Lecture 13

Question 1

what is a smooth pursuit?

Answer 1

conjugate eye movements that allow both eyes to smoothly track a slow moving object so it is kept on the fovea

Question 2

why is there VOR cancelation during combined eye-head tracking in a smooth pursuit?

Answer 2

otherwise the VOR would move the eyes in opposite the direction of the intended gaze

Question 3

why is there OKR cancelation during tracking of moving target against a detailed stationary background?

Answer 3

the optokinetic system will try to hold the gaze on the stationary background, but it is overridden by pursuit

Question 4

what is the stimulus for the smooth pursuit?

Answer 4

perceived motion of the attended target - the object we’re trying to fixate must be perceived as moving

Question 5

what are ineffective stimuli for a smooth pursuit?

Answer 5

auditory or kinesthetic stimuli, or imagined motion are ineffective

Question 6

what is a normal latency for a smooth pursuit?

Answer 6

100-130 msec (longer than VOR and shorter than saccades)

Question 7

what affects the latency of a smooth pursuit?

Answer 7

attention, higher contrast, brighter illumination, and color all improve latency

Question 8

what is a normal velocity of a smooth pursuit?

Answer 8

0.1 to 70 degrees/sec (mostly depending on how fast the target is moving - top athletes may be as high as 130 degree/sec)

Question 9

what is a normal gain for a smooth pursuit (peak eye velocity/peak target velocity)?

Answer 9

1.0 (ideally) - means the velocity of target motion equals the velocity of eye movement

Question 10

what conditions will impair the gain for a smooth pursuit?

Answer 10

if the foveal vision is impaired - amblyopia, central scotoma in strabismus, and macular disease

Question 11

how does prediction affect the smooth pursuit gain?

Answer 11

for prolonged (longer than 1 sec), predictable pursuit targets is about 1.0
if movement is brief - no prediction than the gain goes back to 0.9

Question 12

what happens to smooth pursuit gain with fatigue and aging?

Answer 12

the gain is lowered

Question 13

when is the open loop phase for a smooth pursuit?

Answer 13

(pursuit initiation) - during the latency period and the beginning and guided by target motion (retinal slip velocity)

Question 14

does the open loop depend on initial target velocity?

Answer 14

no - the initial acceleration doesn’t depend on target velocity

Question 15

what is the closed loop phase for a smooth pursuit?

Answer 15

(pursuit maintenance or steady state) after the beginning period - relies on feedback

Question 16

what are the 4 structures involved in higher level control for smooth pursuits?

Answer 16

MT (V5), MST, FEF sem, and DLPN

Question 17

what does the MT/V5 (middle temporal cortex) do?

Answer 17

processes retinal motion - encodes the oculocentric speeds

Question 18

what does the MST (medial superior temporal cortex) do?

Answer 18

processes egocentric motion - combines oculocentric motions

Question 19

what does the FEF sem (frontal eye field pursuit subregion) do?

Answer 19

initiation, steady state and predictive aspects of a smooth pursuit

Question 20

what does the DLPN (dorsolateral pontine nucleus) do?

Answer 20

encodes the horizontal velocity of the fixation target

Question 21

what happens if there is a unilateral posterior cerebral lesion involving the MST?

Answer 21

causes directional pursuit deficit, saccades are not affected - toward the side of the lesion (ipsilateral)

Question 22

what happens if there is a lesion in MT?

Answer 22

causes a scotoma of motion, decreased smooth pursuit speed and dysmetric saccades - both directions of the affected contralateral hemifield

Question 23

what happens if there is a lesion of the FEF sem, DLPN or NOT?

Answer 23

causes deficits in ipsilateral horizontal pursuit

Question 24

what are the 3 types of physiological miniature movements that are not detectable by the naked eye?

Answer 24

microtremor, microsaccades, and microdrift

Question 25

what is a microtremor?

Answer 25

continuous, high-frequency ocular motor activity that underlies both microdrift and microsaccades

Question 26

what is the frequency and average amplitude of a microtremor?

Answer 26

frequency = 50-100 Hz
amplitude = < 1min of arc (usually 5-30 sec of arc) *smallest type of fixational eye movements

Question 27

what is a microdrift?

Answer 27

smooth eye movement to prevent image of stable object from fading

Question 28

what is the velocity of a microdrift?

Answer 28

< 20 min or arc/sec

Question 29

what is a microsaccade?

Answer 29

mini saccades - no known function

Question 30

what is the average amplitude and mean frequency of a microsaccade?

Answer 30

amplitude = 6 min of arc (range < 26 min of arc)
frequency = 120 Hz

Question 31

what areas of the cortex are involved in control of fixation?

Answer 31

supplementary eye fields, parietal eye fields, dorsolateral prefrontal cortex, V5 and V5A

Question 32

what type of neurons are involved in control of fixation?

Answer 32

active process that mostly involves inhibitory neurons - inhibits saccadic eye movements

Question 33

what are 4 types of fixation anomalies (eyes move with larger magnitude than normal fixational movements)?

Answer 33

anomalous slow drift, saccadic intrusions, saccadic oscillations and nystagmus

Question 34

what is anomalous slow drift?

Answer 34

eye slowly drifts away from fixation target - magnitude is much larger than microdrift in normals, up to 1 degree more

Question 35

who typically shows anomalous slow drift?

Answer 35

any condition with abnormal foveal vision = amblyopia is most common cause

Question 36

what are saccadic intrusions?

Answer 36

intermittent or sporadic biphasic disruptions of fixation - saccades during fixation

Question 37

can saccadic intrusions be seen in normals?

Answer 37

milder versions - especially during fatigue and increases with aging (show a square-wave jerk with amplitude 0.5-3)

Question 38

when are saccadic intrusions abnormal?

Answer 38

when there is large amplitude movement - 4 to 50 macro-square wave jerks = associated with cerebellar or brainstem diseases

Question 39

what are saccadic oscillations?

Answer 39

continuous disruptions of fixation - a series of large saccades that straddle fixation

Question 40

what is opsoclonus?

Answer 40

a type of saccadic oscillations without intersaccadic intervals - eyes are never stationary on a target and do not have a rhythm

Question 41

what is jerk nystagmus?

Answer 41

rhythmic oscillations containing a slow phase, magnitudes up to 10 degrees and up to 8 Hz

Question 42

what is pendular nystagmus?

Answer 42

to and from movement without any slow phase (pendular, decreasing/increasing velocity waveform and constant velocity waveform)