Topic 6a: Eye movement Flashcards
Why do we move the eyes?
1) Allow interesting parts of the image to fall onto the high-resolution part of the retina.
2) Converge the eyes at different distances
3) Stabilize the visual image on the retina despite motion of the eye (reafferent) or the scene (exafferent)
Why are eye movements important to study?
- Most common movement
- Allow us to fixate and track objects
Eye movement problems underlie: - diplopia (double vision)
- drift (nystagmus)
- vital for reading (may be one contributor to dyslexia)
Offer a relatively simple example of neural control (less complex than limb movements)
Allow an understanding of cortical-subcortical interactions without confounds of gravity
Show the principles of sensory maps
What is a gaze path?
How do gaze paths look whilst reading?
What does this show about reading?
- The spatial path of the eye as it moves across an image
- Eye movements jump from one location to another, allows a stable image to fall on the retina
- Reader fixates on each word, and sometimes skips common words
- This demonstrate how reading can be automatic and context dependant (both controlled by the CNS) - top down processing
What is gaze stabilisation like whilst viewing an image?
Subjects asked to view top-right image and then asked a different question. All had different gaze stabilisations,
Thus, eye movements are automatic and volitional (self-regulation)
We look at different things depending on the context
How do we control eye muscles?
1) Intra-ocular
2) Extra-ocular
1) Control pupil diameter (changes with brightness)
2) Move the eyeball within the socket (i.e., superior-inferior; medial-lateral). Innervated by specific cranial nerves
A mechanically simple system: the eye behaves like a “spring” (i.e., look forward without input.
What are three functional types of eye movement?
1) Gaze stabilizing mechanisms (old system to make image sharp)
Optokinetic reflex (OKR) (driven by visual motion)
Vestibular-ocular reflex (VOR) (driven by head motion system)
- Gaze shifting mechanisms (new system to scan/track objects)
Vergence movements (to shift focus on near & far objects)
Smooth pursuit (eye movements to fixate on moving objects)
Saccade movements (rapid eye movements between fixation) - Gaze fixation
Eyes must be held stationary between movements
(an active process is required to keep them still)
What does gaze stabilisation mechanism I do? How does it work?
OptoKinetic Reflex (The Slow Way):
- Reflex that maintains gaze position during whole field visual motion
- Minimizes “visual or retinal slip” of the image on the retina to allow us to track moving objects
- Retinal slip stimulates the optokinetic reflex.
What is an Optokinetic Nystagmus?
When is it normal?
When is it abnormal?
Is it fast or slow? Why?
- The alternation of slow drift of the eyes followed by rapid saccades.
- Normal response that stabilises the retinal image when the world drifts past your eyes (e.g., when looking out a train window) because the eyes can only rotate so far
- Abnormal response when caused by lesions that abolish positional control of the eyes, or by abnormal vestibular inputs.
- A slow mechanism because it relies on integrating vision (LGN + V1) and motion (V5) with brainstem compensatory mechanism.
What does gaze stabilisation mechanism II do?
Why is it fast?
How does it work?
Vestibular Ocular Reflex (The Fast Way)
- Reflex that maintains gaze position despite head movement
- Fast (~14 ms) because only 3 neurons in the brainstem are involved.
1. Head movement detected by semi-circular canals
2. Head movement signals go to the vestibular nucleus.
3. Signals cross the midline and innervate the ocular-motor neurons (OMNs), which drive the extra-ocular muscles.
4. Movement to the right abducts the left lateral rectus muscle and adducts the right medial rectus muscle.
What is the direct and indirect pathway? Which is phasic and tonic
During a head rotation to the right a phasic response is seen (because vestibular afferents sense how fast you rotate). This rotates the eye to the left and therefore keeps the image on eyes still
The indirect pathway through the nucleus Prepositus Hypoglossi (PPH) converts the phasic vestibular input into a tonic signal via reverberating neural circuit
What is Vergence?
How does it work?
Think: your eyes converging
Simultaneous movements of both eyes in opposite directions to maintain focus.
Accommodates different viewing distances.
Neural mechanisms of this are not understood
What is Smooth pursuit?
Is it old or new?
Requires suppression of the …
Fast or slow?
- Slow simultaneous movements of both eyes in the same direction.
- Evolutionarily new system to track slow-moving objects against a stationary background.
- Requires suppression of the optokinetic reflex (OKR).
- Is a slow visual “feedback” mechanism because driven by visual motion signals from areas MT & MST (middle temporal & middle superior temporal cortex).
What is saccades?
Simultaneous movements of both eyes in the same direction.
Allow for very rapid gaze shifts.
Constant velocity (up to 600o / Sec).
Need to be fast because vision is degraded during movement.
Thus, saccades minimize the amount of time the eyes are in motion.
What is a saccade?
What part of the brains are involved?
Are saccades voluntary or reflex?
What area controls the relfex?
- Saccade is rapid eye movements
- Produced using parts of the gaze stabilization circuitry, with an extra part that tells the eyes where to look.
- Gaze stabilization circuit controls how to move the eyes.
- Saccade generation circuit controls where to look.
- Controlled by Burst neurons in the PPRF (Paramedian Pontine Reticular Formation)
- Can be voluntary (reading) or reflexive (in response to a stimulus)
- Involuntary saccades are controlled by the Superior Colliculus
What can the Superior Colluculus do?
- Contains a retinotopic map
- Maps visual & auditory inputs to motor outputs
- Orients the head and eyes toward something seen or heard
- Can generate eye movements without cortical involvement (fast reflective system via signals from LGN)
- Turns off “omnipause” neurons, releasing the oculomotor system (PPRF) from inhibition (i.e., releases the eyes from fixation).
