Overview:

A saccade is a rapid eye movement generated to shift visual attention onto a specific target area. Saccades are categorized as ballistic movements, meaning that after it is initiated, corrections are unable to be made. During the eye movement vision is suppressed, however the speed of the movement (up to 700°/s), allows for this momentary suppression to be minimized. Saccades are initiated either through reflexive mechanisms or under voluntary control. Sudden attention to a visual, sensory, or auditory stimulus would be an example of an involuntary saccade. Conversely a voluntary saccade is initiated when desiring to look at a particular object of interest (ex. reading).





Functional Anatomy:

Eye Musculature:
Eye Muscles.jpeg
Figure 1.1
The eye is controlled by six extraocular muscles, working in agonist/antagonist pairs allowing for the eye to move in various directions. These six muscles that attach to the eye consist of: four rectus muscles (superior, inferior, medial, and lateral) and two oblique muscles (superior and inferior). By activating certain combinations of the extraocular muscles the eye is able to rotate upon three axises- horizontal, vertical, and torsional. Horizontal rotation allows for abduction/adduction of the eye. Vertical rotation allows for elevation/depression of the eye. While torsional movements do not change the line of sight, but rotate the eye around it: Intorsion rotates the top of the cornea toward the nose and extorsion rotates it away from the nose (Kandel). These muscles are constantly working together, helping shift gaze by bringing target areas onto the fovea.

Eye Movements.png
Figure 1.2

The actions of the different muscles each contribute to different eye movements (figure 1.2). The medial rectus adducts the eye while the lateral rectus abducts it. The superior rectus elevates and also is involved with intorsion, oppositely the inferior rectus is responsible for depression and slight extorsion of the eye when activated. Both of the oblique eye muscles (superior/inferior) contribute towards movement of the eye along the torsional axis. The superior oblique causes intorsion of the eye, and slight depression. The inferior oblique causes extorsion of the eye, and slight elevation.

Cranial Nerve Innervation:
Extraocular muscles are innervated by three different cranial nerves whose cell bodies form nuclei in the brain stem (Kandel). The oculomotor nuclei (CN III) innervates the medial, inferior, superior recti and the inferior oblique. The abducens nerve (CN VI) innervates only the lateral recuts muscle. And, the trochlear nerve (CN IV) innervates the superior oblique. While sitting relatively close together, these three specific cranial nerve locations can be seen in figure 1.3.


3-4-6-oculomotor-nerve-trochlear-nerve-abducens-nerve-anatomy-en_medical512.jpeg
Figure 1.3


Superior Colliculus:
Superior_col.jpeg
The superior colliculus is a structure in the midbrain which plays a prominent role in the initiation of saccades. It has two distinct functional areas: the superficial layers and the deep (motor) layers. The superior colliculus receives direct retinal projections and information from the frontal eye field regarding visual input. This information comes into the superficial layers, and corresponds to a specific portion of the visual field, maintaining organization. In addition to the visual input, the superior colliculus receives auditory and somatosensory information. This varied input allows the superior colliculus to generate saccades in response to these differing stimuli. For example, a tap on the shoulder or loud noise that catches your attention will excite a certain part of the superior colliculus, generating a saccade oriented toward that stimuli.

The activity level of the superior colliculus is monitored by regions of the basal ganglia. The basal ganglia has tonic inhibition on the superior colliculus, until just prior to a saccade. As visual input is processed, information will eventually reach the frontal eye field. The frontal eye field then sends excitatory signals to the superior colliculus and the caudate nucleus of the basal ganglia. The caudate nucleus then sends inhibitory projections to the substantial nigra pars reticulata, which releases its inhibition on the superior colliculus and allows for the generation of a saccade. The superior colliculus then has projections to horizontal and vertical gaze centers. The horizontal gaze center is located in the paramedian pontine reticular formation, while the vertical gaze center is located in the mesencephalic reticular formation.




Overview of Pathway:

eye field.png
Saccadic eye movements, as mentioned before, can either be voluntary or involuntary and can be evoked from varying stimuli (visual, auditory, somatosensory). This section will focus on voluntary saccades that are generated in the horizontal directions.
As the frontal eye field excites the superior colliculus, its projections correspond to the position of the object of interest. The layers of the superior colliculus are organized in a way, that input projections provide a precise location of the target. Not only does the input to the superior colliculus encode a location for the target, but it also provides an initial direction and amplitude.
The superior colliculus is able to fine-tune and make necessary corrections before sending output projections. From here the superior colliculus sends inhibitory signals to omnipause neurons in the nucleus of the dorsal raphe, and excites long-lead burst neurons in the paramedian pontine reticular formation, the horizontal gaze center. The superior colliculus' projections to these structures is important because both long-lead burst neurons and omnipause neurons have projections acting on excitatory burst neurons. Omnipause neurons are tonically inhibiting the burst neurons, and long-lead neurons are crucial in exciting them. The inhibition of the omnipause neurons from the superior colliculus, triggers disinhibition of the excitatory burst neurons. Long-lead neurons are now able to send excitatory signals to the burst neurons, continuing the saccade as its transmission progresses through the pathway. As burst neurons are excited, they now have two key functions. They provide inhibitory projections to the contralateral burst neuron and send excitatory signals to the motor neuron in the abducens nuclei. The abducens nuclei is then able to excite the lateral rectus of the ipsilateral eye. It also provides input through the medial longitudinal fasciculus, to the medial rectus of the contralateral eye via the oculomotor nuclei (CN III), allowing for convergent eye movement. After the desired image is located on the fovea, tonic neurons become involved in holding the new eye position. The medial vestibular nucleus and nucleus prepositus hypoglossi, are what provide the tonic signal.


Saccades.jpeg

Due to its ballistic characteristics, control centers are unable to make adjustments to saccades after being initiated. Information must travel through the visual processing system before knowing if the saccade was successful. As visual information reaches the striate cortex, this information will travel the dorsal stream and reach structures such as the middle temporal and the posterior parietal cortex. These regions are then able to inform the superior colliculus whether the target area has been fixated on (seen in image below).


brain loop.png



Discharge Rates:

Dishcarge Rates.gif
Saccadic eye movements are generated in whats described as a pulse-step activity. "The discharge frequency of an extraocular motor neuron is directly proportional to the position and velocity of the eye. As the eye velocity goes from 0 degrees/sec to 900 degrees/sec the firing rate of the neuron increases rapidly, described as a pulse activity" (Kandel). As a saccade is initiated, and the eye begins to move, there is a dramatic increase in the oculomotor neuron. For example, in making a leftward saccade, the spike in discharge rate is seen in the left abducens nuclei, as it is providing the muscle with a location with the signal being sent. This spike quickly encodes the position of the target, and helps maintain the lightening fast aspect of the eye movement. The second phase of the of the pulse-step activity is accomplished through the tonic neurons. After the dramatic spike of activity helped quickly fire the eyes, there is a new baseline firing rate that helps maintain the position of the eyes on the target area. These neurons send a steady signal related the the new eye position; they do not generate any saccadic burst signals (Kandel). Lesions in the nucleus prepositus hypoglossi and the medial vestibular nuclei have shown that after saccades have been generated, the eyes tend to drift back to the previous eye position, unable to hold the target area on the fovea.




Summary:

SACCCAAADDDEEESSS.jpeg

Saccadic eye movements are essential in quickly locating new targets in the visual field. So many daily activities rely on the normal functioning of this eye movement. Through the generation of a saccade, many neural interactions are relied on to carry out a successful saccade. One of the key structures in facilitating saccades is the superior colliculus. Receiving so much input information, the superior colliculus is able to integrate that information, and code the amplitude and direction that the saccade must follow. As output information reaches either gaze center, several connections lead to the pulse portion of the movement, leading to a quick and accurate movement of the eyes. This new position is then able to be held by several mechanisms, holding the new eye position and keeping the target area on the fovea. Through visual processing the superior colliculus, is then informed whether the saccade was successful, and if not is able to make corrections as needed.






What does a saccade look like?








Glossary:
Dorsal Raphe Nucleus: located on the midline of the brainstem, house omnipause neurons

Excitatory Burst Neuron: located in the paramedian pontine reticular formation, they excite motor neurons in the abducens nuclei

Extorsion: rotation of the eye away from the nose

Intorsion: rotation of the eye towards the nose

Omnipause Neurons: have tonic inhibition on the burst neurons, turned off by the superior colliculus

Paramedian Pontine Reticular Formation: recieves information from the superior colliculus, houses both the long-lead burst neurons and the excitatory burst neurons.

Striate Cortex: located in the occipital lobe; this region of the brain is responsible for processing visual information





QUIZ:

Multiple Choice:
1. The superior colliculus has output projections to which neurons?
a) Long-lead burst neurons
b) Omnipause neurons
c) Excitatory burst neurons
d) a & b
e) All of the above

2. What is the vertical gaze center?
a) Superior Colliculus
b) Paramedian Pontine Reticular Formation
c) Mesencephalic Reticular Formation
d) Abducens Nuclei

3. Which neuron innervates the lateral rectus?
a) Motor Neuron of abducens nuclei
b) Excitatory Burst Neuron
c) Long-Lead Neuron
d) Omnipause Neuron

4. Through which tract are the abducens nuclei and oculomotor nuclei connected?
a) Tectospinal Tract
b) Medial Longitudinal Fasiculus
c) Corpus Callosum
d) Pyramidal Tract

True/False:
5. T/F: The frontal eye field provides an initial amplitude and direction for the superior colliculus regarding the movement of the saccade
6. T/F: During a saccade adjustments are able to be made to adjust for error or miscalculation
7. T/F: The paramedian pontine reticular formation is the horizontal gaze center


Short Answer:
8. What structures have projections to the superior colliculus and what information do they provide?

9. Once a saccade has been generated how is that eye position held?

Answers:
1. d
2. c
3. a
4. b
5. T
6. F
7. T
8. The frontal eye field, basal ganglia, and posterior parietal cortex, all have projections influencing the superior colliculus. The frontal eye field helps begin the early phase of generating saccades by providing the superior colliculus an initial amplitude and direction regarding the saccade. The basal ganglia has tonic inhibition on the superior colliculus, and must be disinhibited before a saccade is facilitated. And, the posterior parietal cortex and other visual association areas have projections to the superior colliculus informing it of if the saccade was successful.
9. The new eye position is held through a couple mechanisms. One contributing mechanism is through the tonic neurons of the prepositus hypoglossi and the medial vestibular nuclei; they provide the eyes' motor neurons with a new rate discharge, holding the new eye position. Also, the omnipause neurons are again activate and are inhibiting the excitatory bust neurons. And finally, the basal ganglia begins inhibiting the superior colliculus after the saccade has been generated and succesfully located the new target area.

References:
1. Brodal, P. (1992). The central nervous system. NY: Oxford U. Press.
2. Matthews, G. G. (1986). Cellular physiology of nerve & muscle. Palo Alto: BlackwellScientific Publications.
3. Nicholls, John G., Robert Martin, and Bruce Wallace. From Neuron to Brain. 4th ed.Sunderland: Sinauer Associates, 2001.
Print.
4. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition.
5. Sunderland (MA): Sinauer Associates; 2001. How Otolith Neurons Sense Linear Forces. Available fromhttp:/www.ncbi.nlm.
nih.gov/books/NBK10876/
6. Wade, Nicholas J., and Michael Swantson. Visual Perception. New York: Routledge, NY. Print.
7. Thiele A. Neural Mechanisms of Saccadic Suppression. Science [serial online]. March 29, 2002;295(5564):2460.
8. Dragoi, V. Chapter 8: Ocular Motor Control. http://neuroscience.uth.tmc.edu/s3/chapter0.html
9.Kandel, ER. (2000). Principles of neural science. Mc-Graw Hill: New York.