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Vision Science 5 > Lecture 16/17 > Flashcards

Lecture 16/17 Flashcards

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1
Q

what is the function of vergence eye movements?

A

to shift the gaze from near to far (or far to near) so the image of the target is maintained on both fovea

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2
Q

what is the initiation/transient phase of vergence movement?

A

open loop, fast/not precise, responds to large changes (1-10PD) and is controlled by coarse disparity detector neurons

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3
Q

what is the completion/sustained phase of vergence movement?

A

closed loop, slow/negative feedback, responds to small disparity and is controlled by fine disparity detector neurons

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4
Q

what are the 5 types of stimulus to the vergence system?

A

retinal disparity, retinal blur, proximity of targets, tonicity and voluntary innervation

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5
Q

what is retinal disparity and what does it elicit?

A

separation of images of a single object so that they fall on non-corresponding points on the retina
elicits = disparity/fusional vergence

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6
Q

what does crossed binocular disparity stimulate?

A

positive fusional vergence (convergence)

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7
Q

what does un-crossed binocular disparity stimulate?

A

negative fusional vergence (divergence)

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8
Q

what is the vergence demand (VD) of a target?

A

the absolute disparity of the fixation target - how much do you need to converge/diverge eyes to maintain foveal fixation

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9
Q

how do you calculate the vergence demand (PD)?

A

VD = PD/d + prism

PD is in cm, d (target distance) is in m and BO prism (+) and BI prism (-)

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10
Q

what is disparity vergence? how do you calculate it?

A

amount of disparity vergence (DV) is the difference between the vergence demand for two different targets
DV = B - A

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11
Q

how is fixation disparity different than vergence disparity?

A

fixation disparity is very small vs. vergence disparity which has a large transient change and then maintained

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12
Q

why is fixation disparity necessary for fixation?

A

the small binocular disparity stimulates sustained disparity vergence - needed to maintaining the stability of the vergence system

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13
Q

what is the initiation/transient phase for disparity vergence?

A

responds to larger changes of retinal binocular disparity stimuli (step vergence stimulus) = using a prism bar

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14
Q

what is the completion/sustained phase for disparity vergence?

A

responds to small changes of retinal binocular disparity stimuli = testing fusional ranges in phoropter

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15
Q

what does the trajectory look like for pure vergence eye movments?

A

the vergence angles gradually/slowly change (either get larger or smaller)

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16
Q

what is the maximum velocity for vergence?

A

40 degree/sec

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17
Q

why does vergence have a main sequence relationship for velocity?

A

(similar to saccades) = the larger the demand - the faster the vergence movement or peak velocity increases with amplitude of vergence

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18
Q

which is usually faster, convergence or divergence?

A

convergence = there are more neurons stimulating this type of movement

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19
Q

what is the latency for the transient disparity vergence?

A

160 msec

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20
Q

what is the latency for accommodative vergence?

A

200 msec

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21
Q

what can help shorten the latency time for vergence movments?

A

if there are predictable stimuli

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22
Q

what is the duration for a transient disparity vergence?

A

about 500 msec - the exact time depends on the amount of the vergence change (amplitude)

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23
Q

using a 3BI/12BO flipper for vergence facility - what is the typical cpm?

A

15 (but different combinations will have different norms) - this one is best to differentiate between symptomatic vs. asymptomatic

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24
Q

why does retinal blur stimulate vergence?

A

the loss of image sharpness elicits a vergence movement

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25
Q

what is accommodative vergence (AV)?

A

vergence innervation which is produced by blur accommodation signal (no disparity vergence involved)

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26
Q

how are accommodative vergence and blur accommodation related?

A

they are directly proportional - for every diopter of BA innervation you will get “x” PD amount of accommodative convergence

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27
Q

what is the dual interaction model?

A

it characterizes the interaction between accommodation and vergence systems - they innervate each other via cross-talk

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28
Q

what does the AC/A ratio represent?

A

how much vergence is being stimulated if you change 1D of accommodation
change in vergence angle/change in accommodation

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29
Q

how is the AC/A ratio measured?

A

by blur-induced changes of heterophoria (alternating cover test or van graefe phoria test = need to break binocular disparity/fusion)

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30
Q

what are the 2 assumptions for the AC/A ratio?

A

accommodative stimulus equals accommodative response (no errors)
accommodative convergence show linear relationship with accommodative stimulus

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31
Q

how do you find the calculated AC/A?

A

measure heterophoria at distance and near -

PD (cm) + (near - distance)/amount of accommodation change (2.5D for 40cm)

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32
Q

what are the signs for exophoria and esophoria in the calculated AC/A?

A

exo deviation = (-)

eso deviation = (+)

33
Q

how do you find the gradient AC/A?

A

measure heterophoria a certain distance, then add 1D lens to alter accommodation and re-measure
heterophoria (with lens) - heterophoria (no lens)/lens power (1D)

34
Q

what is a stimulus AC/A?

A

used if assuming the accommodative stimulus is equal to accommodative response (no lead or lag) - but patients tend to under-accommodate by 10%

35
Q

does the gradient AC/A overestimate or underestimate the actual AC/A (response)?

A

underestimates

36
Q

based on the dual interaction model, what is the total vergence response?

A 
total = DC + AC
DC = disparity convergence
AC = accommodative convergence
37
Q

what will adding a -1D lens do during the gradient AC/A?

A

it stimulates accommodation and phoria will become more eso or less exo

38
Q

what will adding a +1D lens do during the gradient AC/A?

A

it will relax accommodation and phoria will become more exo or less eso

39
Q

what is a normal AC/A?

A

4/1 (range is 3/1 to 6/1)

40
Q

what are some factors that will affect the AC/A (usually stable)?

A

spectacle lenses if not looking through center, age (increases pre-presbyopia) and CNS and peripheral parasympathetic medications

41
Q

how do you find the accommodative convergence (AC)?

A

take the amount of accommodation (usually 2.5) and multiply by AC/A
if they lag +0.5D - then subtract from 2.5 (2.5-0.5)

42
Q

what binocular anomaly can be seen if there is a low AC/A, normal or exo at distance with greater exo at near?

A

convergence insufficiency

43
Q

what binocular anomaly can be seen with a low AC/A and eso at distance but normal at near?

A

divergence insufficiency

44
Q

what binocular anomaly can be seen if there is a high AC/A, normal or eso at distance with greater eso at near?

A

convergence excess

45
Q

what binocular anomaly can be seen if there is a high AC/A, more exo at distance than exo at near?

A

divergence excess

46
Q

if you have a patient with a high AC/A and has esophoria at near with symptoms, what treatment can you do?

A

plus/minus lenses

47
Q

if you have a patient with normal to low AC/A and has esophoria at near with symptoms, what treatment can you do?

A

usually consider prisms or VT or both

48
Q

what is convergence accommodation (CA)?

A

accommodation innervation which is produced by binocular disparity vergence

49
Q

what is the CA/C ratio?

A

convergence accommodation is quantified by disparity-induced changes of accommodation

50
Q

what is the normal CA/C ratio? when is it highest?

A

1/12 (D/PD) - ranges 0 to 1/4 in young adults

highest = age 10 then drops linerally

51
Q

how does the vergence trajectory change when combined with saccades?

A

it is sped up

52
Q

what 3 areas are involved in the control of vergence?

A

cortical, brainstem and cerebellar control

53
Q

what are the 4 structures involved in cortical control of vergence?

A

occipital lobe: primary visual cortex, temporal lobe: MT/MST, parietal lobe: PPC (posterior parietal cortex) and frontal lobe: FEFsem

54
Q

what are the 3 types of disparity neurons in the primary visual cortex?

A

tuned-zero/near-zero neurons, tuned-far neurons and tuned-near neurons

55
Q

what are tuned-zero/near-zero neurons?

A

respond to small binocular stimuli disparity (fixation disparity) during sustained phase, hold vergence steady and fine-tune precision

56
Q

what are the tuned-far and tuned-near neurons?

A

for large binocular disparities (during vergence jump) during initiation phase to stimulate fast response

57
Q

what 2 areas does the primary visual cortex send information to for vergence control?

A

the parietal and temporal lobes

58
Q

what are the 2 areas in the parietal lobe (processes info about spatial location) involved in vergence control?

A

Lateral intraparietal area (LIP) and parietal eye fields (PEF) within PPC

59
Q

what does the lateral intraparietal area (LIP) do?

A

converts oculocentric direction to egocentric direction - location in 3D space
involved in stimulating vergence and accommodation based on perceived distance

60
Q

what does the parietal eye field (PEF) do?

A

accommodation, vergence, and pupil constriction (near triad)

61
Q

what are the 2 areas in the temporal lobe (processing info about motion) are involved in vergence control?

A

MT and MST

62
Q

what does MT do?

A

perception of steroscopic depth

63
Q

what does MST do?

A

process egocentric motion - involved in initiation of vergence

64
Q

what do both MT and MST do?

A

involved in controlling smooth vergence in space (fixating on a target that moves from far to near)

65
Q

where do the parietal and temporal lobes send their information?

A

to the frontal lobe - FEF (frontal eye fields)

66
Q

what do the FEF do?

A

involved in initiating voluntary eye movements (saccades, smooth pursuits and vergence)

67
Q

what is the sub-region in the FEF for controlling vergence?

A

FEFsem

68
Q

where do all areas of the cortex send information to the mid-brain?

A

the supra-oculomotor area (SOA) = for premotor command

69
Q

what is the supra-oculomotor area (SOA)?

A

a group of neurons within the MRF, 1-2mm dorsal to oculomotor nucleus and EW nucleus

70
Q

what structure is specific to accommodation and vergence activity and generates neural innervation similar to the pulse/step of saccades?

A

supra-oculomotor area (SOA)

71
Q

what are the 3 types of neurons in the SOA that send pre-motor commands to the CN3 nucleus?

A

vergence tonic cells, vergence burst cells and vergence burst-tonic cells

72
Q

which SOA neuron discharges in relation to vergence angle?

A

vergence tonic cells

73
Q

which SOA neuron discharges in relation to vergence velocity?

A

vergence burst cells

74
Q

which SOA neuron discharges in relation to both vergence angle and velocity?

A

vergence burst-tonic cells

75
Q

what is the NRTP in the pons?

A

it may contain neurons important for vergence integration

receives input from FEF and projects to cerebellum

76
Q

will a lesion of the MLF affect vergences?

A

no - vergence innervation bypasses the MLF

77
Q

what happens if there is a lesion to the NRTP?

A

leads to impairment of holding the vergence angle

78
Q

what is the main function of the cerebellum in vergence movements?

A

to store important constants - AC/A and CA/A

79
Q

what happens to vergences if there is a lesion to the cerebellum?

A

affects the accuracy of vergence movements

Vision Science 5 (14 decks)

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