1. Why is it that in most all cases, the left hand side of the fixation disparity curve function tends to climb towards (or into) the ESO fixation disparity side of the X-axis? It is the BI side of the graph. As a patient increases divergence output, they will "get by" with the least divergence effort necessary to maintain fusion. Therefore, the fixation disparity will slip towards ESO.

2. Why is it that in most all cases, the right hand side of the fixation disparity curve function tends to climb towards (or into) the EXO fixation disparity side of the X-axis? It is the BO side of the graph. As a patient increases convergence output, they will "get by" with the least convergence effort necessary to maintain fusion. Therefore, the fixation disparity will slip towards EXO.

3. Define prism adaptation in your own words. How does the amount of prism adaptation possibly affect the slope of the fixation disparity curve in its central region? Did anyone in your group experience significant prism adaptation?

Slow (adaptive) vergence mechanism that maintains a constant fixation disparity over a range of vergence demands. If a 3 BO prism is applied, a prism adapter will respond by converging (and maintaining) exactly three prism diopters. The fixation disparity would remain constant and the slope of the central curve will be flat.

A patient who readily prism adapts would experience a significant eso shift in fixation disparity if forced to wear a 10 BO prism for 10 minutes (like in lab). This effect would be seen if the BO prism was removed and the fixation disparity measurement was taken VERY SOON AFTER! The patient adapts to the BO prism (i.e. converges EXACTLY 10 prism diopters). When the prism is removed, the patient is still adapted to the prism, so they would experience an eso shift in fixation disparity until the adaptation wears out! This mini-experiment points to the importance of removing the prism in between trials while the patient "recalibrates" their fixation disparity.

4. Did you observe any of the following signs while conducting a fixation disparity analysis? What may cause these behaviors?

a. movement of the nonius lines: influx of accommodative vergence (have pt read small letters to stabilize) or changes fast fusional vergence response (if movement is rapid and back and forth)

b. small letters on Disparometer blur: get control of that accommodative system!!!

c. loss of one nonius lines: suppression (determine which eye)

d. "bumps" (i.e. not a smooth curve) on the tails of the F.D. curve: changes in the output of the fast fusional vergence control unit. Adaptive vergence does not play a major role near the extremes of the fixation disparity curve.

5. Did your Maddox rod phoria match the modified Thorington result at near?

Give two reasons why the results are expected to be different.

a. Modified Thorington presents a significant accommodative demand. More accurate accommodation may account for a change in phoria measurement.

b. Modified Thorington presents a significant proximal cue over a simple light. More accurate determination of working distance may account for a change in phoria measurement.

c. Modified Thorington presents a peripheral fusional demand (i.e. the outside border of the card). Fusional cues may account for a change in phoria measurement. Remember, you’re supposed to be dissociated when measuring phorias!

6. When you observe the patient’s eyes during prism bar vergence testing, what two components of that test are you looking for? Break and recovery

7. Can you count on dissociated phorias to predict fixation disparities? Why or why not? It has been shown in our labs and has been established in several research papers that dissociated phorias are a lousy predictor of fixation disparity results; either magnitude (1-20 MAR) or direction (ESO-EXO)