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Vision Science 5 > Lecture 14 > Flashcards

Lecture 14 Flashcards

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1
Q

what is the vestibulo-ocular reflex (VOR)?

A

it stabilizes retinal images during head motion by counter-rotating the eyes at the same speed as the head - but in opposite direction

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2
Q

what are optokinetic eye movements?

A

they stabilize the eyes during tracking of large moving visual scene - causes illusionary sensation of self-rotation in the opposite direction

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3
Q

what is the difference between the 2 types of vestibulo-ocular responses?

A

VOR = momentary movement of eye to compensate for brief head movements
vestibular nystagmus = ongoing eye nystagmoid eye movement related to continuous head motion

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4
Q

what are the 3 types of rotation of the head?

A 
yaw = horizontal
pitch = vertical
roll = torsional
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5
Q

which sensory organs detect head rotational movements (rotational/angular VOR)?

A

the semicircular canals - anterior, posterior, and horizontal

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6
Q

what are the 3 types of translation head movements?

A 
heave = side to side
bob = up and down
surge = forward and backward
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7
Q

what sensory organs detect head translation movements?

A

the utricle and saccule (otolith organs)

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8
Q

what are the sensory neurons for the semicircular canals, how do they work?

A

hair cells sense speed changes in the endolymph and a mechanical bending sends a nerve impulse

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9
Q

how do the utricle and saccule sense a change in direction?

A

they also have hair cells that project into a membrane which has otalith (calcium) crystals

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10
Q

which EOM muscles are connected to the horizontal semicircular canals?

A

the plane of the horizontal canals is 30 degrees up from line of sight - connects ipsilateral MR and contralateral LR

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11
Q

what is the utricular macula?

A

oriented in the horizontal plane - detects horizontal translation and static head tilts

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12
Q

what is the saccular macula?

A

oriented in the vertical plane - detects vertical translation and static head tilts

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13
Q

what is the stimulus for rotational VOR?

A

head angular acceleration (pitch or yawn)

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14
Q

what is the stimulus for translational VOR?

A

head linear acceleration

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15
Q

what is the stimulus for torsional VOR?

A

head tilt (roll)

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16
Q

what are the 3 aspects of a VOR?

A

phase, gain and latency

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17
Q

what is the phase for VOR?

A

temporal difference between head and eye movements = 180 out of phase or zero phase shift (ideal)

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18
Q

what is the gain for VOR?

A

depends on the frequency of head motion - natural head rotation frequency is 0.5-5 Hz and depends on type of VOR
ideal = -1.0

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19
Q

what is the latency for VOR?

A

16 msec (shortest latency = reflex, doesn’t involve higher order control systems)

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20
Q

what happens to the gain of horizontal and vertical VOR in the dark?

A

gain is -0.9 and phase shift is zero - no enhancement of light so it is lower

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21
Q

what VOR situation has the highest/best gain?

A

horizontal and vertical VOR in the light (-1.0)

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22
Q

what is the gain for dynamic torsional VOR?

A

-0.4 to -0.7 (less than -1.0 because the stimulus is not moving as much as VOR)

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23
Q

what is dynamic torsional VOR?

A

tilting the head alternately between the right and left shoulders - mediated by vertical semicircular canals

24
Q

what mediates static torsional VOR?

A

the otolith-ocular reflex from inputs of the utricles

25
Q

what is the gain for static torsional VOR?

A

-0.1 to -0.24 (lowest gain for VOR)

26
Q

what type of neural control is for VOR and what neurons are involved?

A

pure reflex = no conscious control

reflex arc includes 3 or 4 neurons (excitatory and inhibitory)

27
Q

what is the VOR response if the head rotates to the right?

A

stimulates right (ipsilateral) horizontal canal - activates left LR and right MR = both eyes rotate to left (contralateral slow phase)

28
Q

what is the excitatory pathway in the horizontal VOR neural circuit for the 3-neuron arc?

A

signal from horizontal semicircular canal - MVN (2nd order neuron) - contralateral abducens nucleus (CN6) - lateral rectus on that side

29
Q

what is the excitatory pathway in the horizontal VOR neural circuit for the 4-neuron arc?

A

signal from horizontal semicircular canal - MVN (2nd order neuron) - contralateral abducens nucleus (CN6) - cross midline with MLF to innervate MR on contralateral oculomotor nucleus

30
Q

what is the inhibitory pathway for the horizontal VOR neural circuit for the 3-neuron arc?

A

signal from horizontal semicircular canal - (2nd order neuron) in MVN - project to ipsilateral abducens nucleus (CN6) - inhibits LR on that side

31
Q

what is the inhibitory pathway for the horizontal VOR neural circuit for the 4-neuron arc?

A

signal from horizontal semicircular canal - (2nd order neuron) in MVN - project to ipsilateral abducens nucleus (CN6) -MLF to inhibit MR on contralateral CN3

32
Q

what can an acute unilateral vestibular lesion cause?

A

nystagmus (due to horizontal VOR pathway imbalance)

33
Q

which way will the fast phase be for nystagmus if the left horizontal canal is damaged?

A

then the right side is unopposed - causing slow phases that move eyes to left (ipsilateral) and corrective quick phases to the right (contralateral)

34
Q

what is the barany test?

A

stimulate both sides by spinning the patient’s body to one direction (patient is sitting in a chair)

35
Q

what is the caloric test?

A

tests the peripheral vestibular function by injecting water into the external ear canal - alters the tonic state of opposing vestibular drive upon the eye positions

36
Q

which way do the eyes move in the caloric test with cold water? what about warm water?

A

cold (or lesion) = eyes beat in opposite direction to side of lesion
warm (stimulation) = eyes beat in same direction as side of stimulation
“COWS”

37
Q

what is oscillopsia?

A

illusion of the motion of the environment which is brought on or exacerbated by head motion

38
Q

how do you clinically test vestibular function?

A

dynamic visual acuity - test VA with head movement

vestibular function will lose about 5 lines (normal is 1 line)

39
Q

what does VOR have adaptation?

A

in the absence of rapid feedback - VOR must be continuously calibrated by short and long-term adaptations to correct for any errors

40
Q

what is VOR habituation?

A

not the same as adaptation - this is a reduction of response after repetitive stimulation in complete darkness

41
Q

what happens to VOR when wearing hyperopic plus lenses?

A

they have a higher gain - angular gain and translational VOR sensitivity increase

42
Q

what happens to VOR gain with myopic minus lenses?

A

they have a lower gain

43
Q

what are the sites in the cerebellum for VOR adaptation?

A

vestibular nucleus, flocculus and paraflocculus

44
Q

when do you see OKN?

A

it is induced reflexively by motion of a large visual scene - causes an illusionary sensation of self-rotation in the opposite direction

45
Q

what is the function of the optokinetic system?

A

to supplement angular VOR (angular VOR responds to brief, high frequency head rotation vs. OKN which stabilizes retinal image during sustained, low-frequency rotation)

46
Q

what two things balance each other so that no nystagmus occurs if the rotation is suddenly stopped in light?

A

the PVN (post-rotatory VN) + OKAN (optokinetic after-nystagmus)

47
Q

what is optokinetic nystagmus?

A

a jerk nystagmus with slow and fast phases - can be active or passive OKN (depending on how much conscious attention is needed)

48
Q

what is active OKN?

A 
smooth pursuit (slow phase) + voluntary saccades (fast phase)
needs conscious control/attention
49
Q

what is passive OKN?

A

true OKN = reflexive like vestibular nystagmus (fast and slow phases look just like active form)

50
Q

what is the stimulus for active OKN?

A

foveal moving images - same stimuli for smooth pursuits and saccades (can be large or small)
*foveal vision must work well

51
Q

what is the stimulus for passive OKN?

A

movement of large objects (low spatial frequency) - acuity is not important and dominated by peripheral motion

52
Q

what is the latency for OKN?

A

140 msec (slow onset)

53
Q

what is the slow phase gain for OKN?

A

0.8 (gain can be adapted by cerebellum)

54
Q

what can cause an abnormal OKN response?

A

peripheral or central vestibular diseases

development or acquired lesions affecting visual pathway

55
Q

if a patient has macular degeneration, can they still have passive OKN?

A

yes - only active OKN will be abnormal

56
Q

what type of motion can a patient with infantile esotropia see?

A

sees nasal but low sensitivity to temporal motion in each eye

57
Q

what happens if a patient with infantile esotropia looks monocularly at a stationary target?

A

they have an illusion of seeing the world drift towards the nose - creates temporal latent nystagmus (OKN)

Vision Science 5 (14 decks)

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