Chapter 9 - Vestibulo-Ocular Reflex

Question 1

<p>

| What is the latency of the VOR?</p>

Answer 1

<p>

| 5-7 s after a head movement</p>

Question 2

<p>

| Internuclear neurons connect what two nuclei?</p>

Answer 2

<p>

| 6 (Abducens) and 3 (oculomotor)</p>

Question 3

<p>

| What is the ascending tract of dieters?</p>

Answer 3

<p>
Accessory tract to the VOR that sends an excitatory signal from the ipsilateral vestibular nucleus to the oculomotor (CN III) nucleus that then stimulates the medial rectus muscle ipsilateral to the stimulus.</p>

Question 4

What provides the second integration at high frequencies (above 0.5 hz)?

Answer 4

The oculomotor plant.

Question 5

Why is the integrator important functionally?

Answer 5

You would have an appropriate velocity command, but the eyes would drift back without a position command.

Question 6

<p>

| What eye movements share the indirect pathway? (ie, the integrator)</p>

Answer 6

<p>

| Angular VOR, saccades, pursuit (ie all conjugate eye movements)</p>

Question 7

<p>

| What is the approximate lower corner frequency of the aVOR?</p>

Answer 7

<p>

| .02 to .05 hz</p>

Question 8

Define the time constant of the aVOR.

Answer 8

The time for eye velocity to decay to 63% peak value after constant velocity rotation

Question 9

What is the velocity storage integrator?

Answer 9

Accounts for the slower than expected decay in VOR after constant velocity rotation by perseverating the nystagmus

Question 10

<p>

| True/False - the time constant shows little inter subject or interspecies variability?</p>

Answer 10

<p>
False - The VOR time constant varies from seconds to minutes between species and can habituate, change with vestibular pathology or with visual deprivation early in life.</p>

Question 11

What is “dumping” as pertains velocity storage?

Answer 11

Altering the time constant of the VOR by pitch or roll movements.

Question 12

<p>

| What are the two theories distinguishing tilt and translation?</p>

Answer 12

<p>
1) Paige et al. - high-frequency stimuli are recognized as translation and low-frequency as tilt. 2) Angelaki, Merfeld- the semicircular canals assist in distinguishing tilt from translation.</p>

Question 13

What happens if you have a lesion at your Nucleus prepositus hypoglossi (NPH)?

Answer 13

You lose your horizontal neural integrator, and therefore the tonic component of a gaze shift. You are unable to integrate horizontal velocity to eye position, so you make horizontal eye movements with appropriate velocity, but they cannot maintain their position, and drift back.

Question 14

What happens to a slow-phase eye response to constant velocity in darkness?

Answer 14

It decays over several seconds because the canals cannot transduce constant velocity. The rate of this decay is represented as the time constant of the aVOR. In light, OKN is functional at low-frequencies.

Question 15

What anatomic locations are thought to be involved in velocity storage?

Answer 15

Electrical stimulation and lesion studies show that the cerebellar nodulus and ventral uvula are involved. These project to neurons in the vestibular nuclei also involved in velocity storage.

Question 16

Why might you not see nystagmus in darkness after rotation at constant velocity in light?

Answer 16

The post-rotatory nystagmus from the SCC and the optokinetic after-nystagmus are may cancel, since they are of similar magnitude and in opposite directions, with post-rotatory nystagmus the opposite of the stimulus rotation direction and OKAN in the same direction.

Question 17

What is optokinetic after-nystagmus (OKAN)?

Answer 17

The initial quick and then longer gradual decline in OKN after an optokinetic stimulus (must be dark to see).

Question 18

Why is there an initial quick rise in eye velocity and a quick fall a the end of OKN in humans?

Answer 18

Since this is present for vertical and horizontal OKN, but not torsional, thought to reflect pursuit.

Question 19

What structures are responsible for rapid rise and fall in OKN?

Answer 19

middle temporal extrastriate visual cortex (MT/MST), the dorsolateral pontine nuclei (DLPN),
and the flocculus/ventral paraflocculus (F/VPF). These lesions usually affect pursuit as well.

Question 20

Does the slow or fast component of OKN complement the aVOR?

Answer 20

The slow component: OKN builds up at same rate of decline of aVOR, OKAN cancels post-rotatory nystagmus, it has a subcortical retinal slip pathway, it is encoded in canal coordinates, and an intact labyrinth for velocity storage that appears shared.

Question 21

What is the gain and eye movement response to roll in frontal-eyed animals? lateral-eyed animals?

Answer 21

ocular counter-roll is a torsional movement with approximately 10% gain in humans, and vertical movement and 50% gain in lateral-eyed animals.

Question 22

What is a suggested function of the ocular tilt reaction in frontal-eyed animals?

Answer 22

It appears to reset Listing’s plane and the associated primary position and maintains this through an effect on the neural integrator. It may therefore not be related to gaze stability and more to adjsut the coordinates of other eye movements (saccades, pursuit, etc.)

Question 23

True/False: The translational VOR is only useful at distant targets and low-frequencies?

Answer 23

False: It was not until the early 1990s that it
was clearly shown that the TVOR is functionally important only at high frequencies and only during near-target viewing (Paige and Tomko 1991a , b ; Schwarz et al. 1989 ;Schwarz and Miles 1991 ).

Question 24

Accurate compensatory eye movements during the tVOR are dependent on what two factors?

Answer 24

viewing distance and eye position. (the tVOR undercompensates for near-vieweing conditions with gains ~0.5 and overcompensates for far-viewing).

Question 25

What is the complimentary visual stabilization mechanism to the tVOR?

Answer 25

ocular following reflexes (OFR). The OFR is dependent on distance like the tVOR.

Question 26

Latency of the translational VOR?

Answer 26

10-12 ms in monkey, ? longer in humans.