Chapter 2: The Beginning of Perception

Question 1

What is the range of wavelengths for visible light?

Answer 1

• 370nm - 730nm
• Found within the electromagnetic spectrum

Question 2

What’s your field of view?

Answer 2

• The portion of space surrounding you that you can see without moving your eyes
• We have 190 degrees of visibility from side to side

Question 3

Why do we have slightly more visibility downwards compared to upwards?

Answer 3

• The brow bridge makes it difficult to see too high up

Question 4

What’s the trade-off of having eyes on the side of your head?

Answer 4

• Have a greater field of view but worse depth perception (ex. alligators)
• Most predators have eyes on the front of their faces while the opposite is true for prey.

Question 5

What are the three membranes that surround and protect the eye?

Answer 5

1) Sclera - tough, outer membrane. Protective covering, makes up the white part
2) Choroid - middle layer. Contains most of blood vessels. helps circulate oxygen and nutrients
3) Retina - Inner layer. Made up of neurons, including the photoreceptors

Question 6

What are the extraocular muscles?

Answer 6

• 3 different pairs of muscles that surround the eye
• Superior and inferior rectus - up/down
• Medial (toward) and lateral (away) rectus - toward/away from center (center being the nose)
• Superior and inferior oblique - rotation clockwise/counterclockwise. Keeps eyes upright while you spin

Question 7

What’s the cornea?

Answer 7

• A clear, protective bump at the front of your eye
• Sharply refracts light as it enters the eye (including side view)
• Helps focus the light onto the retina
• Fixed in place but still does about 80% of focusing power

Question 8

What’s the purpose of the pupil?

Answer 8

• Note that it’s not really a structure
• Hole/opening in the center of the eye that allows for a certain amount of light to enter the eye.
• Dilation/constriction of the pupil is controlled by muscles in the iris

Question 9

What’s the lens?

Answer 9

• An elastic, crystalline structure that also helps focus light onto the retina
• Does not do as much focusing as the cornea
• Main job is to fine-tune the light onto the retina
• Does about 20% of focusing

Question 10

How does the lens change shape to focus light onto the retina?

Answer 10

• Performed by the ciliary muscles
• Becomes thinner to focus on distant objects (bends light less)
• Becomes thicker to focus close objects (bends light more)

Question 11

What’s the state of the ciliary muscles and the zonule fibres when the lens is relaxed?

Answer 11

• Lens is more spherical
• Ciliary muscles are contracted (thinner). Anchored to the choroid membrane
• Zonule fibres are less taut

Question 12

What’s accomodation?

Answer 12

• The process of changing the shape of the lens, so that light from different objects at different distances focuses correctly on the retina.

Question 13

True or false: The retinal image is the exact same orientation as the perceived image that we see.

Answer 13

• FALSE: The retinal image is upside-down and backward at the fovea

Question 14

What are the five cell layers of the retina?

Answer 14

*Ranked in the order that light moves through them
1) Photoreceptors (rods/cones)
2) Horizontal cells (send signals sideways)
3) Bipolar cells (make ‘through’ connections to ganglions)
4) Amacrine cells (send signals sideways)
5) Ganglion cells (axons combine to form the optic nerve)

Question 15

What are rods?

Answer 15

• Photoreceptors responsible for being extremely receptive to light
• 120 million per eyeball
• Only found in the periphery

Question 16

What are cones?

Answer 16

• Photoreceptors responsible for detecting colour (differences in wavelengths)
• Found primarily in the fovea
• Work best in bright environments
• No convergence, but better acuity
• Three different kinds (red, green, blue)

Question 17

What are the implications of rods having convergence?

Answer 17

• While only a handful of rods may be activated by interactions with photons, they can combine their inputs with other receptors in order to reach the minimum threshold since they all come together at the same ganglion cell
• This also means they have lower acuity since the brain cannot detect where exactly the signal is being fired from.
• The complete opposite is true for cones

Question 18

Why is the fovea centralis important?

Answer 18

• Where the majority of vision occurs
• Involved in directed looking
• highest density of photoreceptors
• Pretty much only cones in this area
• Rods are found in the periphery

Question 19

Where’s the blind spot?

Answer 19

• The area where there are no receptors cause it’s where the optic nerve exits the eyeball and leads to the brain
• Brain ends ups filling in the missing info using the context of the surroundings
• The other eye will also attempt to fill in the missing field of view

Question 20

What’s transduction?

Answer 20

• The transformation of a physical stimulus into a neural signal

Question 21

What and where are the photopigment molecules?

Answer 21

• They are located in the outer segments of rods and cones, each receptor containing thousands of them
• When the retina absorbs a single photon, the photopigment molecules change shape (not the chemical makeup)
• This triggers a cascading chain reaction, causing the release of millions of charged molecules, and ultimately leading to the photoreceptor firing.

Question 22

What term is used to describe when a photopigment molecule changes shape?

Answer 22

• Photoisomerization!
• Receptors have different regeneration rates

Question 23

What does the dark adaptation curve tell us?

Answer 23

• There’s a progressive increase in sensitivity to light
• The dark adaptation curve shows how sensitivity to light changes over time

Question 24

How was the dark adaptation curve developed?

Answer 24

• A participant stares at a black X while a light is shone off to the side
• While staring at the X, the participant uses the method of adjustment to determine minimum threshold for detection of light
• They continue to adjust the light over time

Question 25

How does the dark adaptation curve change if the participant stares directly at the light?

Answer 25

• Cone adaptation occurs, but since cones don’t have very good convergence they can only detect so much light
• Takes cones approximately 6 minutes to regenerate after photoisomerization. Once they all regenerate, it means that at any given time, there will be some available to react to photons. This is where almost an equilibrium is reached.

Question 26

What does the dark adaptation curve look like for someone who is a rod monochromat?

Answer 26

• Initially, when you move into a dark room, there aren’t many rods available for use
• Take approximately 25 minutes for rods to regenerate, that’s why after 25 minutes occurs, there’s a major increase in threshold at this time

Question 27

What does spectral adaptation signify?

Answer 27

• The relationship between sensitivity to light and the wavelength of light
• For humans, the highest sensitivity to light is around 560nm (yellow neon)
• Differences in spectral sensitivity are due to differences in absorption spectra of pigments

Question 28

What’s the Purkinje Effect?

Answer 28

• An explanation for why rods have a lower peak wavelength compared to cones
• Rods’ peak sensitivity is at 500nm which is thought to be because rods are used primarily for seeing in the dusk (note: they still do not identify colour)

Question 29

What are the three types of cones?

Answer 29

• Short-wavelength ‘blue’ cones
• Medium-wavelength ‘green’ cones
• Long-wavelength ‘red’ cones
  *The ratio of types of cones varies from person to person

Question 30

If there’s only three types of cones, how do we respond to the whole colour spectrum?

Answer 30

• Each colour/hue has its own specific cone response pattern for each wavelength
• Each cone will absorb different amounts of light, and then the brain puts together info from all three cones and then perceives a colour to represent the firing pattern

Question 31

What’s strabismus?

Answer 31

• Disorder of the extraocular muscles
• The two eyes do not coordinate to point in the same direction, resulting in a double-image and impaired depth perception

Question 32

Is strabismus the same as lazy eye?

Answer 32

• No it is not, although if left untreated it can lead to ‘lazy eye’ also known as amblyopia where signals from one eye become completely ignored by the brain

Question 33

What’s myopia?

Answer 33

• Also known as nearsightedness
• Light from distant objects is focused in front of the retina (i.e., can’t see far objects)
• Often caused by slight elongation of the eye
• The longer the eye, the worse the myopia
• The lens will focus light where it thinks it should be, not were it actually fovea is

Question 34

What’s hyperopia?

Answer 34

• Also known as farsightedness
• Light from nearby sources is focused behind the retina (i.e., can’t see objects up close)
• Often caused by a shortened eye length
• Can also be caused by aging where the lens loses elasticity (presbyopia), cannot bend light to the same degree

Question 35

What’s astigmatism?

Answer 35

• The lens or cornea is misshapen, causing a misplaced focal point and/or more than one focal point on the retina
• Causes blurriness and/or double vision

Question 36

What’s cataracts?

Answer 36

• Progressive clouding of the lens, caused by the breakdown of proteins
• Associated with aging, pupils start to appear grey

Question 37

What’s macular degeneration?

Answer 37

• Damage to the photoreceptors in the central areas of the retina (fovea and parafovea)
• The damage may spread due to a number of reasons

Question 38

What’s retinitis pigmentation?

Answer 38

• An inherited (genetic) condition of gradual degeneration of photoreceptors, starting in the periphery
• Often leads to night blindness and tunnel vision
• Field of view also diminishes but doesn’t always lead to total blindness

Question 39

How do night vision devices generally work?

Answer 39

• The few photons in a dark environment that interact with the device are funnelled into an intensifier tube, which are then converted to electrons
• The electrons strike a phosphorous screen which glow depending on the electron energy levels
• More photons = more electrons interacting with the phosphorous = brighter area