Co-ops VS117
4/14/98

Today: Fixation and Saccades
Next Week: Pursuits - Image stabilization

For each of the above mentioned eye movements you should be able to define:
1) Purpose of the movement
2) Behavioral Characteristics/ Dynamics
3) Neural Control

- Today Chapter 12 and 14 in the reader - Next Week Chapter 15
- Also there was a handout #8 (it was 5 pages double sided)

- Fixational Eye Movements:

- When we try to fixate on a stationary target or track a moving target, we are exhibiting behavior that is exclusive to animals with foveas .
- Fixation is not simply aligning the fovea with a given target, it also involves the process of holding the image on the fovea
- Fixation is extremely important to Predators: Schor gave the example of a cat stalking its prey
- Fixation is required so that we can vastly improve upon our visual acuity of a given object that we are viewing
- In reality, the eye is never held perfectly still when fixating on a target. It is constantly moving around the foveated target.

- There are Three Movements that Occur during Fixation:

-This is a Diagram of approximately a one tenth degree area. If a person is asked to fixate on a point in that area’s center, the following fixational eye movements would result if you tracked the eye position carefully. Solid lines represents quick eye movements, where as dashed lines represent slow drifting eye movements.

-This is a graphical trace of these fast and slow eye movements in both the horizontal and vertical planes. Again note that the eyes are always moving.
-It turns out that you can classify these eye movements that are seen in Figures one and two into three distinct eye movements:

1) Microtremors: These movements are extremely small, so small in fact that they can not be seen with most eye movement monitors (or in the traces shown above). These tremors are only about a half minute of arc and occur at about 50-60 times per second. Microtremors are the result of random, individual twitch fiber activation in the extraocular muscles that move the eye. Also unique is the fact that these movements are independent in the two eyes

2) Microsaccades: These are the quick eye movements that were tracked in the two above figures. Microsaccades are approximately six minutes of arc (~ 1/10 a degree) and occur about three times per second. Unlike microtremors microsaccades are a yoked eye movement.

3) Drifts: These are the slow movements in the above figures. Drifts are also about 1/10 a degree and occur at a velocity of one minute of arc per second.

Drifts and microsaccades constantly act together to correct random errors in fixation. Drifts serve to correct small vergence errors that occur during binocular fixation. Microsaccades act to correct small monocular errors in fixation.

Clinical Correlate: Normal ocular fixation movements can not be seen with the unaided eye. Usually we see these movements with the aid of a positive lens or some other magnifying system. Any movement that can be seen by the unaided eye is considered abnormal.

-Interestingly, you can train people to eliminate microsaccades altogether from their fixational behavior. It turns out that microsaccades are actually learned behavior that come as a result of such tasks as reading and other long term close visual inspection of fine detail. When reading, we want to fixate on a particular word and then quickly jump to the next. Monkeys, who don’t usually undertake fine detail tasks of this manor, do not exhibit microsaccades. However, you can get monkeys to show this behavior by making them do tasks that require long-duration foveation. When drifts control fixation without these quick movements it is referred to as "slow control"

-Below are examples of fixation anomalies:

1) Latent Nystagmus: This condition appears when one eye is occluded and not during binocular fixation. The jerk nystagmus is characterterized by the direction of its slow phase: drift toward the occluder is followed by a quick movement away from the occluder. This pattern of a quick movement followed by a slow movement back to straight ahead is referred to as a "jerk nystagmus." In latent nystagmus the eyes move conjugately and is believed to be a developmental problem in the OKN system. We will learn more about this when we cover OKN (chapter 15)

2) Congenital Nystagmus: This is also a jerk nystagmus but is usually associated with albinism. The biology of this association is not quite known. Albinos tend to have decreased visual acuity due to abnormal photo pigment in the retinal receptors. The visual acuity is also decreased by the "retinal smearing" that results from the nystagmus. The prescribed treatment for these congenital nystagmus movements is the prescription of prism that position the eye in a "null position." It turns out that the nystagmus is greatly reduced when the eyes are positioned slightly converged and in a specific gaze direction. The prescribed prism holds the eyes in this position.

3) Eccentric Fixation: In this condition a non-foveal retinal locus is used as the preferred position of the retinal image during fixation. The eye still behaves similar to one with normal foveation, remaining relatively steady. Eccentric Fixation can occur when there is damage to the foveal area of the retina and so the patient can improve visual acuity by moving focus into the periphery. This condition also occurs in cases of strabismus ( associated with amblyopia) The eye with reduced vision tries to compensate by focusing on non-foveal position.
- Clinical Correlate: Symptoms of eccentric fixation are reduced visual acuity with no apparent pathology.

-Diagram showing the probability of where the eyes will be positioned around a fixation point. The eyes will be inside the white region 100% of the time. Eighty percent of the time the eye will be within the gray region. Finally, the eye will be inside the black area 50% of the time.

- Note that in the Figure, that the eye movements graphed are movements that occur under lighted conditions. There are also fixation movements in the Dark. The eyes are able to hold this position relatively steady, within approximately 2 degrees, or 20x less controlled compared to in lighted conditions. To do this the visual system utilizes proprioceptive cues from stretch receptors in the extraocular muscles. An example would be that you are able to hold your gaze within two degrees of the position from which you heard a noise in the dark. However, this gaze holding ability is far less steady than when the brain uses visual feedback cues to control the muscles under lighted conditions.

Saccades

-Dr. Shor likened saccades to a visual grasping or catching mechanism. In this analogy , the fovea would be the catchers glove and the extraocular muscles move the fovea at a high velocity to get in position to "catch" the light from the fixation target.
- One of the most common uses of saccades is the task of reading

- Readout of a machine we have down in clinic that records a patient’s saccade movements while reading. Note the machine also picked up regressions where the reader goes back to pick up a word again.

- We also use saccades in recognition (scan paths). When we view pictures of people, you can record where the visual axises are crossed as they scan the pictures. Note that fixation is concentrated in the eye, nose, and mouth regions. These are areas of high expression from which we can recognize familiar faces. Saccades are the primary movements during these tasks of object recognition.

- Saccades are really the only movement we process that allows for shifts in fixation from one stationary target to another. (Remember we do have tracking movements to foveate moving objects)

- Saccades also correct for errors in pursuit movements. So if you are trying to follow a particular target, and it gets ahead of your visual axis, you can use a "catch-up saccade" to return the moving object to foveal fixation.

- Saccades can also correct for tracking that is progressing too quickly and the line of slight gets too far ahead of the object

-The term "saccade" comes from the french word describing the bridled movement of a horse’s head. Similarly, the lateral and medial recti bridle the eye and force its movement.

- This diagram shows the orchestrated head and eye movements that take place when fixation is shifted from one target to another. Note that the eye moves first and the head, which is larger and therefore has greater inertia, rotates more gradually. Also note that as the head turns, the eye must rotate back to primary position when the head eventually completes its turn. Remember from earlier in the year that this counter rotation with head movement is mediated by the VOR.

- Interestingly, our vision is extremely impaired during saccadic eye movements for a couple of reasons:

1) They are so Fast (~ 1000 degrees/sec during a 15 degree saccade) So fast that there is "retinal smearing." It is just too quick for any recognition to take place.

2) Masking: This refers to a phenomena that occurs when you look at something stationary, then move extremely quickly to another stationary object. When this occurs the newly fixated object gets transmitted to the brain so quickly that is covers or "masks" the perception of the first object. Masking is dependent on the time it takes retinal images to be transmitted back to the visual cortex (~10 msec). However, the speed of transmission depends on the speed which the object comes into the visual field. Objects that appear quickly are transmitted more quickly than stationary objects.

3) Saccadic Suppression: The visual system comes observably less sensitive slightly before and during saccades. If you flash a light source at a subject during a saccade, the threshold is significantly larger (increases by about 20%)
4) When you make such a fast accelerating eye movement, the viscous vitreous humor is jerked into motion and tugs slightly on the retina. This tugging hinders visual sensitivity. Sometimes during saccades you can observe this retinal force by the visual field suddenly getting lighter as the eye turns. The reason is that the retina is falsely stimulated by this tugging force. This phenomena is more prominent in middle aged people as their vitreous goes through the normal age related liquification process. The more the vitreous liquifies, the greater the shearing force on the retina. In fact the retina can actually tear during these moments of increased force. Patients will report saying they see these flashes of light when they make quick eye movements. If tearing of the retina does occur, a retinal specialist can use a laser to "spot weld" the retina in place to avoid further tearing. If a small tear is left untreated, it can enlarge and fluid can accumulate behind the tear. This is referred to as a detached retina and can result in blindness. In cases where a patients complains of these symptoms you want them to report once a week for a month or six weeks. This is how long that it usually takes for the whole process to run its course. You want to make sure that the slight tear remains small and does not progress into a retinal detachment.

- So why is it beneficial for the visual system to reduce sensitivity during a
saccade?

- When you make a saccade, and end up looking at a new target, you must recalculate visual direction. (Remember VS 118?) You must combine information about head rotation, eye version, eye convergence, and all the other visual cues to calculate head centric direction. Most importantly, these calculations take time. So when the saccade is in progress and all this upper level processing of directional cues is in progress, the visual system is essentially blind. Want Proof of this? Go home and look in the mirror while shifting your gaze from the left eye to the right eye. You don’t see your eyes move!!

Dynamics of a Saccade:

1) Latency: How long does it take to initiate a saccade once you decide to move the eye?

-This is a diagram showing the chronology of a saccadic eye movement relative to a stimulus moving from one position to another. Studies show that this initiation latency is ~ 200 msec. If the target moves back to the original position directly after the first move, another 200 msec. Latency is seen before the eye follows. You can’t respond to the 2nd stimulus until this second period is observed.

- Of all the ocular movements, this is the largest of the latency periods. During this refractory period, the visual system is completely committed to performing the saccade and the other eye movements are ignored. The length of this refractory period can be shortened if the stimulus is predictable, for example a constantly moving target (express saccades). Essentially the latency period is still there, but you can initiate the next move in advance before the object gets there.

2) Saccades are Ballistic Movements: Ballistic movements are ones that can not be modified during the latency period. Just like once you pull the trigger on a rifle and the bullet is fired, there is no way to modify its course, velocity, and direction

3) Velocity: The velocity of the saccade depends on the magnitude of the saccade. Two degree saccades move at approximately 100 degrees/sec while 15 degree saccades move at ~ 1000 degrees/sec. Note that these high velocities can be obtained because the eye movement is rotary and not translational.

- Interestingly you can predict the speed of the saccade by the magnitude of the eye turn in degrees and vise versa (main sequence diagram). This plot shows the fixed relationship between the those two aspects of saccades. A neurologist can measure the magnitude and velocity of saccades and if they fall off of this curve it is considered abnormal.

4) Duration of Saccades: As with velocity, the duration of saccades is also predictable by the magnitude of the eye’s movement in degrees. Large saccades have longer duration.

- If you look at the duration of a saccade relative to its magnitude and it falls to far from this plot it is considered abnormal

*** The predictability of Magnitude, Velocity, and Duration of Saccades, make diagnosis of pathologies anywhere in the pathway for control of these movements possible.
5) Accuracy: Saccades are extremely accurate for eye movements under 10 degrees in magnitude. For movements larger than 10 degrees the visual system gets conservative. In these large movements, the eye completes the task by using two separate movements. The first saccade gets the eye relatively close to the visual target. The second saccade corrects for whatever residual error persists. The first saccade always falls short of the target leaving a short corrective rotation. Movements that underestimate the total distance like the one described above are known as Hypometric Movements.
- There are also Hypermetric movements in which the eye initially rotates too far, but these are considered abnormal. Hyper and Hypometric
movements are collectively called Dismetric saccades.
- There is functional significance in the fact that the visual system chose undershoot rather than overshoot the target in large saccades. By undershooting, the visual system makes less work for itself. It only has to worry about the magnitude of the resultant error, and not the direction. If the visual system was random in its overshooting or undershooting, both direction and amplitude would have to be determined every time.
- Glissades: Often when we undershoot the magnitude of the total saccade, we will glide the rest of the way completing the rotation, instead of initiating another saccade. (More on this later in the course)

Neural Control of Saccades:

- We’ll start our way at the level of the extraocular muscles and then work our way centrally.
- The progression:
-Extraocular Muscles
- Brain Stem
- Colliculus
- Cortex
- The majority of out knowledge with regard to Neural Control is at the level of the brain stem

- ExtraOcular Muscles:

- Diagram of muscle activity of the left eye moving to the left. In this figure the electromyogram is recording the activity in the medial and lateral recti. Note that at the left of the graph when the eye is at rest, there is a baseline contraction level in both muscles. Remember from VS106 that this baseline contraction of all the extraocular muscles is responsible for the physiological position of rest. When the saccade is initiated there is a burst of activity in the lateral rectus and simultaneous inactivation of the medial rectus. This is graphical evidence for DeCartes-Sherington’s Law that states that movements require reciprocal innervation of antagonistic muscle pairs. Also note that once the eye movement is complete (The right side of the graph) a higher amount of activation is required to hold the eye in the new position. The lateral rectus is straining against the elastic stretching force of the medial rectus in its extended position. Finally notice that the burst in activity of the medial rectus is required to stop the saccade.

- Brain Stem: (also covered in next lecture)
- For saccades to take place, the eyes must overcome two forces:
1) viscosity: Muscles have viscosity in them as a result of the
inertia required to get filaments to slide past one another
2) Elasticity: Muscles also have elastic properties. The more you
stretch the muscles the more force it takes to hold them in
position
- Viscosity effects motion. An example of this would be stirring a glass of honey compared to tea. The faster you stir the honey, the more force that is required. Because saccades are so fast, they are heavily effected by viscosity

- To overcome these two forces you have two innervations of the extraocular muscles:
1) Burst Innervation: Actually during the saccade. These are high frequency discharges that aid in getting by viscosity.
2) Step Innervation: Follows the Saccade. These involve a constant change in innervation due to tonic cells. This innervation fights the elasticity that results from the new position.
- Burst innervation determine Velocity and Magnitude of the saccade. Duration of the Burst innervation determines the magnitude of the movement. Long bursts result in large magnitude movements. Velocity is determined by the frequency of the burst. A high frequency of innervation results in a high velocity.
- The area of the brain that integrates vertical movements is the Nucleus of Cahal and is located near the RIMLF (anterior to III cn). The area responsible for horizontal movements is called the prepositives and is located near the sixth nerve nucleus.
- Pause Cells: These are the "clutches" for the brain stem. They inhibit burst cells while they are active. Pause cells keep the burst cells quiet until the appropriate time and are located near the PPRF. Pause cells allow for exact activation of all Burst cells to get the highest force and velocity possible in the saccade
- All of the above mentioned processes are required for accurate and fast saccades

- Now lets go even higher. What causes the Brain stem to Fire?
- Superior Colliculus: this structure transforms position error on the retina into an eye movement command. This structure contains a map of the retina. The front of the colliculus represents the fovea and as you move caudally you get representation of different horizontal eccentricities. Medial represents upward and lateral downward saccades. So when the image of an object falls off fovea, the colliculus initiates a saccade to return it to the fovea. The rostral pole of the colliculus stimulated fixation by activating pause cells in the brainstem.