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Vision Science 5 > Lecture 5/6 > Flashcards

Lecture 5/6 Flashcards

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1
Q

how is fixation disparity different from binocular disparity?

A

fixation disparity is a misalignment of the visual axes and binocular disparity is non-correspondence of the retinal regions stimulated by a target located off the horopter

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2
Q

what is Hering’s law of binocular visual direction?

A

the visual direction of fused images which fall on slightly disparate on retinal points is the average of the two visual direction

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3
Q

what is is fixation disparity angle under exo fixation disparity?

A

aL > 0 and aR < 0 = n is positive (crossed binocular disparity)

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4
Q

what is the fixation disparity angle under eso fixation disparity?

A

aL < 0 and aR > 0 = n is negative (uncrossed binocular disparity)

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5
Q

how are fixation disparity and heterophoria’s related?

A

FD and heterophoria is usually correlated (exophoria = exo FD and esophoria = eso FD) - but do not always agree

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6
Q

what are the testing conditions for fixation disparity?

A

test under binocular conditions using prisms to find the associated phoria

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7
Q

what are the testing conditions for heterophorias?

A

test under monocular conditions using prisms to find the dissociated phoria (need to break binocular fusion)

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8
Q

what is an associated phoria?

A

the amount of prism needed to eliminate fixation disparity (horizontal associated phoria is usually smalelr than dissociated phoria)

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9
Q

what are the two common features shared by most clinical tests for FD?

A

binocular fusion lock (seen by both eyes with angular width of 1.5) and two nonius lines (seen monocularly)

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10
Q

what are some tests that assess FD and the associated phoria?

A

Bernell lantern, malett box, wesson card, sheedy disparometer, and Saladin card

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11
Q

what is fixation disparity?

A

a very small deviated of the visual axes during normal binocular fusion - the fixated object is still seen as fused

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12
Q

can a fixation disparity exist if the patient has a strabismus?

A

no - it is a deviation that only exists during binocular fusion

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13
Q

which clinical test is the most accurate for the measurement of FD?

A

sheedy disparometer (Saladin card uses the same method)

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14
Q

using the wesson card, are the polarized lines deviated or centered?

A

they are centered - the lines mark the intended fixation point but each eye’s visual axis misses it (appear deviated due to FD)

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15
Q

using the sheedy disparometer, are the polarized lines deviated or centered?

A

they are actually deviated - each line falls on the visual axis of that eye and appear centered due to FD

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16
Q

what does the sheedy disparometer display?

A

the location of the visual axes of each eye (actual position of OS and OD)

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17
Q

what does the wesson card display?

A

the perceived location of the fixation point relative to each visual axis (apparent location of OS and OD)

18
Q

what does introducing prism do when a patient has a fixation disparity?

A

used to shift the images to match their visual axis (image will follow the apex)

19
Q

what does BI prism stimulate?

A

negative fusional vergence (divergence)

20
Q

what does BO prism stimulate?

A

positive fusional vergence (convergence)

21
Q

if BI prisms are introduced in front of both eyes - what happens to the image for the OD and what does the eye do in response?

A

BI prism moves the image temporally (nasal retina) (for OD) - creates a small disparity, within panum’s area which stimulates disparity vergence (divergence) and foveal fixation is restored

22
Q

what do you do if a patient has a 2’ exo FD?

A

start adding BI prism - causing the image to shift temporal (decreasing the exo FD) - eventually with the right amount of BI prism (4 BI) the FD will be zero

23
Q

what happens to a 2’ exo FD if you introduce too much BI prism?

A

more BI prism will shift the images further to the right - beyond the visual axis and the patient will now have an eso FD

24
Q

what is forced vergence?

A

using prisms to force the eyes to diverge or converge to eliminate an FD (using BI or BO)

25
Q

what is the x-axis in the forced vergence fixation disparity curve?

A

the prism that you use to neutralize the associated phoria (BI or BO) where FD = 0

26
Q

what is the y-axis in the forced vergence fixation disparity curve?

A

the patient’s fixation disparity (Eso or Exo) without any prism

27
Q

what do the ends of the curve plotted represent in the forced vergence fixation disparity curve?

A

the break points for BI and BO prisms - after these points the patient is outside Panum’s area and will have diplopia

28
Q

what is the forced vergence fixation disparity curve?

A

characterizes how the vergence system will respond to the demand - more dynamic measurement than a single phoria or vergence measure

29
Q

what is the slope at the y-intercept indicate in the forced vergence fixation disparity curve?

A

how “good” the vergence system is (flatter is better/stronger)

30
Q

what does a flat central part (stays around zero) indicate in the forced vergence fixation disparity curve?

A

there is no fixation disparity

31
Q

what does a greater/longer slope indicate across the middle section in the forced vergence fixation disparity curve?

A

fixation disparity is slowly changing with addition of prism (eyes tend to lag behind prism when they stimulate vergence)

32
Q

what is the type 1 FD curve?

A

the more common and considered a normal response (can be EXO or ESO)

33
Q

what is the type 1 eso FD curve?

A

eso FD with no prism - then add BO prism to decrease eso FD until it is zero

34
Q

what is the type 1 exo FD curve?

A

exo FD without prism - add BI to decrease until it becomes zero

35
Q

what is the type 2 FD curve?

A

usually found in patients with large eso-phorias (curve is all in eso portion)

36
Q

why does the type 2 curve stay the same no matter how much BO prism is given?

A

the eyes are too good at converging and they stop following the prism

37
Q

what is the type 3 FD curve?

A

usually found in patients with large exo-phorias (curve is all in exo portion)

38
Q

why does the type 3 curve stay the same no matter how much BI prism is given?

A

the eyes are too good at diverging and they stop following the prism

39
Q

what is the type 4 FD curve?

A

the patient has a small exo FD - very rare (5% of population)

40
Q

what happens to the type 4 curve when BI increases?

A

the FD quickly converts from exo to eso - the eyes are not following the prism very closely

41
Q

what everyday task may increase FD, associated phoria or the FD curve slope?

A

sustained near visual tasks (reading) produce stress in the binocular system

Vision Science 5 (14 decks)

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