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Visual Science: Binocular Vision > Binocular Vision: Lecture 7: Fixation Disparity > Flashcards

Binocular Vision: Lecture 7: Fixation Disparity Flashcards

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1
Q
  1. What does Stereopsis depend on? (2)
    a. Especially which one?
    b. Why is this?
  2. What would happen if we had Errors in Motor Fusion?
  3. What does Heterophoria represent?
A
  1. Sensory, and Motor Fusion

a. Motor Fusion
b. Because disparities needed for Stereopsis are very SMALL DIFFERENCES in the Image position in the two eyes

  1. It would GREATLY distort the Depth info we get from Stereopsis, or ELIMINATE Binocular Depth Perception
  2. Fusional Vergence Demand once fusion is attempted, which is to say, how much more the eyes must converge or diverge from their dissociated position to regain binocular vision (motor fusion)
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2
Q
  1. What does ESOPHORIA need to Maintain fusion?
  2. What about Exophoria?
  3. What is an UNCOMPENSATED HETEROPHORIA called?
  4. What does a Maddox Rod consist of?
    a. What does it do?
A
  1. Negative Fusional Vergence (Divergence) (NFV)
  2. Positive Fusion Vergence (Convergence)
  3. Strabismus
  4. A Series of High Powered Cylindrical Lenses
    a. Dissociate Fusion
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3
Q
  1. What is Fixation Disparity?
  2. The level of innervation maintained by the Vergence Eye Movement System is determined by what 2 things?
    a. What does Fixation Disparity serve as a STIMULUS FOR? Why?
  3. Larger fusional Vergence requires what?
A
  1. a Small, Purposeful Error in Vergence
  2. Magnitude and Sign of a Vergence Error Signal
    a. for the VERGENCE SYSTEM; to maintain its innervation level.
  3. Larger amts of Fixation disparity
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4
Q
  1. The Magnitude of Fixation Disparity is DIRECTLY related to what?
  2. Why is measuring Fixation Disparity useful?
  3. What is a Fixation Disparity of more than a few minutes of arc an indicator of?
A
  1. to the Magnitude of the Fusional Vergence response required for Binocular Vision
  2. it tells us how well the Disparity Vergence System is working
  3. of Patients w/Potential Binocular Vision Problems
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5
Q
  1. Why are Fixation Disparities important?

2. What are NORMAL Fixation Disparities? (Exo and Eso)

A
  1. They help us (doctors) determine the Correct amt of Prism to prescribe to correct horizontal and Vertical Phorias
  2. 6 arc minutes EXO and 4 Arc minutes ESO
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6
Q
  1. Fixation Disparity is measured clinically at distance and near w/simple targets. What 2 things do these targets consist of?
  2. A Portion of the Target, Known as FUSION LOCK, is polarized/unpolarized and seen by one/both eyes.
    a. Another part is Polarized/unpolarized and seen only by the right/left eye.
    b. Another part is what? And visible only to what eye(s)?
  3. What degree of Fusion Lock is used in MOST of the TESTS?
    a. This is about the size of what?
    b. For a 40 cm test distance, a 1.5 degree circle has a diameter of about what?
A
  1. a. Some binocularly visible details that serve as a Binocular Fusion Lock

and

b. 2 Monocularly seen Nonius Lines (via Polaroid Filters)
2. UNPOLARIZED; BOTH EYES
a. Polarized; Right eye
b. Cross-polarized; Left Eye
3. 1.5 degrees of fusion lock
a. of the Rod-Free Fovea
b. 1 cm

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7
Q
  1. Binocularly viewed target will be fused and seen singly. One or both of the 2 Nonius Lines will be perceived as what?
  2. What will Fixation Disparity create?
    a. Why?
A
  1. Out of alignment, even though they’re actually physically aligned w/the binocularly viewed target
  2. Misalignment of the Nonius Lines
    a. They no longer stimulate corresponding Retinal Points
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8
Q
  1. Mallet Unit Fixation Disparity Test: What does it do?

2. Other 2 tests

A
  1. help determine relationship of the 2 eye’s visual axes to each other.
  2. Wesson Fixation Disparity Card; and Bernell Test Lantern (Crude method of measuring fixation disparity)
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9
Q
  1. What is Fixation disparity related to and usually correlated with?
    a. Esp at Near/distance?
    b. Why is that?
  2. Amt of prism needed to eliminate fixation disparity is called what?
    a. Because Fusion is present, the two eyes are WHAT?
  3. Which is usually smaller: Horizontal Associated Phoria or Horizontal Dissociated Phoria measurement?
    a. Why?
A
  1. Heterophorias
    a. NEAR
    b. cuz an Esophoria usually will have an ESO Fixation Disparity, and an Exophoric Patient will more likely have an Exo fixation disparity to maintain PFV activity
  2. ASSOCIATED PHORIA
    a. ASSOCIATED!
  3. Horizontal Associated Phoria
    a. Cuz of the Additional influence of Prism Adaptation under Associated Conditions
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10
Q

Sheedy Disparometer

  1. What does it have?
  2. What does the patient do?
A
  1. a Set of Targets w/LINES offset by predetermined amts in either the ESO or EXO direction
  2. They select the target that APPEARS ALIGNED. It’s labeled with the amt and direction of the Fixation disparity
    * Saladin Near Point Card (look it up…)
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11
Q

Vergence Adaptation

  1. The response of the vergence system to prism or lenses varies on the individual. MANY PATIENTS will exhibit what?
    a. What is this?
  2. When a Prism is first introduced before a Binocularly Fixating individual, what happens?
A
  1. Vergence Adaptation
    a. An Applied prism or lens has a full effect at first, then has less apparent effectiveness over time, sometimes over the course of only a few minutes or even seconds
  2. a Fast, Immediate response is generated in the FAST or DISPARITY VERGENCE SYSTEM
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12
Q

Vergence Adaptation (2)

  1. What is the Goal of the FAST RESPONSE?
  2. What has to remain in order to allow the fast system to continue to function?
A
  1. To get rid of RETINAL DISPARITY caused by the prism and to restore single binocular vision
  2. Fixation Disparity
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13
Q

Vergence Adaptation (3)

  1. Continued output of the fast disparity vergence system provides the stimulus for activation of the 2nd vergence System component: What is it?
  2. What does the Slow System increase the levels of?
    a. When this system is activated to provide the vergence needed to compensate for the prism, less output of what system is needed?
A
  1. the SLOW or VERGENCE ADAPTATION SYSTEM
  2. Tonic Vergence
    a. of the Fast System
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14
Q

Forced Vergence Fixation Disparity Curve (1)

  1. What is a Fixation Disparity Curve?
    a. It’s more indicative of how a patient will respond where?
A
  1. more a DYNAMIC measure than a single phoria or vergence measurement
    a. in the real world to visual tasks, esp at nearpoint
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15
Q

Forced Vergence Fixation Disparity Curve (2)

  1. How does it test the Binocular Fusion?
  2. Fixation disparity curve tells us what?
A
  1. By Increasing Vergence demand (introduce Prism), then Measure how Fixation Disparity Changes as a FUNCTION of the POWER and DIRECTION of the PRISM
  2. How well the vergence system copes with demands on it.
    * From -5 to +5, there is very little change
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16
Q

Forced Vergence Fixation Disparity Curve (3)

  1. X-Axis: What is it?
  2. Y-Axis: Shows what 2 things about FIXATION DISPARITY?
  3. How would the curve shift if an Eso Fixation Disparity was present?
A
  1. Amt of prism used to induce NFV and PFV
  2. Amt and Direction of Fixation Disparity
  3. It would shift UPWARD (opp. for EXO)
17
Q

Forced Vergence Fixation Disparity Curve (4)

  1. How is Fixation Disparity usually measured?
  2. Base-in prism stimulates what?
    a. This produces what type of Fixation Disparity?
  3. Base-out..(opp)
  4. Shape of the Fixation Disparity Curve is USUALLY…?
A
  1. No Prism; 3 PD BI, 3 PD BO, 6 PD BI, and 6 PD BO
  2. NFV (Divergence)
    a. ESO
  3. Opp…
  4. Sigmoidal or S-Shaped
18
Q

Forced Vergence Fixation Disparity Curve (5)

  1. What does the Y-Intercept tell us?
  2. What does the X-Intercept tell us?
A
  1. the Amt of Fixation Disparity w/o Added Prism

2. the Amt of Prism that Produces ZERO Fixation Disparity (ASSOCIATED PHORIA)

19
Q

Forced Vergence Fixation Disparity Curve (6)

  1. If pt has 0 Fixation Disparity, and can’t converge more than 10PD BO and Diverge more than 10 PD BI:
    a. What happens as more BI prism is added?

b. BO starts to be increased, and the fixation point appears to move how?
c. As More BO is added, what happens?

A
  1. a. The eyes can’t follow the target and the image begins to slide off the center of Panum’s Area, causing a slight ESO Fixation Disparity
    b. Inward before each eye
    c. the Eyes no longer follow the target, and the image starts to slide off the center of Panum’s area, causing a slight EXO Fixation disparity
20
Q

Forced Vergence Fixation Disparity Curve (7)

  1. Type I Curve
    a. How does Fixation disparity change?
    b. Which region of the Curve is FLATTER and which is STEEPER?
A
  1. a. Gradually w/added prism until it nears the limits of their Fusional Vergence Ranges
    b. Flatter Central Region and Steeper Peripheral Region
21
Q

Forced Vergence Fixation Disparity Curve (8)

  1. Type I Curve
    a. Why is the Central Region FLATTER?
    b. Due to the effective range of Vergence adaptation being LIMITED, beyond this range in the peripheral zones of the fixation disparity curve, what EMERGES?
    i. WHY?
    c. So what is the Tail portion of the Fixation Disparity Curve?
A
  1. a. due to Vergence Adaptation. With a Relatively Smaller fixation disparity increase w/increasing prism power
    b. TAILS Emerge w/Fixation disparity increasing at a FASTER RATE w/PRISM
    i. Cuz more is NEEDED to maintain FAST (Disparity) Vergence System Activity
    c. A Region beyond the Effective Range of vergence Adaptation
22
Q

Forced Vergence Fixation Disparity Curve (9)

  1. Type II Curve
    a. % of Population at Distance or Nearpoint

b. Has a flat region where?
c. Where is it SHARPLY CURVED?

d. Most Patients with TYPE II CURVES have what?
i. They show poor adaptation to what kind of Prism?

A
  1. a. 25%
    b. on its BO (Convergent) Side that may not cross the X-Axis
    c. On the BI Side
    d. ESOPHORIA
    i. to Base-in Prism
23
Q

Forced Vergence Fixation Disparity Curve (10)

  1. TYPE III Curve
    a. % of population tested at Nearpoint have this?

b. % of population at Distance?
c. Flat on what side?
d. Steep on what side?
e. They have what (eso or ExO)
f. What can they handle well?
g. What can they NOT handle well?

A
  1. a. 10%
    b. 0%
    c. Base-in (Divergent) side
    d. Base-out side
    e. HIGH EXOPHORIA
    f. FORCED DIVERGENCE well
    g. Forced Convergence
24
Q

Forced Vergence Fixation Disparity Curve (11)

  1. Type IV Curves
    a. % of population at distance or nearpoint?
    b. They show what kind of change in Fixation disparity?
A
  1. a. 5%
    b. LITTLE Change in Fixation disparity w/increasing vergence demand in ANY PORTION of the CURVE, even near the fusional Limits (i’m guessing that’s 20 PD)
25
Q

Forced Vergence Fixation Disparity Curve (12)

  1. Which are usually steeper: Vertical vergence of Horizontal Vergence Fixation Disparity Curves?
    a. Why?
  2. What is considered a reasonable starting point for most patients for an Associated Phoria Prism Prescription?
A
  1. Vertical Vergence Fixation Disparity Curves
    a. Due to the difference in Robustness b/w the vertical and horizontal vergence systems
  2. 1/2 of the associated phoria prism prescription
26
Q

Forced Vergence Fixation Disparity Curve (13)

  1. What 2 things contribute to Fixation Disparity?
  2. What produces stress on the ENTIRE Binocular Vision System, including Vergence?
    a. How have these effects been shown after 20 minutes of reading?
A
  1. Accommodation and Fusional Vergence
  2. Sustained Nearpoint visual tasks
    a. Increased Fixation disparity, increased associated phoria, and increased slope in the forced vergence fixation curve

Visual Science: Binocular Vision (20 decks)

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