What are saccades and why do we need them?

The process of controlling eye movement is a very complicated task that involves many different systems, which all must work together in harmony in order to In order to maintain control throughout the variety of visual tasks that one must perform every day. Typically, the goal of almost all eye movement is to foveate on an object, or to keep an image on the foveal region of the retina. This is because the fovea is the region of the retina with the highest density of cone cells; photo-receptors responsible for high acuity vision mostly in well lit environments. One of the most common eye movements that is performed on a daily basis is the saccade. Saccadic eye movements are rapid, conjugate eye movements that change the visual axis from one fixation to another with maximal speed. This allows an individual to change the object or point on which they are foveating quickly while minimizing retinal disparity, by keeping both eyes focused on the same point. The speed of a saccade is necessary because vision, particularly the magnocellular visual pathway, is suppressed during the time period of a saccade such that an individual is almost blind for the duration of the movement (Burr 1994). As a result, saccades occur at incredibly high speeds, even up to 800deg/s, in order to minimize the amount of time that vision is suppressed (Squire 2003). Lastly, it is important to note that saccades cannot be performed slowly because the speed is pre-set by the paramedian pontine reticular formation (Squire 2003), and saccades can be reflexive, in response to a sudden visual or auditory stimulus, or voluntary as an individual shifts their gaze to an object or point of interest in tasks such as searching, reading or just shifting their gaze to look at something new.

The structures involved in saccadic eye movements

Extrinsic_eye_muscles_3.jpg
http://medical-transcriptionist-reference.blogspot.com/2012/05/eye-muscles.html



Ultimately, in order to perform any movement in the body, a muscle must be excited to contract. The eye has six muscles that control its movement, a lateral and medial rectus, which are responsible for horizontal movements, an inferior and superior rectus which are responsible for vertical movements. In addition to this the human eye has a superior oblique muscle, which depresses and intorts the eye, and an inferior oblique, which elevates and extorts the eye. The responsibility of controlling the eye muscles is shared by several nuclei. The lateral recti are controlled by the abducens nuclei through cranial nerve 6 (Ramat 2006).

Nuclei of eye movment.jpg
Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia A-S, White LE, editors. Neuroscience. 2nd ed. Sunderland, MA: Sinauer Associates (2001)

The inferior oblique, superior rectus, and medial rectus are all controlled by the oculomotor nuclei through cranial nerve 3 (Ramat 2006). And lastly, the superior oblique is controlled by the trochlear nuclei through the trochlear nerve, which is cranial nerve 4 (Ramat 2006). It is also important to understand that the nuclei that control eye movement are interconnected, which allows the eyes to move together for conjugate gaze. The most clear example of this is in the case of horizontal eye movements. In order to maintain a conjugate gaze in a horizontal movement, the abducens nuclei are connected to the to the oculomotor nuclei through the medial longitudinal fasciculus (MLF). This is important because the abducens only has a direct excitatory affect on the lateral rectus muscles which means that, when acting alone, it could only effectively abduct the ipsilateral eye. However, because the abducens is connected to the oculomotor nuclei through the MLF, the abducens is also able to control the contralateral medial rectus muscles through excitatory input to the oculomotor nucleus on the contralateral side. The connection between the abducens and the oculomotor nuclei via the MLF that allows for the maintenance of a conjugate gaze during a horizontal saccadic eye movement.


Sup. C location.jpg
http://biology4isc.weebly.com/nervous-system.html



Another important structure for the control of saccadic eye movements is the superior colliculus (SC). The superior colliculus sits just under the thalamus as with the other members of the midbrain (mesencephalon).
In addition to this, the superior colliculus has visual layers, which map the external world relative to the individual, this helps to accurately orient head and eye movements during a saccade. The mapping of the SC is such that different regions of the cortex represent specific regions of the visual field, because of this, excitation of any given part of the colliculus will elicit a motor response and the eyes will saccade to the region of the visual field associated with that location in the visual field associated with the portion of the colliculus that was stimulated. However, some research shows that the SC maps the distance between two points in the visual field than the amplitude of the saccade necessary to cover that distance (Bergeron and Guitton 2003).The rostral portion of the superior colliculus is the part of the "visual map" that is associated with the foveal region of the visual field. Therefore, under circumstances when the eyes are not moving, and an individual is foveating on a specific fixation point, it is the foveal region of the SC that is active. However, when another area of the colliculus is excited, either by voluntary control from the frontal cortex, or by reflex, the foveal region is silenced until a saccade is performed and the eyes have a new fixation.


superior colliculus.png
The key to understanding the control of saccadic eye movement is to understand how the superior colliculus is able to influence the nuclei that innervate the eye muscles. There are several specific neurons that are important for the production of a saccadic eye movement: The excitatory burst neuron, the cell body of which is in the horizontal gaze center in the paramedian pontine reticular formation (PPRF). The excitatory burst neuron is ultimately the link between the SC and the nuclei that have control over the eye muscles because it excites the ipsilateral abducens nucleus, thereby exciting the ipsilateral lateral rectus. Also,through this mechanism, a conjugate gaze can be maintained throughout the movement because the abducens and oculomotor nuclei are linked via the medial longitudinal fasciculus which allows for simultaneous contraction of the lateral rectus in one eye and the medial rectus in the other causing the eyes to turn in the same direction. It is also important to note that the PPRF inhibits the contralateral abducens nucleus through an inhibitory burst neuron so that the eyes will turn in the desired direction, or the direction of the stimulus in the case of a reflexive saccade. Also in the PPRF are long lead burst neurons which are responsible for aiding in the excitation of the excitatory burst neuron and is excited by the superior colliculus. Additionally influencing the excitatory burst neuron is the omnipause which is located in the nucleus of the dorsal raphe. The omnipause neuron is responsible for providing tonic inhibition on the excitatory burst neuron, so that the eyes are not jumping around when they should not be performing a saccade. While the omnipause neuron helps to the eyes stay fixed on a new location and inhibits the production of a saccade, the tonic neuron actively sends input to the abducens nucleus to keep the eyes in their location. The tonic neuron is located in the nucleus prepositus hypoglossi and participates in keeping the eyes at a new location after a saccade by changing their firing rate. The tonic neurons fire a some given rate when an individual is foveating on a fixation point. After a saccade is performed the tonic neuron's firing rate on the abducens nucleus changes in order to keep the eyes fixed on a new location.

Another very important structure in the control of saccades is the cerebellum. The cerebellum plays a crucial role in affecting the speed and accuracy of saccades (Robinson and Fuchs 2001). specific regions responsible for contributing to the initiation and modification of saccadic eye movements are the oculomotor virmis and some regions of the flocculus.
Cerebellum-6.jpg
http://medicalpicturesinfo.com/cerebellum/

Additionally, the cerebellum is involved in adaptation of saccades by changing the amplitude of a saccadic movement. When an issue arises such as a weakness in one of the eye muscles, or a lesion in one of the motor nuclei that would cause a disconjugation during a saccade. This would allow for correction if one eye was not performing the necessary movement during a saccade and was causing the eyes to rotate to land on different fixation points (Robinson and Fuchs 2001). Without the cerebellum, one would still be able to perform saccadic eye movements, but they would be slow and inaccurate. For a more complete review of the Cerebellum read:

http://kin450-neurophysiology.wikispaces.com/Cerebellum
http://kin450-neurophysiology.wikispaces.com/Cerebellum+II

The last important structure to understand is the basal ganglia (BG). This is a group of nuclei that are found in the midbrain and are responsible for assisting in the facilitation of desired movement and inhibition of unwanted movement.There are two pathways in the BG, the direct pathway, which is associated with

the facilitation of wanted movement, and the indirect pathways, which is associated with the inhibition of unwanted movement. For the purposes of understanding saccadic eye movements, it is important to understand that cortical input comes into the BG through the striatum, which is composed of two nuclei,the caudate and the putamen, and exits through the output nuclei, which are the globis palladis internis and the substantia nigra pars reticulata. When the information being processed by the BG is vision related, it enters primarily through the Caudate and exits through the substantia nigra pars reticulata. For a more complete understanding of the role of the basal ganglia in movement read these wiki pages dedicated to its function:

http://kin450-neurophysiology.wikispaces.com/Basal+Ganglia
http://kin450-neurophysiology.wikispaces.com/Basal+Ganglia+II




Input & Output pathways (neuronal connections)

Eye tracking little girl.jpg
Alfred Yarbus'study mapping eye movementsShows the path of saccades searching a face


There are two primary pathways that are means of eliciting a saccade, reflexive and voluntary movement. In the case of a reflexive horizontal saccadic eye movement, some stimulus must reach the superior colliculus and cause a region other than the rostral region (associated with the fovea) to become excited. If the stimulus is visual, then a stimulus excited photo receptors on the retina and the information will travel along the optic tracts directly to the superior colliculus where it excites a part of the colliculus that is different from the foveal region. When the activity in the SC shifts to the region associated with the stimulus, output signals from the SC cease their excitation the omnipause neuron, and increase excitation of the long-lead burst neuron. The collective affect of this is the disinhibition and excitation of the excitatory burst neuron, which in turn excites the ipsilateral abducens nucleus. The abducens then excites the ipsilateral lateral rectus and excites the contralateral oculomotor nuclei through the medial longitudinal fasiculus; the oculomotor nuclei then excites the medial rectus so to produce the saccade with conjugate gaze.

In the case of a voluntary saccades, the visual information from M type ganglion cells travels through the “where” pathway in the dorsal stream ultimately projecting to the lateral inferior parietal cortex. This information is constantly happening anytime an individual is looking around. However, after being processed, the information is also being sent from the dorsal stream to the frontal eye fields, which is a part of the cortex
Volentary saccade .png
http://neuroscience.uth.tmc.edu/s3/chapter08.html
that knows and remembers where an individual is, and where objects are around them. The Frontal eye fields have projections that activate the caudate nucleus in the basal ganglia. This information will ultimately output from the substantia nigra pars reticulate with the affect of facilitating a wanted saccadic eye movement and inhibition any unwanted movement. The substantial nigra's output has an inhibitory affect on the excitatory burst neuron, so when the caudate nuclei inhibits the substantial nigra through the direct pathway affect is to disinhibit the superior colliculus, and a saccade can take place.

Summary

The purpose of this page was to be an informational study tool to help students to better understand the mechanisms of horizontal saccadic eye movements therefore, to summarize I am going to attempt to create a comprehensive study guide of the content on the page. First, to understand the mechanisms of saccadic eye movements there are several structures along with their physiological roles that one needs to be able to identify and understand.


The neurons associated with a saccade

1. The excitatory burst neuron (found in the PPRF)
  • Output to the abducens nuclei to excite eye movement
2. The omnipause neuron (Found in the Dorsal Raphe)
  • Inhibits the excitatory burst neuron when foveating on an object in order to inhibit a saccade from occurring
3. The long lead burst neuron (found in the PPRF
  • Facilitates the excitation of the excitatory burst neuron when activation of the SC shifts from the foveal region.
4. The Tonic neuron (found in the prepositus hypoglosi)
  • Play a role in maintaining fixation after a saccade by changing the firing rate of its output on the abducens nuclei
  • Also plays a role in inhibiting a saccade on the contralateral side.
5. The inhibitory burst neuron (found in the PPRF)
  • Inhibits the excitatory burst neuron on the contralateral side

The major nuclei involved:


1. The Superior Colliculus
- The superior colliculus is mapped such that the visual field is represented with different regions of the colliculus representing very specific parts of the visual field. The rostral portion of the SC is the part associated with the foveal region of the visual field.
- The SC has input to the onmipause neurons and to the long lead burst neurons


2. The Basal Ganglia
- the basal ganglia has a vital role in the facilitation of desired movement and the inhibition of unwanted movement through the direct and indirect pathway respectively.

3. The motor nuclei affecting eye muscles involved in horizontal saccades
  • The Abducens nuclei: Directly innervates the ipsilateral lateral rectus and indirectly innervates the contralateral medial recti through connection to the oculomotor nuclei via the medal longitudinal fasciculus in order to maintain conjugate gaze.
  • The Oculomotor nuclei: innervates the ipsilateral medial rectus muscle.

4. The cerebellum
  • Flocculus and Virmis participate in determining the speed and accuracy of a saccade
  • Without the Cerebellum saccades are slow and inaccurate



The eye muscles responsible for horizontal saccades

  • The medial rectus muscle: adduction of the eye
  • The lateral rectus muscle: abduction of the eye

Self assessment of understanding:


1. Saccades can be voluntary or reflexive? T/F

2. The superior colliculus has a map of the visual field? T/F

3. The visual information necessary to perform a voluntary saccade travels directly to the Superior colliculus via the optic tracts? T/F

4. Which if the following neurons innervates the abucense nuclei?
a. Excitatory burst neuron
b. long lead burst neuron
c. omnipause neuron
d. tonic neuron

5. Without the Cerebellum you would not be able to perform saccades. T/F

6. Once the Superior Colliculus is activated, what is the process that occurs in order to perform a horizontal saccade?

7. What is the role of the Cerebellum is saccadic eye movements?

8. The excitatory burst neuron resides in which of the flooding nuclei?
a. The Dorsal Raphe
b. The abducens
c. the VL of the Thalmus
d. the Paramedian pontine reticular formation
e. the prepositus hypoglossi

9. What are the parts of the basal ganglia associated with initiating a voluntary saccade and why?


Answers:
1. T
2. T
3. F
4. a&d
5. T
6. The a region other than the rostral portion is activated, this inhibits affectively silences the omnipause neuron, excites the long lead neuron. The affect of this is the excitation of the excitatory burst neuron which excites the ipsilateral abducens nucleus, and inhibits the contralateral abducens through the inhibitory burst neuron. once the saccade is performed the tonic neuron changes its output firing rate on the abducens to keep the eyes at their new location.
7. The cerebellum is responsible for affecting the gain, amplitude, velocity, and accuracy of saccades
8. The paramedian pontine reticular formation
9. the Caudate and the substantia nigra pars reticulata are the nuclei associated with the initiation of a voluntary saccade. We know this because they are the major nuclei associated with the visual system in the BG and because the BG is associated with the facilitation of wanted movement and the inhibition of unwanted movement.

Glossary:

Superior colliculus: a structure in the midbrain that is responsible for saccadic eye movements
Basal ganglia: a group of nuclei associated with the facilitation of wanted movement and the inhibition of unwanted movement
Cerebellum: located "behind the brain" it is associated with the modification of movement, and the gain and amplitude and speed of saccades
Long lead burst neuron: helps to excite the excitatory burst neuron
Omnipause neuron: tonically inhibits the excitatory burst neuron when the rostral portion of the SC is active
Excitatory burst neuron: excites the abducens nucleus ultimately to initiate a saccade
Inhibitory burst neuron: inhibits the contralateral excitatory burst neuron
Tonic neuron: adjusts its output firing rate on the abducens to keep the eyes fixated at a new location after a saccade
Abducens nucleus: innervates the lateral rectus, and the oculomotor nuclei through the MLF
Oculomotor nucleus: Innervates the inferior rectus and oblique muscles, superior rectus and medial rectus.
Lateral rectus: muscle that abducts the eye
Medial rectus: muscle that adducts the eye
Medial longitudinal fasciculus: a tract of white matter that connects the abducens nucleus and the oculomotor nucleus.


Extra readings and resources

http://www.brown.edu/Departments/Engineering/Courses/122JDD/Lcturs/sacc05.html - A lecture on saccades including anatomy, causes and other details from Brown University.
http://www.tutis.ca/Senses/L11EyeMovements/L11EyeMovements.swf - A short animated tutorial of saccadic eye movements. Amusing and informational.
http://neuroscience.uth.tmc.edu/s3/chapter08.html - An online neuroscience textbook chapter on the control of eye movement.

Video: https://www.youtube.com/watch?v=FEJfAxJEP6E - this shows what saccades look like (although it is kind of creepy)
Video: https://www.youtube.com/watch?v=P6uTlnyNaTs - Another video showing what a saccade actually looks like both vertical and horizontal
Video: https://www.youtube.com/watch?v=NAz_g3FDPjw - A video explaining conjugate gaze

References

Burr, D. C., Morrone, M. C. & Ross, J. Selective suppression of the magnocellular visual pathway during saccadic eye movements. Nature 371, 511–513 (1994).

Bergeron, A., Matsuo, S., & Guitton, D. (2003). Superior colliculus encodes distance to target, not saccade amplitude, in multi-step gaze shifts. Nature Neuroscience, 6(4), 404-413.

Jarbus, A. L., & Yarbus, A. L. (1967). Eye movements and vision. New York: Plenum Press.

Kunimatsu, J., & Tanaka, M. (2016). Striatal dopamine modulates timing of self-initiated saccades. Neuroscience, 337(1), 131-142.

Purves D, Augustine GJ, Fitzpatrick D, Hall WC, LaMantia A-S, White LE, editors. Neuroscience. 2nd ed. Sunderland, MA: Sinauer Associates (2001)

Robinson, F. R., & Fuchs, A. F. (2001). The Role of the Cerebellum in Voluntary Eye Movements. Annual Review of Neuroscience, 981-1004. Retrieved December 20, 2016.

Ramat, S., Leigh, R. J., Zee, D. S., & Optican, L. M. (2006). What clinical disorders tell us about the neural control of saccadic eye movements. Oxford: Oxford University Press. doi:10.1093/brain/awl309

Squire, L. R. (2003). Fundamental neuroscience. San Diego, CA: Academic.