Eye movements
What are the five eye movements?
Conjugated eye movements Eyes moves in the same direction -Saccades: quick, ballistic -Smooth pursuit: slow, visually guided -Optokinetic: smooth pursuit and saccades, visually guided -Vestibulo-ocular: slow, vestibular guided Disconjugated eye movements Eyes move in opposite direction -Vergence (convergens and divergence): visually guided near and far fixations
Describe the optokinetic nystagmus eye movement. Nystagmus?
Example: Watching a train pass the eye field. The train generates a very distinct pattern of linear motion from one side to the next. There are various elements of the train passing in one direction or the other through you visual field. It is really difficult not to be grabbed by the visual stimulus. Concept We tend to make a saccadic eye movement to some feature on the train, maybe a window or a door, and then as the object passes in front of us we make a smooth pursuit eye movement. But very quickly our eye reach the limit of excursion in the orbit and it causes us to make a quick saccade like movement where we reset and acquire the next object that allows us to track using our smooth pursuit system. Nystagmus There is a slow movement and there is quick reflexive movement in the opposite direction. Note that this nystagmus should be distinguished from the vestibular nystagmus.
What is the function of eye movements?
Eye movements adjust fixation... -when visual targets move -during head movement -when acquiring a new target
What is the functional difference between the frontal eye fields and the superior colliculus?
Frontal eye fields = intentional saccades Superior colliculus = Express saccades, saccades that are stimulated by the environment
Where is the vertical gaze center located?
It is located in the midbrain. Like the PPRF (the horizontal gaze center) it is network of interneurons that will coordinate the output of the appropriate motor neurons in the ocular motor nuclei that are governing superior and inferior rectus muscles.
What is the function of the substantia pars reticulata?
Modulate the thalamus and the superior colliculus, to have normal eye movements.
Which nerves innervate which muscles?
Note that the trochlear nerve innervates contralaterally
Describe saccades.
Quick, ballistic eye movements that allows us to a change fixation from one target to another. In the graph we see and illustration of a couple of key points regarding saccadic eye movements. Imagine a visual target comes on the rise of the red line, what we notice is that it takes a bit of time for us to generate the saccade. Typically about 200 ms are necessary between the onset of the target and the onset of the eye movement that allows us to fixate that target. The eye movement itself is a rather sudden and sharp movement where we change fixation from one position to another. This allows us to acquire the target. The 200 ms delay is important because let us say that in the middle of this 200 ms period of time, some other target came on that we might choose to look at. What would happen is that we could not generate a new saccade until the completion of the previous saccade. There would be another 200 ms delay before the acquisition of the next saccade target. The moving of the eyes takes 150 ms, and during this time we are functionally blind.
Describe the smooth pursuit eye movement
Smoot pursuit eye movements typically require a visual target. A visual target is moving across the visual scene at some particular velocity, as indicated by the red lines. If we would want to follow one of these moving object we would have tot do a saccade to that target. Once we acquire the target then we can track very nicely the movements of the target
Describe the amplitude of the eyes
The amplitude of movement, of the eyes is encoded in the rate and duration of activity in the appropriate alpha motor neurons. This is illustrated in the panel to the left. Imagine that we want to shift our eye from looking straight to look off to the left. For the left eye that is going to require activation of our lateral rectus muscle. The lateral rectus muscle is going to contract and that is going to allow the eye to roate towards the left, to abduct. If were to record from the alpha motor neuron that innervate the lateral rectus muscle, what we fins is that these motor neurons start firing action potential just as we are beginning to actually contract the lateral rectus muscle. Vary soon after the onset of contraction, within just a few ms, there is an adjustment in eye position. If we are just making a very small deviation from our neutral resting position, then that motor neuron might go quiet again. If we want to continue further out to our left, we will need to induce another burst of action potentials in this abducens motor neuron that is going to cause this eye to deviate even further. If we get beyond some critical distance where we actually need to pull our eye in order to keep fixation at that particular angel, relative to the neutral position of our orbit, there may be needed a sustained activity. If we would take the eye back a little towards the neutral position the neuron would go silent during that period. This reflects the activity of some kind of interneuron that will suppress the activity of the abducens alpha motor neuron. In this way, the signals that are coordinated between the motor neurons that adduct the eye and abduct the eye can be coordinated. They can work in synergy with one another. When the eye is adducted, the abducens neurons fall silent and vice versa for the ocular motor neurons that innervate the medial rectus muscle.
Describe direction for the eyes, the pathway.
The direction of motion is specified by which pair of extraocular muscles that are activated. These muscles are coordinated by circuits of interneurons that are found in the core of the brainstem, the reticular formation. The reticular formation is a large collection of circuits. These are circuits that coordinate the horizontal movements of the eyes and the vertical movements of the eyes. The illustration shows the circuitry that coordinates the horizontal movement of the eyes. That circuitry of the reticular formation is known as the PPRF, the paramedian pontine reticular formation. One way to think about this circuit is a set of interneurons that are integrating signals that may be command signals from other motor structures or they may be sensory signals. The output of this circuitry interneurons is to coordinate the appropriate firing of abducens motor nucleus neurons and ocular motor nucleus neurons that govern the lateral and medial rectus muscles respectively so that our eyes can move in a conjugate fashion along the horizon. The paramedian pontine reticular formation is what we call the horizontal gaze center. The pathway is showed in the picture, similar to the vestibuloocular reflex.
Describe the frontal eye fields.
The frontal eye fields are a region here in the anterior part of the premotor cortex in the posterior part of the superior frontal and middle frontal gyri. They are probably just outside of the precentral gyrus and region that we call the Brodmanns area 8. The frontal eye fields are important in directing our gaze towards a visual target that we actually choose to look at. We make a decision, we generate a motion plan, which is common to what is going on in the primary motor cortex and then we execute that plan. The execution of that plan involves descending projections from the frontal eye fields to the ipsilateral superior colliculus and to the contralateral gaze centers of the reticular formation. The paramedian pontine reticular formation in the pons for horizontal movement and a vertical gaze center in the midbrain. The fact that the frontal eye field projects to both the superior colliculus and the gaze centers in the brainstem suggests that there may be some redundacy to the means by which circuits of upper motor neurons control eye movements.
How does the optokinetic system and the vestibuloocular reflex stabilize our vision?
The illustration shows us the gain of the optokinetic system and the vestibular ocular system across most frequencies, and what we fins is that for the optokinetic system the gain is quite high when the motion is quite slow. The optokinetic system help to stabilize our gaze when there is very slow drift in the visual field. When the rate of movement begins to increase then our optokinetic system can no longer keep the pace that ok becuase out vestibular ocular system now kicks in. Once we are approaching movements at a fairly moderate pace then our vestibuloocular reflex can maintain the fixation control while there is motion, in this case of the head. These systems work in a complementary fashion.
What input does the superior colliculus get and which functions is depending on these inputs.
The superior colliculus receives input not just from the visual system, but also from the auditory system and the somatic sensory system. What the superior colliculus really wants to do is direct our gaze towards a novel stimulus, a stimulus that captures our interest or our attention.
Describe the upper motor neuronal control of eye movement.
There are essentially two principal parts of the brain that are involved in generating the command signals that are conveyed into the gaze centers in the reticular formation of the brainstem. At that level we can consider that to be the level of our lower motor neurons that are coordinating the output from our alpha motor neurons that actually execute the movement. The two main stations that are relevant for controlling saccades from the prospective of upper motor neurons, are the frontal eye fields, which are part of the premotor cortex, and the superior colliculus, which is on the dorsal aspect of the midbrain. What these structures do is that they begin to fire just before the generation of a saccadic eye movement. That timing suggests that they have a role to play in generating the command signals about what kinds of saccade to make, both in terms of the amplitude of the movement as well as the direction. What we find in both the frontal eye field and the superior colliculus is a motor map. This motor map for generating saccadic eye movements is informed by sensory maps that are present in both of these structures. That sensory map represents location in the contralateral visual hemifield such that if one where to record form neurons in the frontal eye field of the superior colliculus one could define a receptive field that would be present somewhere off in the contralateral world. If one were then to stimulate the same site from which you would record what would happen is that the animal or the person would generate a saccadic eye movement to precisely the location of the receptive field of that column of neurons that were just sampled.
Which ones are the vergence eye movements and what is their function?
There are two varieties of vergence eye movements. There is convergence and then there is divergence. Imagine the situation where you are changing you fixation from some point in the distance and you decide now to hold up your finger and look at what might be at the end of your finer. When you do that you are changing fixation the point b to point a. Such a change in fixation is going to involve a rightwards roation of the left eye and leftward rotation of the right eye, both eyes are in adduction. This is activity that is driven bu the ocular motor nerve causing contraction of the medial rectus muscles in both orbits. That is what is necessary to go to a near point of fixation. This is called convergence. If one was going to go from the near point of fixation to a distant, divergence is necessary. Divergence is going to cause the left eye to rotate to the left and the right eye to rotate to the right. This will bring our point of fixation from point a out to something distant, point c for example.
