Chapter 2

Question 1

electromagnetic spectrum

Answer 1

spectrum where energy is described as its wavelength, ranging from short wave gamma rays (10^-12m) to long length radio waves (10^4m). Visible light is a band in this spectrum and is identifiable by colour (short - blue, mid - green, long - yellow, orange, red)

Question 2

photon

Answer 2

small packet of light energy

Question 3

cornea

Answer 3

transparent, fixed covering of the front of the eye that accounts for 80% of the eye’s focusing power

Question 4

lens

Answer 4

transparent, ciliary muscles adjust curvature of lens, accounts for 20% of focusing power of the eye,

Question 5

retina

Answer 5

a layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed; layered with 5 types of neurone - signals generated in receptors travel to bipolar cells and then to ganglion cells with long axons to transmit messages out retina; horizontal and amacrine cells connect neurons across retina

Question 6

accommodation

Answer 6

change in lens shape that occurs when ciliary muscles in front of eye tighten, increasing curvature of the lens so that it gets thicker (increased curvature = increased light bending)

Question 7

near point

Answer 7

distance at which you can no longer accommodate

Question 8

myopia

Answer 8

inability to see distant objects clearly; refractive myopia - cornea/lens bend light too much, axial myopia - eyeball is too long

Question 9

far point

Answer 9

distance at which light becomes focused on retina (a myopic can see an object clearly at far point)

Question 10

laser assisted in situ keratomileusis

Answer 10

LASIK - cut a flap, sculpt cornea with laser, flap replaced

Question 11

hyperopia

Answer 11

far sighted - focus is on a point behind the retina, the eyeball is too short

Question 12

visual pigments

Answer 12

present in outer segments of rods and cones; opsin - long protein, retinal - smaller, light sensitive component; when it absorbs one photon of light, retinal component changes shape (bent to straight) in a process called isomerization. Triggers chemical reaction that activates thousands of charged molecules to create electrical signals in receptors, amplifying the effect of isomerization

Question 13

isomerization

Answer 13

process by which one molecule is changed into another molecule with exactly the same components in a different organization

Question 14

dark adaptation

Answer 14

eye adapts to darkness by increasing sensitivity to light

Question 15

rods and cones

Answer 15

visual receptors with inner and outer segments interspersed throughout peripheral retina (+ rods [120 million] than cones [6 million-cones in fovea]); rods most sensitive to light at 500 nm, cones most sensitive to light at 560 nm

Question 16

fovea

Answer 16

small area on retinal that has only cone receptors (50,000), looking directly at an image it falls here

Question 17

macular degeneration

Answer 17

destroys cone rich fovea and area around it producing a blind region in central vision so that an object can not be seen if looked at directly

Question 18

retinitis pigmentosa

Answer 18

hereditary degeneration of retina attacks peripheral rod receptors and results in poor peripheral vision

Question 19

blind spot

Answer 19

has no receptors (brain “fills it in”, other eye compensates, located on edge of visual field) - location where optic nerve leaves eye

Question 20

dark adaptation curve

Answer 20

function relating sensitivity, light and time in the dark and beginning when light is extinguished

Question 21

light adapted sensitivity

Answer 21

sensitivity in light

Question 22

method to measure dark adaptation

Answer 22

• observer looks at small fixation point centred on fovea while paying attention to flashing test light on peripheral retina
• in light, observer adjusts strength of flash with a knob so that it can barely be seen (threshold - converted to sensitivity high threshold = low sensitivity)
• lights are turned off and experiment is repeated, observer must continually reduce strength of flash as dark adaptation occurs
• adaptation occurs rapidly for 3-4 mins (cones) and levels out, at 7-10 minutes further adaptation occurs (rods) until observer has been in darkness for 20-30 mins

Question 23

dark adapted sensitivity

Answer 23

sensitivity at the end of dark adaptation (100000 x greater than light adapted sensitivity)

Question 24

measuring cone adaptation

Answer 24

must measure response of fovea

Question 25

measuring rod adaptation

Answer 25

must use subjects with no cone receptors (people with rare defect called rod monochromats)

Question 26

detached retina

Answer 26

retina detaches from pigment epithelium, retinal and opsin can’t recombine, blindness in area served by the separated retina

Question 27

spectral sensitivity

Answer 27

responses to visual spectrum ( sensitivity to light as a function of light’s spectrum)

Question 28

Purkinje shift

Answer 28

at night we are more sensitive to short wavelength light (blue/green)

Question 29

spectal sensitivity curves

Answer 29

• use monochromatic light to determine threshold at different wavelengths
• threshold for light is lowest in the middle of the spectrum
• 1/threshold=sensitivity which produces curve

Question 30

absorption spectrum

Answer 30

plot of amount of light absorbed vs wavelength of light (rods - 500 nm, cones - short @419nm, med @ 531nm, long @ 558 nm, add together for 560 nm); closesly matches spectral sensitivity curve

Question 31

lateral geniculate nucleus (LGN)

Answer 31

signals through optic nerve end up in this group of neurons before arriving in visual receiving area of hte brain

Question 32

resting potential

Answer 32

difference of charge inside and outside of neuron cell (resting potential -70 mV) measured by electrode tip inside and outside neuron

Question 33

action potential

Answer 33

1 ms - charge in axon reaches +40 and returns generating a message

Question 34

propagated response

Answer 34

message travels without decreasing in size

Question 35

refractory period

Answer 35

after an action potential, a period when a neuron can not generate an action potential regardless of strength of stimuli

Question 36

spontaneous activity

Answer 36

rate of action potentials generated in absence of stimuli

Question 37

synaptic vesicles

Answer 37

packets of neurotransmitters in presynaptic cell

Question 38

excitatory response

Answer 38

depolarization (+ positive)

Question 39

inhibitory response

Answer 39

hyperpolarization (+ negative), important for inhibiting action potentials so brain has time to process

Question 40

neural convergence

Answer 40

occurs when a number of neurons synapse into a single neuron (happens often in retina - 126 million receptors and only 1 million ganglion cells); rod signals converge more than cone signals, cones in fovea each have private lines to one ganglion cell; rods are more sensitive to light in darkness (less light required for response), cones are better for detail vision

Question 41

convergence and summation

Answer 41

due to convergence, rods can summate excitation more quickly

Question 42

acuity

Answer 42

ability to see detail - moving eyes to search is using fovea where acuity is highest

Question 43

visual evoked potential

Answer 43

recorded by disc electrodes placed on head over visual cortex - researchers alternate grated and grey stimuli - electrical response is VEP