Lecture 13 Flashcards
what is a smooth pursuit?
conjugate eye movements that allow both eyes to smoothly track a slow moving object so it is kept on the fovea
why is there VOR cancelation during combined eye-head tracking in a smooth pursuit?
otherwise the VOR would move the eyes in opposite the direction of the intended gaze
why is there OKR cancelation during tracking of moving target against a detailed stationary background?
the optokinetic system will try to hold the gaze on the stationary background, but it is overridden by pursuit
what is the stimulus for the smooth pursuit?
perceived motion of the attended target - the object we’re trying to fixate must be perceived as moving
what are ineffective stimuli for a smooth pursuit?
auditory or kinesthetic stimuli, or imagined motion are ineffective
what is a normal latency for a smooth pursuit?
100-130 msec (longer than VOR and shorter than saccades)
what affects the latency of a smooth pursuit?
attention, higher contrast, brighter illumination, and color all improve latency
what is a normal velocity of a smooth pursuit?
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)
what is a normal gain for a smooth pursuit (peak eye velocity/peak target velocity)?
1.0 (ideally) - means the velocity of target motion equals the velocity of eye movement
what conditions will impair the gain for a smooth pursuit?
if the foveal vision is impaired - amblyopia, central scotoma in strabismus, and macular disease
how does prediction affect the smooth pursuit gain?
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
what happens to smooth pursuit gain with fatigue and aging?
the gain is lowered
when is the open loop phase for a smooth pursuit?
(pursuit initiation) - during the latency period and the beginning and guided by target motion (retinal slip velocity)
does the open loop depend on initial target velocity?
no - the initial acceleration doesn’t depend on target velocity
what is the closed loop phase for a smooth pursuit?
(pursuit maintenance or steady state) after the beginning period - relies on feedback
what are the 4 structures involved in higher level control for smooth pursuits?
MT (V5), MST, FEF sem, and DLPN
what does the MT/V5 (middle temporal cortex) do?
processes retinal motion - encodes the oculocentric speeds
what does the MST (medial superior temporal cortex) do?
processes egocentric motion - combines oculocentric motions
what does the FEF sem (frontal eye field pursuit subregion) do?
initiation, steady state and predictive aspects of a smooth pursuit
what does the DLPN (dorsolateral pontine nucleus) do?
encodes the horizontal velocity of the fixation target
what happens if there is a unilateral posterior cerebral lesion involving the MST?
causes directional pursuit deficit, saccades are not affected - toward the side of the lesion (ipsilateral)
what happens if there is a lesion in MT?
causes a scotoma of motion, decreased smooth pursuit speed and dysmetric saccades - both directions of the affected contralateral hemifield
what happens if there is a lesion of the FEF sem, DLPN or NOT?
causes deficits in ipsilateral horizontal pursuit
what are the 3 types of physiological miniature movements that are not detectable by the naked eye?
microtremor, microsaccades, and microdrift
what is a microtremor?
continuous, high-frequency ocular motor activity that underlies both microdrift and microsaccades
what is the frequency and average amplitude of a microtremor?
frequency = 50-100 Hz amplitude = < 1min of arc (usually 5-30 sec of arc) *smallest type of fixational eye movements
what is a microdrift?
smooth eye movement to prevent image of stable object from fading
what is the velocity of a microdrift?
< 20 min or arc/sec
what is a microsaccade?
mini saccades - no known function
what is the average amplitude and mean frequency of a microsaccade?
amplitude = 6 min of arc (range < 26 min of arc) frequency = 120 Hz
what areas of the cortex are involved in control of fixation?
supplementary eye fields, parietal eye fields, dorsolateral prefrontal cortex, V5 and V5A
what type of neurons are involved in control of fixation?
active process that mostly involves inhibitory neurons - inhibits saccadic eye movements
what are 4 types of fixation anomalies (eyes move with larger magnitude than normal fixational movements)?
anomalous slow drift, saccadic intrusions, saccadic oscillations and nystagmus
what is anomalous slow drift?
eye slowly drifts away from fixation target - magnitude is much larger than microdrift in normals, up to 1 degree more
who typically shows anomalous slow drift?
any condition with abnormal foveal vision = amblyopia is most common cause
what are saccadic intrusions?
intermittent or sporadic biphasic disruptions of fixation - saccades during fixation
can saccadic intrusions be seen in normals?
milder versions - especially during fatigue and increases with aging (show a square-wave jerk with amplitude 0.5-3)
when are saccadic intrusions abnormal?
when there is large amplitude movement - 4 to 50 macro-square wave jerks = associated with cerebellar or brainstem diseases
what are saccadic oscillations?
continuous disruptions of fixation - a series of large saccades that straddle fixation
what is opsoclonus?
a type of saccadic oscillations without intersaccadic intervals - eyes are never stationary on a target and do not have a rhythm
what is jerk nystagmus?
rhythmic oscillations containing a slow phase, magnitudes up to 10 degrees and up to 8 Hz
what is pendular nystagmus?
to and from movement without any slow phase (pendular, decreasing/increasing velocity waveform and constant velocity waveform)
