19: Accommodation Flashcards
what does the stimulus response function show?
it shows the steady state accommodation response to a steady object placed at a range of target distances
what is the shape of the stimulus response function
sigmoidal (S shaped)
what is the slope of the central linear region of the stimulus response function?
0.9-1.0 for an adequate target
SRF: accommodative lag for _____ objects
nearer
SRF: accommodative lead for ____ objects
farther
what is the fulcrum of the SRF?
the fulcrum position is the target dioptric vergence for which the response exactly matches the stimulus
define accommodative far point
is the most distal retinal conjugate for any target vergence
define accommodative near point
is the most proximal retinal conjugate for any target vergence
what is the typical standard deviation of micro fluctuations
typically less than 0.4 D
the total response time to a step =
the same of the reaction time and the movement time
step response:
reaction time=
is the length of time during which there is no immediate change in response
what does the reaction time in the step response represent?
neural processing time and ciliary body latency time
what is the range of reaction time in the step response?
for non-predictable target motion it falls in the range of 280-425 ms
typically, the accommodative response is a:
rising or falling exponential
accommodation has a maximum speed of up to ____ in young humans
17 D/s
in far range of viewing, increasing accommodation is: ________ (faster/slower)
faster
what are Heath’s 4 components of accommodation?
- reflex
- proximal
- convergence
- tonic
describe ‘reflex’ accommodation
refers to accommodation response to defocus
why is ‘reflex’ not the best word for reflex accommodation
the response is not a true reflex: attention is necessary for a good accommodation response
what are some examples of a stimulus to accommodation
blur and longitudinal chromatic aberration (LCA) are stimuli for accommodation
in reflex accommodation, if conditions were met to remove blue and LCA, what would occur?
many individuals would continue to focus but with lower gain
what is proximal accommodation?
is the accommodation response to the perceived distance of an object
list some cues to distance
pictorial depth cues, relative size and looming, motion parallax, and stereo depth cues
accommodation to proximity is ____ accurate than to defocus
less
what is the stimulus response slope for proximal accommodation
0.3-0.6
what is voluntary accommodation?
refers to the ability to change accommodation voluntarily independent of the external stimulus
describe convergence accommodation
is accommodation due to convergence (or divergence) of the eyes, and it is part of the near response
what is the CA/C ratio
the amount of convergence accommodation per unit of convergence
what are typical CA/C ratio values
0.6-0.7 D/MA
what is tonic accommodation?
is defined as the accommodation response in the absence of any accommodative (and vergence) stimuli
what is the intermediate resting position hypothesis?
holds that the resting position of accommodation is not the far point but some intermediate focus level
what are the population norm values for dark focus
mean dark focus of 1.5D, with a range of 0-4 D
what is modified mohindra retinoscopy used for?
a clinical method for determining dark focus
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
parasympathetic innervation is mainly by ____ receptors
M3 muscarinic receptors
sympathetic innervation is mainly by ____ receptors
B2 adrenergic receptors in the ciliary muscle
increased parasympathetic innervation leads to _______ accommodation
increased
describe parasympathetic innervation system for accommodation
is rapid and accounts for the fast observable dynamics of accommodation, such as those in the step response
describe sympathetic innervation system for accommodation
has a slow action (it requires minutes to reach its intended level)
increased sympathetic innervation leads to _____ accommodation
decreased
what forms the triad of the near response?
accommodation, convergence, and pupil responses
accommodation is less accurate for _____ pupils
smaller
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
a fully corrected hyperopic eye must exert _____ ocular accommodation than the fully corrected myopic eye at any near distance
slightly more
what is the accommodative unit?
the ratio of ocular refraction in the spectacle-corrected ametropic eye to that in the emmetropic eye
define presbyopia
the age-related loss of accommodative amplitude
subjective methods generally _____ (over/under) estimate the objective amplitude of accommodation
overestimate
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)
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
changes to the linear zone in the SRF of accommodation with age
linear zone becomes progressively smaller
ciliary body changes with age
-ciliary body moves anteriorly and inward with age
but ciliary body force and activity does not decrease with age
zonular fiber changes with age
the attachment of the anterior lens move forward in the 40s
crystalline lens changes with age
increased thickness, decreased radii of curvature, and a forward movement of its center of mass in the eye
lens capsule changes with age
becomes thicker, less elastic, and more brittle
lens substance changes with age
becomes less elastic with age
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
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
disaccomaodation theory of presbyopia is based on the:
observation that the presbyopia lens appears much like an accommodated young lens
what doesn’t fit with the disaccomaodation theory of presbyopia
the lens diameter, which does not decrease with age but stays constant
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
what is the schachar’s theory of accommodation
increased zonular fiber tension leads to increased accommodation
(opposite of Helmholtz-Fincham model)
schachar’s theory of presbyopia
increased equatorial diameter with age leads to decreased zonular fiber tension and hence decreased accommodative amplitude
