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AC/A RatioAccommodation response
produces a convergence response
Accommodative convergenceAC/A expressed in ∆/D
CA/C RatioLess commonly measured in
practiceConvergence response produces
an accommodation responseConvergence accommodationCA/C expressed in D/∆
AccommodationBlur driven responseOther optical cues such as chromatic
aberration contributeNon-optical cues such as size, rate of
change, time to contactVergence
VergenceDisparity driven responseEye movements usually complex
with version and vergence interactions
Non-optical cues contributeAccommodation
Pupil ResponseMiosis observed for near viewingNo drive from accommodationNo drive from vergenceNon-optical cues such as perceived
distance are main drivers
AC/AResponse AC/A: amount of
accommodative convergence produced by an actual change of 1D of accommodation
Stimulus AC/A: amount of accommodative convergence produced by a change of 1D in the stimulus to accommodation
Measurement of AC/ADifficult to measure response
AC/AStimulus AC/A used in practiceNot appropriate to measure in
older age groups Useful in younger patients with
BV problems
ExamplePD 60mm; Distance 3SOP; Near 12SOP
AC/A = 6 + ((12 - 3)/3)AC/A = 6 + (9/3)AC/C = 9:1
Very High AC/A producing anomalous phoria at near
Gradient Test
-15
-10
-5
0
5
10
15
-4 -2 0 2 4
Stimulus to Accommodation (D)
Induced Phoria (∆)
AC/A
Other MethodsSynoptophore Method: Similar to
gradient but using synoptophore
Fixation Disparity Method: Indirect method and not appropriate clinically
Significance of AC/AHas effect on near phoriaLow AC/A leads to large xopHigh AC/A leads to large sopMay be associated with
convergent strabismusHyperopia and high AC/A
associated with convergent squint
Innate or Acquired?No relationship between Rx and
AC/AAC/A similar in mon and binocularStudies of twins show similar AC/AAC/A unchanged by vision training