is focused behind the retina; farsightedness
We are born with severe myopia, due to developed cornea and lens, but small (not fully developed) eyes. Newborns can, therefore, see things very close to them.
-Minus spherical correction for Myopia -Plus spherical correction for Hyperopia

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Myopia

When light entering the eye is focused in front of the retina, and distant objects cannot be seen sharply; nearsightedness

We are born with severe myopia, due to developed cornea and lens, but small (not fully developed) eyes. Newborns can, therefore, see things very close to them.

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Minus spherical correction for

Myopia

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Plus spherical correction for

Hyperopia

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Astigmatism

A visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea (requiring cylinder correction).

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Presbiopia

“old sight”

Hardening of the crystalline lens.
The lens becomes sclerotic (harder) and the capsule that encircles the lens (enabling it to change shape) loses its elasticity.

This means older people cannot focus on things as closely, which is why older people tend to hold things further away when reading.
Eventually, it’s time to get plus lenses (reading glasses, with spherical correction).

People with myopia (often, those who read a lot or use the computer at work) tend to avoid or postpone the effects of presbyopia (need for plus prescription) because they are also myopic (in other words: one problem: nearsightedness, tends to counteract the other problem: hardened lenses).

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Camera parts analogous to the eye

F-stop (aperture): Iris/pupil. Regulates the amount of light coming into the eye (also depth of field)

Focus (focal length): Lens. Changes shape to change focus

Film (sensor): Retina. Records the image

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Using the ___, doctors can view the back surface of patients’ eyes, called the fundus

ophthalmoscope

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Rods

Photoreceptors that are specialized for night vision

Respond well in low liht conditions
Do not process color

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Rods

Photoreceptors that are specialized for night vision

Respond well in low light conditions
Do not process color

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Cones

Photoreceptors that are specialized for daylight vision, fine visual acuity, and color

Respond best in high luminance conditions

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Duplex Retina

retina with cones and rods for day & night vision

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Duplex Retina

retina with cones and rods for day & night vision

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Light passes through several layers of cells before reaching the rods and cones

Light activates a photoreceptor, which then signals the horizontal and bipolar cells that synapse with it

Bipolar cells are connected to amacrine and ganglion cells

Ganglion cells have axons that leave the retina through the optic disc (blind spot)

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Light activates a photoreceptor, which then signals the _____ cells that synapse with it

horizontal and bipolar cells

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Bipolar cells are connected to _____ cells

amacrine and ganglion cells

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Ganglion cells have axons that leave the retina through the ______.

optic disc (blind spot)

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Photoreceptor Potential

Photoreceptors communicate by graded potential, not through action potentials (spikes) of most neurons

Photoreceptors have no neural input synapses; they only receive information from photons and noise

Amount of neurotransmitter glutamate in the photoreceptor-bipolar cell synapse at any time is inversely proportional to the number of photons being absorbed by the photoreceptor

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Pigment Epithelium:

Provides nutrients to photoreceptors.
Usually opaque, sometimes reflective.

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Photoreceptors

Rods: dim light, larger, more numerous, more sensitive, not in fovea;.

Cones: better acuity, daylight, mostly in fovea, few in periphery, usually 3 types (S, M, & L)

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Horizontal Cells

located between photoreceptors and bipolar cells.

Perform lateral inhibition.

Responsible for center-surround receptive fields

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Bipolar Cells

bridge between photoreceptors and ganglion cells.

2 types:
midget, 1:1 cone:ganglion
(giving low convergence).
diffuse: connected to several photoreceptors (giving high convergence)

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Amacrine Cells

less well understood.

Seem to integrate information from groups of bipolar cells and communicate to ganglion cells.

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Ganglion Cells

Connection between eye and brain. Recieves information from bipolar cells (and amacrine cells) and sends info via the optic nerve.

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Why are photoreceptors in the back of the retina?

The consensus is that the pigment epithelium provides nutrients to photoreceptors.

Since photoreceptors must be near the opaque pigment epithelium, and many other interconnected neurons, and all the other cells are transparent, the ordering becomes more intuitive.

In general, most mammals and birds have this type of eye design. Marine life and insects have different architectures.

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Retinal Information Processing

The distribution of rods & cones is not constant over the retina

Cones process color; rods do not

This means that you have very poor color vision in your periphery.
It may seem as if your entire field of view has full-resolution color, but it does not (we use top-down processing to fill-in color, where cones are absent)

In a human retina, there are roughly:
90 million rods
4-5 million cones

We measure the size of visual stimuli by how large an image appears on the retina (not how large the object is)

The standard way to measure retinal size is in terms of “degrees of visual angle”

The visual angle of an object is a function of both its actual size and distance from the observer

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Blue, green, and red represent the S-, M-, and L-cones, respectively, of a living human being in a patch of retina at 1 degree from the fovea

cones

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Rule of thumb

If you hold your thumb out at arms length, the width of your thumb is about 2 degrees of visual angle

Matlab Solution: 1/tand(.5)/2 = 57.29
Which means, at 57.29 cm from the eye, 1 cm is 1 degree.

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The visual angle of an object is a function of ____ and ____

The visual angle of an object is a function of both its actual size and distance from the observer