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Visual Optics > 19: Accommodation > Flashcards

19: Accommodation Flashcards

1
Q

what does the stimulus response function show?

A

it shows the steady state accommodation response to a steady object placed at a range of target distances

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2
Q

what is the shape of the stimulus response function

A

sigmoidal (S shaped)

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3
Q

what is the slope of the central linear region of the stimulus response function?

A

0.9-1.0 for an adequate target

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4
Q

SRF: accommodative lag for _____ objects

A

nearer

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5
Q

SRF: accommodative lead for ____ objects

A

farther

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6
Q

what is the fulcrum of the SRF?

A

the fulcrum position is the target dioptric vergence for which the response exactly matches the stimulus

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7
Q

define accommodative far point

A

is the most distal retinal conjugate for any target vergence

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8
Q

define accommodative near point

A

is the most proximal retinal conjugate for any target vergence

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9
Q

what is the typical standard deviation of micro fluctuations

A

typically less than 0.4 D

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10
Q

the total response time to a step =

A

the same of the reaction time and the movement time

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11
Q

step response:

reaction time=

A

is the length of time during which there is no immediate change in response

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12
Q

what does the reaction time in the step response represent?

A

neural processing time and ciliary body latency time

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13
Q

what is the range of reaction time in the step response?

A

for non-predictable target motion it falls in the range of 280-425 ms

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14
Q

typically, the accommodative response is a:

A

rising or falling exponential

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15
Q

accommodation has a maximum speed of up to ____ in young humans

A

17 D/s

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16
Q

in far range of viewing, increasing accommodation is: ________ (faster/slower)

A

faster

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17
Q

what are Heath’s 4 components of accommodation?

A
  1. reflex
  2. proximal
  3. convergence
  4. tonic
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18
Q

describe ‘reflex’ accommodation

A

refers to accommodation response to defocus

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19
Q

why is ‘reflex’ not the best word for reflex accommodation

A

the response is not a true reflex: attention is necessary for a good accommodation response

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20
Q

what are some examples of a stimulus to accommodation

A

blur and longitudinal chromatic aberration (LCA) are stimuli for accommodation

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21
Q

in reflex accommodation, if conditions were met to remove blue and LCA, what would occur?

A

many individuals would continue to focus but with lower gain

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22
Q

what is proximal accommodation?

A

is the accommodation response to the perceived distance of an object

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23
Q

list some cues to distance

A

pictorial depth cues, relative size and looming, motion parallax, and stereo depth cues

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24
Q

accommodation to proximity is ____ accurate than to defocus

A

less

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25
Q

what is the stimulus response slope for proximal accommodation

A

0.3-0.6

26
Q

what is voluntary accommodation?

A

refers to the ability to change accommodation voluntarily independent of the external stimulus

27
Q

describe convergence accommodation

A

is accommodation due to convergence (or divergence) of the eyes, and it is part of the near response

28
Q

what is the CA/C ratio

A

the amount of convergence accommodation per unit of convergence

29
Q

what are typical CA/C ratio values

A

0.6-0.7 D/MA

30
Q

what is tonic accommodation?

A

is defined as the accommodation response in the absence of any accommodative (and vergence) stimuli

31
Q

what is the intermediate resting position hypothesis?

A

holds that the resting position of accommodation is not the far point but some intermediate focus level

32
Q

what are the population norm values for dark focus

A

mean dark focus of 1.5D, with a range of 0-4 D

33
Q

what is modified mohindra retinoscopy used for?

A

a clinical method for determining dark focus

34
Q

path of parasympathetic innervation for accommodation

A

from accommodative neurons in the Edinger-Westphal nucleus via the oculomotor nerve to the ciliary ganglion and to the ciliary muscle via the short ciliary nerves

35
Q

parasympathetic innervation is mainly by ____ receptors

A

M3 muscarinic receptors

36
Q

sympathetic innervation is mainly by ____ receptors

A

B2 adrenergic receptors in the ciliary muscle

37
Q

increased parasympathetic innervation leads to _______ accommodation

A

increased

38
Q

describe parasympathetic innervation system for accommodation

A

is rapid and accounts for the fast observable dynamics of accommodation, such as those in the step response

39
Q

describe sympathetic innervation system for accommodation

A

has a slow action (it requires minutes to reach its intended level)

40
Q

increased sympathetic innervation leads to _____ accommodation

A

decreased

41
Q

what forms the triad of the near response?

A

accommodation, convergence, and pupil responses

42
Q

accommodation is less accurate for _____ pupils

A

smaller

43
Q

the accommodation control system for defocus can be made ‘open loop’ with what?

A

with a 0.5 mm pinhole

because depth of focus is so large that any changes in accommodation have no affect on perceived image quality

44
Q

a fully corrected hyperopic eye must exert _____ ocular accommodation than the fully corrected myopic eye at any near distance

A

slightly more

45
Q

what is the accommodative unit?

A

the ratio of ocular refraction in the spectacle-corrected ametropic eye to that in the emmetropic eye

46
Q

define presbyopia

A

the age-related loss of accommodative amplitude

47
Q

subjective methods generally _____ (over/under) estimate the objective amplitude of accommodation

A

overestimate

48
Q

what is the difference between subjective and objective measurements of amplitude of accommodation

A

subjective (calculated as 15 - one quarter of patient’s age) over estimates the actual accommodative ability due to depth of field effects (object is more accurate and with auto refractor)

49
Q

changes in age of response time (latency) and response speed of accommodation

A

response time becomes longer with age but response speed is unaffected by age for targets within the linear zone

50
Q

changes to the linear zone in the SRF of accommodation with age

A

linear zone becomes progressively smaller

51
Q

ciliary body changes with age

A

-ciliary body moves anteriorly and inward with age

but ciliary body force and activity does not decrease with age

52
Q

zonular fiber changes with age

A

the attachment of the anterior lens move forward in the 40s

53
Q

crystalline lens changes with age

A

increased thickness, decreased radii of curvature, and a forward movement of its center of mass in the eye

54
Q

lens capsule changes with age

A

becomes thicker, less elastic, and more brittle

55
Q

lens substance changes with age

A

becomes less elastic with age

56
Q

lenticular theory of presbyopia

A

the loss of elasticity of the lens substance is the major force of presbyopia. the lens substance becomes more difficult to deform with age

57
Q

geometric theory of presbyopia

A

changes in the geometry of the lens, zonular fibers, and ciliary body reduce the transfer of forces from ciliary body to the lens

58
Q

disaccomaodation theory of presbyopia is based on the:

A

observation that the presbyopia lens appears much like an accommodated young lens

59
Q

what doesn’t fit with the disaccomaodation theory of presbyopia

A

the lens diameter, which does not decrease with age but stays constant

60
Q

what would the disaccomaodation theory of presbyopia rely on to prevent the individual from becoming highly myopic

A

must rely on a large decrease in lens refractive index

61
Q

what is the schachar’s theory of accommodation

A

increased zonular fiber tension leads to increased accommodation
(opposite of Helmholtz-Fincham model)

62
Q

schachar’s theory of presbyopia

A

increased equatorial diameter with age leads to decreased zonular fiber tension and hence decreased accommodative amplitude

Visual Optics (22 decks)

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