4. Accommodation, Vergence, and ZCSBV - Eye Movements & Binocular Vision Summer 2026

What is the equation for accommodative demand?

1/distance

What is the accommodative demand for a far away target in a fully corrected patient?

0

True or False:

The accommodative demand formula is a generalization

true

The accommodative demand equation DOES NOT necessarily describe what?

the accommodative demand for an object for a given patient

What needs to be factored into the accommodative demand equation?

-refractive error

-refractive correction

True or False:

We can accommodate and we can relax accommodation

true

What is the absolute limit to accommodation?

fully relaxing accommodation -- we cannot accommodate less than 0D

True or False:

Sometimes the spec rx that we give patients DOES NOT match the refraction

true

When would we not give patients the spec rx that matches their refraction?

D/t aniseikonia

Usually d/t accommodation

What is the distance accommodative response in a 2D hyperope wearing +2.00D specs?

0

What is the amount of blur at distance in a 2D hyperope wearing +2.00D specs?

clear

What is the near accommodative response in a 2D hyperope wearing +2.00D specs?

+2.50

What is the amount of blur at near in a 2D hyperope wearing +2.00D specs?

clear

What is the distance accommodative response in a 1D hyperope wearing no specs?

+1.00

What is the amount of blur at distance in a 1D hyperope wearing no specs?

clear

What is the near accommodative response in a 1D hyperope wearing no specs?

+3.50

What is the amount of blur at near in a 1D hyperope wearing no specs?

clear

What is the distance accommodative response in a -1D myope wearing -2.00 specs?

+1.00 (build in add)

What is the amount of blur at distance in a -1D myope wearing -2.00D specs?

clear

What is the near accommodative response in a -1D myope wearing -2.00 specs?

+3.50

What is the amount of blur at near in a -1D myope wearing -2.00D specs?

clear

What is the distance accommodative response in a -3D myope wearing -2.00 specs?

0.00

What is the amount of blur at distance in a -3D myope wearing -2.00D specs?

-1.00 blur

What is the near accommodative response in a -3D myope wearing -2.00 specs?

+1.50

What is the amount of blur at near in a -3D myope wearing -2.00D specs?

clear

Disjunctive binocular movements

vergence

With vergence, eyes will go in the (same/opposite) direction

opposite

Vergence movements around the vertical axis (horizontal movements)

-convergence

-divergence

What is convergence?

inward movement of both eyes

What is divergence?

outward movement of both eyes

What is zero lateral vergence posture?

when the lines of sight are parallel such as when viewing an object at infinity

What is a vergence angle?

angle between the lines of sight

Lateral Vergence Angle Equation

Verg Angle = pd / D

With the lateral vergence angle equation, if pd and D are both the same units, the angle unit will be in _____

radians

With the lateral vergence angle equation, if pd is in cm and D is in m, the angle unit will be in _____

prism diopters (cm/m)

Can radians be converted to degrees?

Yes

What are meter angles and are they typically used clinically?

-1/D

-Meter angles are not typically used clinically

What are meter angles useful for?

In understanding some relationships of units -- we can look at accommodation and vergence responses in similar units

How to convert meter angles to prism diopters?

meter angles * pd (cm) = PD

Lateral Vergence Angle (Pic)

Lateral Vergence Angle (Pic)

What are the vergence movements around the horizontal axis (vertical movements)?

-hypovergence

-hypervergence

What is hypervergence?

If the right eye goes up and the left eye goes down, this is a R hypervergence

What is hypovergence?

If the right eye goes down and the left eye goes up, this is a R hypovergence

Are vergence disorders common?

Yes

What is THE MOST COMMON binocular vision disorder?

convergence insufficiency

What can we do to correct convergence insufficiency?

-vision therapy

-prism in specs

What is the benefit of vision therapy for convergence insufficiency?

improves fusional compensation

What are the 4 components to total horizontal ocular vergence?

-fusional (disparity) vergence

-accommodative vergence

-proximal vergence

-tonic vergence

What is the stimulus for fusional (disparity) vergence?

binocular disparity

What is the equation for disparity?

Disparity = Target position - Eye position

Positive disparity (eyes are underconverged) is a stimulus for ________ fusional convergence

positive

To get rid of fusional convergence, what would we have to do?

get rid of fusional convergence

When we break fusion, we are left with a _____ on alternating cover test

phoria

True or False:

When you uncover an eye, fusional vergence will come back

true

What is the latency for the fusional vergence system?

160ms

What are the 2 components to fusional (disparity) vergence?

-dynamic

-static

What is dynamic fusional (disparity) vergence?

Fusional vergence used to change vergence posture (fixate a target at a different distance)

What is static fusional (disparity) vergence?

Holds vergence posture (compensates for the phoria)

What is the only components of the vergence system that is controlled by a feedback loop?

fusional (disparity) vergence

Accommodative vergence is driven by __________

accommodation

What is the Rate of accommodative convergence equation?

Accommodative convergence / accommodation (AC/A)

What is the unit for accommodative convergence?

PD / d

How is the AC/A ratio measured in clinic?

-calculated AC/A

-gradient AC/A

What is the avg AC/A ratio?

4 pd/D

What is proximal vergence?

stimulus is the perceived distance of the target

Can proximal vergence affect clinical testing?

Yes -- mirrored room, instrument (phoropter)

What is tonic vergence?

-ocular vergence posture when the output of the other 3 stimuli driven vergence components is 0

-vergence posture in the absence of vergence stimuli

What are the 4 components to ocular accommodation?

-blur (defocus) accommodation

-vergence accommodation

-proximal accommodation

-tonic accommodation

What is the stimulus for blur (defocus) accommodation?

retinal defocus

True or False:

blur (defocus) accommodation is driven by a feedback loop

true

What is convergence accommodation driven by?

by the act of converging

What is the rate of convergence accommodation equation?

convergence accommodation / convergence (CA/C)

What is the typical units on the CA/C ratio?

D / pd

What is a normal CA/C value?

0.1-0.4 D/pd

Is CA/C typically measured clinically?

No

Does the CA/C contribute substantially to normal binocular vision?

Yes

What is tonic accommodation?

Base line value in the absence of any stimulus to accommodation

What is the avg value for tonic accommodation?

1.5D

What is a phoria?

vergence posture of the eyes in the absence of fusional vergence (latent deviation)

What question does a phoria value answer?

-"How close does accommodative, proximal, & tonic vergence come to the required vergence angle?"

Whats left over when accommodative, promixal, and tonic accommodation are in play is accounted for by _______

fusional vergence

What does the phoria demonstrate?

What fusional vergence must ovecome when the person is looking with both eyes

The bigger the phoria, the (more/less) strain you are putting on fusional vergence

more

What are avg values for near phoria?

3pd exophoria

What are avg values for distance phoria?

1pd exophoria

How is a phoria measured clinically?

-alternating cover test

-VG phorias

-Modified Thorington

What is an esophoria?

person uses negative fusional convergence (turn eyes out) to compensate for the tendency to turn eyes in

What is an exophoria?

person uses positive fusional convergence (turn eyes in) to compensate for the tendency to turn eyes out

In order to measure a phoria, what vergence components need to be held stable?

-accommodative vergence

-proximal vergence

To measure a dissociated phoria, you need to eliminate __________

fusional vergence

REVIEW: How do we eliminate fusional vergence?

-occlusion (CT)

-vertical prism (VG)

To measure a phoria, we have to measure what?

how far the covered eye drifted away from the fixation target

What does the phoria represent?

how much fusional vergence must be used to maintain fusion

To assess the impact of the phoria, what must we do?

measure the vergence ranges

One method for analyzing the workload that patients have for visual tasks at differing distances?

Graphical analysis of the ZCSBV

What does this graph (see pic) show?

For a Target at 40cm, the accommodative demand will be 2.5D and the vergence demand for a 60cm PD will be 15PD

How to increase the convergence demand of a target?

BO prism

What happens if you increase the convergence demand of a target?

Patient will have to use positive fusional vergence to fuse the target

What does positive fusional vergence create?

positive convergence accommodation

In some patients, increasing positive convergence accommodation will create ______

blur