Saccadic Eye Movements Flashcards
What are saccades?
accurate, high velocity, non-ballistic
What is the purpose of saccades?
to give samples of a scene
How big is a saccade?
less than 15 degrees, when larger involve the head and body
What is the neurology of a saccade?
saccade is generated by a pulse-step combo with an effect copy sent for a closed loop feedback system
Why do we need an efferent copy?
create a closed loop system, serves as comparator, tells perception that the world is stable
Explain the controller signals for saccadic eye movements
the agonist and antagonist pulses move in opposite directions and following Sherington’s law despite differences in power; step signal works to maintain the new position
Describe eye position of a saccadic eye movement
during pulse see the position change, during step eye held in the new position– made a 10 deg movement in the graph
Describe the eye velocity of a saccadic eye movement
peak velocity is at 1/2 thru the eye movement
Which has a faster velocity: short saccade or long saccade?
long saccades reach a faster velocity because it takes longer to reach the halfway point and they are able to reach a faster speed
When is the active state tension highest?
at the end of the pulse signal
What does a muscle force graph show for a saccade?
there is a spike for the pulse and then muscle forces drop to a lower baseline once step stabilizes the eye
What is found in the nucleus of a muscle performing an eye movement action?
the motor neurons
Where are other neurons found that are involved in saccades?
in paramedian pontine reticular formation
Where are tonic neurons found?
vestibular nucleus and nucleus prepositus
Describe the action of pause neurons
pause neurons function to keep the eyes stable, they signal at a constant rate the whole time except they shut off for the pulse signal
What two neural firing complexes are involved in the pulse signal?
long lead and short lead
What is a long lead?
The first part of the pulse signal, it’s firing rate begins to increase to signal a saccade
What is a short lead?
the second part of the pulse signal with a quick firing burst to signal a saccade
Describe the action of a tonic neuron
fires at a consistent pace until the movement then fires a bit more rapidly to maintain position after a step signal
Describe the action of a motor neuron
slow baseline, then fires to move muscle, then returns to a stable, yet higher baseline
What is different about neural firing when eyes are not starting in primary gaze?
neurons look different, elastic forces will return the eye to primary gaze instead of neurology
What is a normometric saccade?
aka orthometric, single saccade that lands appropriately on-target, normal gain and characteristics
What is a dysmetric saccade?
some problem with the gain or characteristics: hypometric, hypermetric, pulseless, glissadic
What is a hypometric saccade?
dysmetric– too short
What is a hypermetric saccade?
dysmetric– too long, overshoot
What is a pulseless saccade?
dysmetric– pulse doesn’t work
What is a glissadic saccade?
dysmetric– pulse ok but not great, glides instead of jumps
What are the three saccadic eye movement categories?
saccadic refixations, micro saccades, and saccadic oscillations and nystagmus
What are the two saccadic refixations?
normometric and dysmetric
What are the two normometric saccadic refixations?
main sequence saccade (majority) and low velocity long duration saccade (still end up at target)
What are the two dysmetric saccadic refixations?
multistep and single step
What are the three multistep dysmetric saccadic refixations?
corrective, closely spaced, and overlapping
What are the two single step dysmetric saccadic refixations?
hypometric and hypermetric
What are the two hypometric (undershoot) single step dysmetric saccadic refixations?
slow or pulseless saccade and glissadic undershoot
What are the two hypermetric (overshoot) single step dysmetric saccadic refixations?
glissadic overshoot and dynamic overshoot
What are the clinical correlates of glissadic undershoot?
nerve or muscle paresis, fatigue, internuclear ophthalmoplegia, myasthenia
What are the clinical correlates of pulseless saccadic refixations?
spinocerebellar degeneration
What are the clinical correlates of glissadic overshoot?
abductor overshoot in internuclear ophthalmoplegia
What are the clinical correlates of dynamic overshoot?
none
What are the clinical correlates of low velocity long duration saccadic refixations?
progressive supranuclear palsy, Wilson’s disease, Huntington’s chorea
What are the clinical correlates of multistep saccadic refixations?
cerebellar disease, drug intoxication, fatigue, brainstem dysfunction
What is the difference between old and young patients and their pursuits with catch up saccades?
older patients make more catch up saccades/sec while younger patients more appropriately calculate distance for better pursuit capability
What is latency?
the amount of time between the decision to act and the actual movement starting aka reaction time
What is the latency for saccades?
180-200 msec +/- 30 msec
What actions make up the latency period for saccades?
neurosensory delay retina to cortex 50 msec; neuromotor delay w/ higher centers and midbrain 30 msec; computational delay 50 msec; cognitive decision making 50 msec
What is the latency and velocity for saccades?
200 msec latency and 1000 deg/sec
What is the latency and velocity for a smooth pursuit?
125 msec and 50 deg/sec
What is the latency and velocity for a VOR?
15 msec and 300 deg/sec
What is the latency and velocity for vergence?
160 msec and 10 deg/sec
What factors increase saccadic latency?
very small and large target eccentricities, target uncertainty, increased target complexity, older age, inability to disengage, decreased motivation
What factors decrease saccadic latency?
normal size eccentricities, target predictability, simple target, young age, uninteresting targets, increased motivation
What are saccadic suppressions?
higher velocities (longer saccades), elevation of visual threshold, occurs during saccades and microsaccades, central neural inhibition
What are saccadic omissions?
visual masking (forward of backward), primarily responsible for lack of gray out
What is best for visual masking?
high contrast and contoured stimuli ex: looking between two faces the background is blurred
What is the absolute saccadic refractory period?
the time the eyes are unable to generate a new saccade within the latency time, once program created motion is carried out
What happens when a target is moved to a new position and back within the saccadic latency period of 200 msec?
the eye will move to the new location and remain there for the full 200 msec regardless of the duration of time the target actually stayed at the new position
T/F if a new stimulus is shown within a very brief period, the first shift in position is often ignored
true, with a short time period in the new locations, the saccade is sometimes overridden (relative saccadic refractory period)
T/F the bigger the saccade the faster it moves
true
What is the result of frontal eye field damage?
impairment of saccadic motion of the side opposite the lesion
What side do the eyes drift to with FEF damage?
eyes turn toward the side of the lesion
What direction may a patient with FEF damage tilt their head?
head tilt the opposite direction of eye movement/damage
T/F the sequence in which you take in information helps to build your perceptual representation of the scene
true
Why is neural inhibition important?
it is a basic mechanism of production of movement and main mechanism of sensorimotor train ie inhibiting distractors and focusing on a task; it is also the source of perceptual mechanisms of filtering and selection and plays a decisive role in certain cognitive functions such as decision making
What does Berthoz say about saccades?
saccades are a decision to act, not a response to a stimulus
