Lecture 12 Flashcards
what is a saccade?
the fastest eye movement - it rapidly redirects our line of sight
what is the most rudimentary of all saccades?
quick phases - automatic resetting movements in response to spontaneous drift of the eyes
what are reflexive saccades (visually guided)?
made in response to the sudden appearance of a novel visual stimulus
what are higher-level voluntary saccades?
elective saccades made as part of purposeful behavior (to command, predictive, memory guided and antisaccades)
list in order the hierarchy of saccades from most lowest to highest
quick phases, visually guided (reflexive) saccades, to command, predictive/anticipatory, memory guided and antisaccades
what is a normal latency of a saccade?
200 msec (+/- 50msec)
what is latency of a saccade affected by?
patient motivation, attention and target predictability
what is the normal velocity of a saccade?
30-700 degree/sec for amplitude ranging from 0.5 to 40 degrees
what is the maximum speed for a saccade?
1000 degree/sec
how are velocity and amplitude related in saccades?
peak velocity increases with amplitude - a larger amplitude has a faster saccade
what is a normal duration of a saccade?
30-100 msec for amplitude ranging from 0.5 to 40 degrees
how are duration and amplitude related in saccades?
directly related - the higher amplitude, the saccade goes on for a longer period of time
what is the amplitude of a normal saccade?
less than 15 degrees - larger gaze changes usually stimulate head movement with eye movement
what is hypometria?
undershooting saccades - tends to occur with larger amplitude saccades (less than 10% of overall amplitude)
what is hypermetria?
overshooting saccade - tends to occur with small amplitude saccades
are saccadic dysmetrias (hypometria or hypermetria) normal?
small degree dysmetria’s are normal (most commonly hypometria)
which condition has a reading disability due to significant overshooting (hypermetria)?
spinocerebellar degeneration
what is a normal pattern for diagonal saccades?
the horizontal and vertical components are similar and trajectory is almost straight
what do diagonal saccades look like if the brainstem mechanism for horizontal or vertical components is imparted?
very strongly curved trajectory
what happens to latency of saccades over time?
it increases 1-2 msec each year (expect to see slower saccade initiation in geriatric patients)
what happens to peak velocity over time?
it decreases 1 deg/sec/year
do saccadic gain and accuracy decrease with age?
no they do not change - they can be reduced with fatigue or drugs that affect CNS
what are some stimuli for saccades?
visual targets (oculocentric), auditory and somatosensory (egocentric) and imagined target or direction - all are converted to egocentric direction
what is the pulse in the pulse-step innervation of saccades?
motor neurons and the agonist EOM exhibit a burst of high-frequency phasic activity moving the eye quickly from one position to another
what is the step in the pulse-step innervation of saccades?
(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
what does pulse innervation determine?
the saccadic velocity
what is the function of the omnipause neurons located in the RIP in the pons?
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
what structure performs the pulse-step innervation?
the velocity-to-position neural integrator NI (gaze-holding mechanism)
what is the structure for horizontal movements in the NI?
NPH-MVN (nucleus propositus hypoglossi and medial vestibular nucleus)
what is the structure in the NI for vertical and torsional movements?
INC (interstitial nucleus of Cajal)
what happens if there is a lesion of the PPRF?
slow or absent horizontal saccades to the same side (diagonal will also be abnormal) - decreased pulse height (firing rate)
what happens if there is a lesion of the RIP?
disruption of the omnipause neurons - saccadic oscillation, including opsoclonus and flutter (cannot hold a steady gaze - NI is leaky)
after an excitatory burst neuron in the PPRF, where does the pulse innervation go to?
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)
what happens if there is a lesion of the MLF?
ipsilateral aDduction palsy (INO) - right MLF lesion results in adduction palsy for the right eye
what happens if there is a lesion of the NPH-MVN?
causes horizontal gazed-evoked nystagmus because of failure of gaze holding (leaky integrator)
after receiving visual information in the occipital lobe and after visualspatial integration in the PPC, what can trigger a saccade?
PEF = triggered reflexively FEF = triggered intentionally
what 4 structures are involved in higher level control for saccades?
PPC, PEF, FEF and SC
what does the PPC (posterior parietal cortex) do?
calculates egocentric direction
what do the PEF (parietal eye fields) do?
initiates reflexive, visually guided saccades
what do the FEF (frontal eye fields) do?
initiates voluntary saccades
what does the SC (superior colliculus) do?
target selection, initiation of saccades, contributes to their speed, and saccade trajectory
what 2 structures does the SC send motor command to?
PPRF bust neurons and omnipause neurons
what might the vermis and fastigial nuclei do (in cerebellum)?
control the size of the saccadic pulse (lesion will cause saccadic pulse dysmetria)
what might the flocculus and paraflocculus do (in cerebellum)?
appropriately matching the saccadic step to pulse (lesion causes post-saccadic drift)
