Lecture 12

Question 1

what is a saccade?

Answer 1

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

Question 2

what is the most rudimentary of all saccades?

Answer 2

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

Question 3

what are reflexive saccades (visually guided)?

Answer 3

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

Question 4

what are higher-level voluntary saccades?

Answer 4

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

Question 5

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

Answer 5

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

Question 6

what is a normal latency of a saccade?

Answer 6

200 msec (+/- 50msec)

Question 7

what is latency of a saccade affected by?

Answer 7

patient motivation, attention and target predictability

Question 8

what is the normal velocity of a saccade?

Answer 8

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

Question 9

what is the maximum speed for a saccade?

Answer 9

1000 degree/sec

Question 10

how are velocity and amplitude related in saccades?

Answer 10

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

Question 11

what is a normal duration of a saccade?

Answer 11

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

Question 12

how are duration and amplitude related in saccades?

Answer 12

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

Question 13

what is the amplitude of a normal saccade?

Answer 13

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

Question 14

what is hypometria?

Answer 14

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

Question 15

what is hypermetria?

Answer 15

overshooting saccade - tends to occur with small amplitude saccades

Question 16

are saccadic dysmetrias (hypometria or hypermetria) normal?

Answer 16

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

Question 17

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

Answer 17

spinocerebellar degeneration

Question 18

what is a normal pattern for diagonal saccades?

Answer 18

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

Question 19

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

Answer 19

very strongly curved trajectory

Question 20

what happens to latency of saccades over time?

Answer 20

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

Question 21

what happens to peak velocity over time?

Answer 21

it decreases 1 deg/sec/year

Question 22

do saccadic gain and accuracy decrease with age?

Answer 22

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

Question 23

what are some stimuli for saccades?

Answer 23

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

Question 24

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

Answer 24

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

Question 25

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

Answer 25

(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

Question 26

what does pulse innervation determine?

Answer 26

the saccadic velocity

Question 27

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

Answer 27

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

Question 28

what structure performs the pulse-step innervation?

Answer 28

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

Question 29

what is the structure for horizontal movements in the NI?

Answer 29

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

Question 30

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

Answer 30

INC (interstitial nucleus of Cajal)

Question 31

what happens if there is a lesion of the PPRF?

Answer 31

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

Question 32

what happens if there is a lesion of the RIP?

Answer 32

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

Question 33

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

Answer 33

``` 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) ```

Question 34

what happens if there is a lesion of the MLF?

Answer 34

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

Question 35

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

Answer 35

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

Question 36

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

Answer 36

``` PEF = triggered reflexively FEF = triggered intentionally ```

Question 37

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

Answer 37

PPC, PEF, FEF and SC

Question 38

what does the PPC (posterior parietal cortex) do?

Answer 38

calculates egocentric direction

Question 39

what do the PEF (parietal eye fields) do?

Answer 39

initiates reflexive, visually guided saccades

Question 40

what do the FEF (frontal eye fields) do?

Answer 40

initiates voluntary saccades

Question 41

what does the SC (superior colliculus) do?

Answer 41

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

Question 42

what 2 structures does the SC send motor command to?

Answer 42

PPRF bust neurons and omnipause neurons

Question 43

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

Answer 43

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

Question 44

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

Answer 44

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