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Vision Science 5 > Lecture 12 > Flashcards

Lecture 12 Flashcards

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1
Q

what is a saccade?

A

the fastest eye movement - it rapidly redirects our line of sight

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2
Q

what is the most rudimentary of all saccades?

A

quick phases - automatic resetting movements in response to spontaneous drift of the eyes

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3
Q

what are reflexive saccades (visually guided)?

A

made in response to the sudden appearance of a novel visual stimulus

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4
Q

what are higher-level voluntary saccades?

A

elective saccades made as part of purposeful behavior (to command, predictive, memory guided and antisaccades)

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5
Q

list in order the hierarchy of saccades from most lowest to highest

A

quick phases, visually guided (reflexive) saccades, to command, predictive/anticipatory, memory guided and antisaccades

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6
Q

what is a normal latency of a saccade?

A

200 msec (+/- 50msec)

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7
Q

what is latency of a saccade affected by?

A

patient motivation, attention and target predictability

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8
Q

what is the normal velocity of a saccade?

A

30-700 degree/sec for amplitude ranging from 0.5 to 40 degrees

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9
Q

what is the maximum speed for a saccade?

A

1000 degree/sec

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10
Q

how are velocity and amplitude related in saccades?

A

peak velocity increases with amplitude - a larger amplitude has a faster saccade

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11
Q

what is a normal duration of a saccade?

A

30-100 msec for amplitude ranging from 0.5 to 40 degrees

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12
Q

how are duration and amplitude related in saccades?

A

directly related - the higher amplitude, the saccade goes on for a longer period of time

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13
Q

what is the amplitude of a normal saccade?

A

less than 15 degrees - larger gaze changes usually stimulate head movement with eye movement

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14
Q

what is hypometria?

A

undershooting saccades - tends to occur with larger amplitude saccades (less than 10% of overall amplitude)

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15
Q

what is hypermetria?

A

overshooting saccade - tends to occur with small amplitude saccades

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16
Q

are saccadic dysmetrias (hypometria or hypermetria) normal?

A

small degree dysmetria’s are normal (most commonly hypometria)

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17
Q

which condition has a reading disability due to significant overshooting (hypermetria)?

A

spinocerebellar degeneration

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18
Q

what is a normal pattern for diagonal saccades?

A

the horizontal and vertical components are similar and trajectory is almost straight

19
Q

what do diagonal saccades look like if the brainstem mechanism for horizontal or vertical components is imparted?

A

very strongly curved trajectory

20
Q

what happens to latency of saccades over time?

A

it increases 1-2 msec each year (expect to see slower saccade initiation in geriatric patients)

21
Q

what happens to peak velocity over time?

A

it decreases 1 deg/sec/year

22
Q

do saccadic gain and accuracy decrease with age?

A

no they do not change - they can be reduced with fatigue or drugs that affect CNS

23
Q

what are some stimuli for saccades?

A

visual targets (oculocentric), auditory and somatosensory (egocentric) and imagined target or direction - all are converted to egocentric direction

24
Q

what is the pulse in the pulse-step innervation of saccades?

A

motor neurons and the agonist EOM exhibit a burst of high-frequency phasic activity moving the eye quickly from one position to another

25
Q

what is the step in the pulse-step innervation of saccades?

A

(after a saccade) motor neurons and the agonist EOMs assume new, higher level of tonic activity, holding the eye in new position against orbital elastic force

26
Q

what does pulse innervation determine?

A

the saccadic velocity

27
Q

what is the function of the omnipause neurons located in the RIP in the pons?

A

to tonically inhibit excitatory burst neurons (PPRF and riMLF) so that we hold our gaze steady when we fixate - this is inhibited during a saccadic movment

28
Q

what structure performs the pulse-step innervation?

A

the velocity-to-position neural integrator NI (gaze-holding mechanism)

29
Q

what is the structure for horizontal movements in the NI?

A

NPH-MVN (nucleus propositus hypoglossi and medial vestibular nucleus)

30
Q

what is the structure in the NI for vertical and torsional movements?

A

INC (interstitial nucleus of Cajal)

31
Q

what happens if there is a lesion of the PPRF?

A

slow or absent horizontal saccades to the same side (diagonal will also be abnormal) - decreased pulse height (firing rate)

32
Q

what happens if there is a lesion of the RIP?

A

disruption of the omnipause neurons - saccadic oscillation, including opsoclonus and flutter (cannot hold a steady gaze - NI is leaky)

33
Q

after an excitatory burst neuron in the PPRF, where does the pulse innervation go to?

A 
abducens (CN6) nucleus for ipsilateral lateral rectus and abducens on contralateral via MLF for medial rectus 
the horizontal (NPH-MVN) generates a saccadic step to hold the eyes in new position (gaze holding)
34
Q

what happens if there is a lesion of the MLF?

A

ipsilateral aDduction palsy (INO) - right MLF lesion results in adduction palsy for the right eye

35
Q

what happens if there is a lesion of the NPH-MVN?

A

causes horizontal gazed-evoked nystagmus because of failure of gaze holding (leaky integrator)

36
Q

after receiving visual information in the occipital lobe and after visualspatial integration in the PPC, what can trigger a saccade?

A 
PEF = triggered reflexively 
FEF = triggered intentionally
37
Q

what 4 structures are involved in higher level control for saccades?

A

PPC, PEF, FEF and SC

38
Q

what does the PPC (posterior parietal cortex) do?

A

calculates egocentric direction

39
Q

what do the PEF (parietal eye fields) do?

A

initiates reflexive, visually guided saccades

40
Q

what do the FEF (frontal eye fields) do?

A

initiates voluntary saccades

41
Q

what does the SC (superior colliculus) do?

A

target selection, initiation of saccades, contributes to their speed, and saccade trajectory

42
Q

what 2 structures does the SC send motor command to?

A

PPRF bust neurons and omnipause neurons

43
Q

what might the vermis and fastigial nuclei do (in cerebellum)?

A

control the size of the saccadic pulse (lesion will cause saccadic pulse dysmetria)

44
Q

what might the flocculus and paraflocculus do (in cerebellum)?

A

appropriately matching the saccadic step to pulse (lesion causes post-saccadic drift)

Vision Science 5 (14 decks)

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