Chapter 2: Beginnings Of Perception

Question 1

Define Electromagnetic Spectrum

Answer 1

The continuum of energy produced by electric charges and is radiated as waves (short gamma -> long radio waves) in nanometers

Question 2

Define Wavelength

Answer 2

The distance between the peaks of electromagnetic waves

Question 3

What range of the electromagnetic spectrum can be seen by the human eye?

Answer 3

400nm (blues) -700nm (reds)

Question 4

Light enters the pupil and is focused onto the retina by the Cornea and Lens. What percentage of focusing is each responsible for?

Answer 4

Cornea: 80%
Lens: 20%

Question 5

What is Accommodation?

Answer 5

The process of the lens adjusting its shape depending on the distance of the stimulus. Ciliary muscles tighten and the lens thickens.

Question 6

Define Near Point

Answer 6

The distance at which your lens can no longer adjust to focus near objects

Question 7

Define Far Point

Answer 7

Distance at which the spot of light becomes focused on the retina

Question 8

What is Presbyopia (“Old Eye”)?

Answer 8

A condition where the distance of the near point increases due to hardening of the lens and weakening of the ciliary muscles

Question 9

What is Myopia? What are the 2 types of myopia and their causes?

Answer 9

Inability to see distant objects clearly because the image is focused in front of the retina

A. Refractive Myopia: cornea or lens bends the light too much
B. Axial Myopia: eyeball is too long

Question 10

What is Hyperopia? What is its cause?

Answer 10

Inability to see nearby objects clearly because the focus point is behind the retina, caused by an eyeball that is too short

Question 11

Define Isomerization

Answer 11

The process of a retinal changing shape, sticking out of the opsin

Question 12

Define Enzyme Cascade

Answer 12

The idea that one visual pigment activates a chain reaction (of millions of molecules)

Question 13

What are the Visual Receptors? What are their visual pigments composed of?

Answer 13

The visual receptors are cones and rods. Their visual pigments are composed of:
A. Opsin: a long strand of protein
B. Retinal: one, single light sensitive molecule

Question 14

Describe the distribution of Rods and Cones on the retina. How many of each are there on average?

Answer 14

There are 120 million rods and 6 million cones on the retina on average.

The fovea contains only cones, while the peripheral retina contains more rods than cones.

Question 15

What is the Blind Spot? Why don’t humans actually notice it?

Answer 15

The blind spot is where the optic nerve leaves the eye

We don’t notice it because:

• The brain fills in informtation
• One eye covers for the other’s blind spot
• It’s located at the edge of vision

Question 16

Define Macular Degeneration

Answer 16

The degeneration of the fovea and surrounding area (common in elders)

Question 17

Define Retinitis Pigmentosa

Answer 17

The degeneration of peripheral rods and potentially foveal cones (genetic)

Question 18

What is Dark Adaptation?

Answer 18

The phenomenon where extended exposure to the dark increases light sensitivity

Question 19

Outline the 3 experiments used to test dark adaptation. How long does it take to reach rod and cone max sensitivity?

Answer 19

A. Cones: Observer is light adjusted, lights are turned off, once dark adapted observer adjusts a test light until she can just see it (3-4min max)

B. Rods + Cones: observer looks at a fixation point but pays attention to a test light on the side
- Cone sensitivity reaches max before rod sensitivity, but when dark adapted, cone sensitivity follows rod sensitivity

C. Rods: Observer must be a rod monocromat (no cones) (25min max)

Question 20

What is the Rod-Cone Break?

Answer 20

The place where rods begin to determine the dark adaptation curve

Question 21

Define Spectral Sensitivity

Answer 21

An observer’s sensitivity to light at each wavelength across the spectrum

(Threshold for light is lowest in the middle of the spectrum)

Question 22

How is spectral sensitivity measured? Where on the spectrum are rods and cones most sensitive to light?

Answer 22

Cone: a monochromatic light is used on the fovea
- 560nm (419nm, 531nm, 558nm)

Rods: observer is dark adapted, then a monochromatic light is used

• 500nm
• more sensitive to short wavelength light

Question 23

What is the Purkinje Shift?

Answer 23

Enhanced sensitivity to short (blue) wavelengths during dark adaptation

Question 24

What is the Absorption Spectrum?

Answer 24

The plot of the amount of light absorbed vs wavelength

Question 25

What are the 5 neurons that make up the retina and each of their functions?

Answer 25

(neurons = cell body + dendrites + axons)

A. Receptor cells: respond to specific kinds of energy
B. Bipolar cells: vertical
C. Ganglion cells: long axons, vertical
D. Horizontal cells: connect receptors
E. Amacrine cells: connect bipolar cells, connect ganglion cells

Question 26

How can you record electrical signals in neurons?

Answer 26

Small electrodes can be used to record electrical signals in neurons. A recording electrode is placed inside nerve fiber and a reference electrode is placed outside the nerve fiber. The electrode inside is -70mV compared to the outside (resting potential) and electrical signals are measured as the difference between the inside and outside charges.

Question 27

Define Action Potential

Answer 27

Change in electrical charge because of passage of an impulse

size is constant but more frequent firing increases stimulus intensity

Question 28

Define Refractory Period. How long is a typical refractory period?

Answer 28

Period after firing

1ms

Question 29

What is the typical firing rate of neurons in impulses/sec?

Answer 29

500-800 imp/sec

Question 30

What is a Synapse? How do electrical signals travel across synapses?

Answer 30

A small space between neurons

Neurotransmitters are released from the vesicle of one neuron and received by receptor sites of another neuron

Question 31

What are the 2 types of Neurotransmitters?

Answer 31

Excitatory Transmitters: cause depolarization, so neurons become more positive and likelihood of AP increases

Inhibitory Transmitters: cause hyperpolarization, so neurons become more negative and likelihood of AP decreases

(Excitatory and inhibitory transmitters can be present simultaneously, causing random firing rather consistent firing/no firing)

Question 32

After a signal moves through all structures, where does it move and why?

Answer 32

A signal goes through all structures, then directly back…possibly to reduce light scatter that would affect rods and cones

Question 33

What is the make-up of rod and cone:ganglion cell connections?

Answer 33

126 million rods and cones converge to 1 million ganglion cells

• 120 rods to 1 ganglion cell
• 6 cones to 1 ganglion cell
• In fovea, 1 cone to 1 ganglion cell

Question 34

How does the make-up of visual receptor-ganglion cell connections influence vision?

Answer 34

Because more rods are connected to a single ganglion, there is a higher chance of response, higher sensitivity to light under dim conditions, and vision is blurry

Because there is a 1:1 convergence in fovea, there is high vision acuity