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sensory systems > the eye > Flashcards

the eye Flashcards

1
Q

photoreceptor

A

cell specialised for light detection

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2
Q

photopigment

A

protein + light absorbing factor

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3
Q

light absorbing factor

A

photosensitive co-factor bound by a protein

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4
Q

xeopus: african clawed frog eyes

A

have optical apparatus that make photosensitive cells particularly sensitive to light coming from some directions

eyes make each photoreceptor cell respond to light from a restricted range of directions, which corresponds to light originating in a particular portion of the visual scene

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5
Q

xenopus skin

A

Photosensitive cells in their skin: respond to light, but not the direction it is coming from

detect ambient light levels

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6
Q

cup eyes

A

little pits lined with heavily pigmented cells (screening pigments), and photoreceptor cells sitting within them

Pigmented cells shield the photoreceptors from light coming in from some directions: they cast shadows across the pit

they can now determine which direction light is coming from

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7
Q

basic function cup eyes

A

render photoreception direction dependent

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8
Q

planaria use for cup eyes

A

negative phototaxis: to seek shade

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9
Q

the insect compound eye

A

Pigment cells form a tube surrounding a group of photoreceptors (ommatidia), there are thousands in each eye

light can only enter from one end; light can reach the photoreceptor from the direction the tube is pointed

many in different directions allows them to make up the whole visual scene

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10
Q

ommatidia

A

Pigment cells form a tube surrounding a group of photoreceptors

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11
Q

why do ommatidia have a cornea and a protective lens?

A

these act to maximise the efficiency of light transmission to the photoreceptors

do not play a major function in making light direction selective

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12
Q

what shape are ommatidia collecting area?

A

cone shaped, gets wider teh further from the ommatidia entrance (wider at the eye surface)

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13
Q

feature of neighbouring ommatidia and function

A

only have partially overlapping collecting cones
so by comparing activity across ommatidia it would be possible to determine the location of bright/ dim areas

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14
Q

examples of uses of the insect compound eye

A

Example 1: moth to a flame
* Light from blub falls withing the collecting cone of this ommatidium, but not its neighbouring ones

Example 2: finding flowers
* Light reflected from the flower will fall more within collection of cones of some ommatidia than others. These can tell the moth which direction to fly

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15
Q

acuity

A

capacity for seeing distinctly the details of an object

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16
Q

what do opticians use to measure acuity?

A

Snellen acuity charts measure acuity

the smaller the letters the harder to distinguish details, therefore to read smaller letters you need higher acuity

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17
Q

what does high acuity vision require?

A

an array of pixels each receiving light from a restricted range of points in visual space

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18
Q

what increases effective acuity?

A

The more independent representations of light (pixels) you can squeeze into an image, the higher the spatial resolution of that image and the higher the effective acuity of someone viewing it

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19
Q

each ommatidium represents how many pixels?

A

one

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20
Q

size of mirror eyes

A

1mm

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21
Q

design of mirror eyes

A

concave mirror that focus image onto an array of photoreceptors

  • Eye has an aperture at the front to let light in
  • Light enters aperture, passes through photoreceptors, and falls on a reflective surface
  • Reflective surface is concave so when light bounces off it is reflected inwards to a focus point on the photoreceptors
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22
Q

photoreceptors relationship to pixels in mirror eye

A
  • Each photoreceptor receives light preferentially from a different point in visual space and can be regarded as a separate pixel

this overcomes the fundamental limits to acuity of a compound eye

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23
Q

relationship between acuity and distance compound eye

A

inverse correlation

as distance increases, acuity worsens

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24
Q

at a longer the distance from the eye the rays appear…

A

more parallel

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25
Q

viewing long distances (mirror eye)

A

concave mirror bends the light to a single point on the photoreceptors
mirror focuses a clear image onto the photoreceptors

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26
Q

in a mirror eye at a shorter distance the rays appear..

A

more divergent

27
Q

viewing shorter distances mirror eye

A

rays divergent when they hit the mirror therefore they are not reflected onto the photoreceptors

acuity is very poor

28
Q

acuity and focus in mirror eye

A

object in focus at some viewing distances, but not at others
acuity is strong dependent upon viewing distance

29
Q

what is invariant in the mirror eye

A

focal length of the mirror

30
Q

what is acuity limited by in the compound eye?

A

the collecting area of teh ommatidium

31
Q

how many times has the lens eye developed independently?

A

three times

32
Q

animals that have the lens eye?

A

all vertebrates
invertebrates: spiders, squid, octopus

33
Q

what shape are lens eyes?

A

sperical

34
Q

what is the general passage of light onto the retina (lens eye)?

A

light to enters through an aperture, which passes through a lens, that focuses it onto an array of photoreceptors on the retina

35
Q

what does the focus of the lens result in?

A

the projection of an image of the visual scene onto the photoreceptors

36
Q

acuity of lens eye

A

very high acuity

37
Q

lens eye photoreceptor pixel relationship

A

each photoreceptor receives light from a different point in visual space: becomes an independent pixel

38
Q

how is light focused on to the retina (basic)

A

refraction

39
Q

refraction

A

focusing divergent rays from a point in visual space into a single point on the retinal surface

40
Q

what does the degree of refraction depend on?

A

distance from the object

41
Q

refraction from the cornea is…

A

constant, invariant

42
Q

what does refraction adjustment depend on

A

the lens, the lens is adjustable

43
Q

what is the lens connected to?

A

a ring of muscle called ciliary body by suspensory ligaments

44
Q

role of suspensory ligaments on the lens

A

hold the lens in place and pull it flat

the lens is elastic and wants to form a sphere, the suspensory ligaments stop it doing this

45
Q

role of the ciliary muscles

A

pull the suspensory ligaments taught in order to maintain an appropriate lens shape

46
Q

lens for distant objects

A

Light rays near parallel, require little refraction to focus

Lens thin and therefore has low refractive power

caused by the ciliary muscles relaxing and pulling the lens flat

47
Q

lens for close objects

A

Lights rays diverging, require greater refraction to focus

Lens thick and has high refractive power

ciliary muscles contract, releasing tension in the suspensory ligaments, allowing the elastic lens to get rounder

48
Q

accommodation

A

an involuntary response to changing distance of view

49
Q

changing from distance to close vision

A
  • Ciliary muscles contract and suspensory ligaments relax
  • Lens thickens and refractive power increases
50
Q

how can you externally see change from distance to close vision?

A

pupil constriction

51
Q

refractive errors:

A

problems with corneal or lens refraction

52
Q

what happens when a perfect image is not formed on the retina?

A

we observe a reduction in acuity

53
Q

what is myopia

A

short sightedness, difficulty viewing distant objects
image is focussed in front of the retina

54
Q

why can we still see near objects with myopia?

A

the accommodative change in the lens can compensate for the problem

55
Q

what can cause myopia?

A

eye too deep (corneal:retinal distance too large)
effective refractive power too large

56
Q

how can we correct myopia?

A

*Diverging (concave) lens corrects

makes parallel rays from distant objects more divergent as they hit the cornea

57
Q

what is hyperopia/ hypermetropia?

A

long sightedness
difficulty viewing close objects
image not yet focussed by the time the light reaches the retina

58
Q

what causes hyperopia?

A

eye too shallow (corneal:retina distance too small)
refractive power of the cornea too low

59
Q

why can people with hyperopia still view distant objects?

A

accommodative changes in the lens compensate

60
Q

how can we correct for hyperopia

A
  • Converging (convex) lens corrects

make diverging rays from close objects more parallel as they hit the cornea

61
Q

presbyopia

A

most widely experienced

loss of accommodation ability during aging

As people get older their lenses become less elastic
Even when ciliary muscles are full constricted and suspensory ligaments lose, the lens stays relatively thin (lacks elasticity to form full sphere when not under tension)

why older people have difficulty seeing things close up

62
Q

what does speed of increase of myopia suggest?

A

speed of this increase suggests strong environmental influence
-time spent indoors with artificial light through childhood and adolescence, as time spent outdoors is negatively correlated with myopia incidence

63
Q

what is genetically heritable in the lens eye? what can this cause?

A

eye depth is heritable to some extent
myopia is heritable

64
Q

what function does metabotropic glutamate receptors perform in vision?

A

they allow retinal ganglion cells to be excited by increases in light

sensory systems (17 decks)

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