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Visual Optics > 19: Accommodation > Flashcards

19: Accommodation Flashcards

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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
what is the stimulus response slope for proximal accommodation
0.3-0.6
26
what is voluntary accommodation?
refers to the ability to change accommodation voluntarily independent of the external stimulus
27
describe convergence accommodation
is accommodation due to convergence (or divergence) of the eyes, and it is part of the near response
28
what is the CA/C ratio
the amount of convergence accommodation per unit of convergence
29
what are typical CA/C ratio values
0.6-0.7 D/MA
30
what is tonic accommodation?
is defined as the accommodation response in the absence of any accommodative (and vergence) stimuli
31
what is the intermediate resting position hypothesis?
holds that the resting position of accommodation is not the far point but some intermediate focus level
32
what are the population norm values for dark focus
mean dark focus of 1.5D, with a range of 0-4 D
33
what is modified mohindra retinoscopy used for?
a clinical method for determining dark focus
34
path of parasympathetic innervation for accommodation
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
parasympathetic innervation is mainly by ____ receptors
M3 muscarinic receptors
36
sympathetic innervation is mainly by ____ receptors
B2 adrenergic receptors in the ciliary muscle
37
increased parasympathetic innervation leads to _______ accommodation
increased
38
describe parasympathetic innervation system for accommodation
is rapid and accounts for the fast observable dynamics of accommodation, such as those in the step response
39
describe sympathetic innervation system for accommodation
has a slow action (it requires minutes to reach its intended level)
40
increased sympathetic innervation leads to _____ accommodation
decreased
41
what forms the triad of the near response?
accommodation, convergence, and pupil responses
42
accommodation is less accurate for _____ pupils
smaller
43
the accommodation control system for defocus can be made 'open loop' with what?
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
a fully corrected hyperopic eye must exert _____ ocular accommodation than the fully corrected myopic eye at any near distance
slightly more
45
what is the accommodative unit?
the ratio of ocular refraction in the spectacle-corrected ametropic eye to that in the emmetropic eye
46
define presbyopia
the age-related loss of accommodative amplitude
47
subjective methods generally _____ (over/under) estimate the objective amplitude of accommodation
overestimate
48
what is the difference between subjective and objective measurements of amplitude of accommodation
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
changes in age of response time (latency) and response speed of accommodation
response time becomes longer with age but response speed is unaffected by age for targets within the linear zone
50
changes to the linear zone in the SRF of accommodation with age
linear zone becomes progressively smaller
51
ciliary body changes with age
-ciliary body moves anteriorly and inward with age | but ciliary body force and activity does not decrease with age
52
zonular fiber changes with age
the attachment of the anterior lens move forward in the 40s
53
crystalline lens changes with age
increased thickness, decreased radii of curvature, and a forward movement of its center of mass in the eye
54
lens capsule changes with age
becomes thicker, less elastic, and more brittle
55
lens substance changes with age
becomes less elastic with age
56
lenticular theory of presbyopia
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
geometric theory of presbyopia
changes in the geometry of the lens, zonular fibers, and ciliary body reduce the transfer of forces from ciliary body to the lens
58
disaccomaodation theory of presbyopia is based on the:
observation that the presbyopia lens appears much like an accommodated young lens
59
what doesn't fit with the disaccomaodation theory of presbyopia
the lens diameter, which does not decrease with age but stays constant
60
what would the disaccomaodation theory of presbyopia rely on to prevent the individual from becoming highly myopic
must rely on a large decrease in lens refractive index
61
what is the schachar's theory of accommodation
increased zonular fiber tension leads to increased accommodation (opposite of Helmholtz-Fincham model)
62
schachar's theory of presbyopia
increased equatorial diameter with age leads to decreased zonular fiber tension and hence decreased accommodative amplitude

Visual Optics (22 decks)

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