Smooth Pursuit of Eye Movement
Jenna Joy Levchuck


Smooth pursuit movements focus the stimulus on the fovea. The smooth pursuit is under the control of the observer. The observer can decide whether or not to continue to follow the moving stimulus. Only advanced observers can perform smooth pursuit of eye movement without a moving target. If most were to attempt this, they would simply make a saccade. When an object moves, the image is still on the fovea (7). An example of this is when watching a tennis match and the racket makes contact with the ball, the eyes will continue to follow and track the ball in the air. The maximum velocity for the smooth pursuit movement is 100 degrees per second.

An example of smooth pursuit eye movement
http://www.youtube.com/watch?v=sKrvQgoR2uk


Anatomy

1. Temporal eye field neurons in medial superior temporal gyrus and middle temporal gyrus(in monkeys)- These neurons begin and guide smooth pursuit. They determine the direction and velocity of the smooth pursuit of eye movement. They send their axons to the dorsolateral pontine nucleus. These match up with the superior temporal-inferior parietal areas in humans.
2. Frontal eye field neurons- Also start smooth pursuit of eye movement and send their axons to the dorsolateral pontine nucleus.
3. Dorsolateral pontine nucleus- Determines what speed and direction the eye movement needs to be to match the speed and direction of the moving visual target. Axons decussate and end in the contralateral cerebellum.
4. Cerebellum- sends its axons to the vestibular nuclei
5. Vestibular nuclei- propels axons to abudcens, trochlear and oculomotor nuclei by way of the medial longitudinal fasciculus. They regulate smooth pursuit of eye movement for the temporal eye field. They also help to organize the atagonist muscles involved in smooth pursuit (3).


Input and Output pathways-

The system is extremely similar to the optokinetic system. The control of smooth pursuit eye movements is managed by neurons in the PPRF(Paramedian Pontine Reticular Formation). The actual pathway to the PPRF has not been identified (9), however it has been suggested that for a nonhuman, the temporal eye field transmits signals to the dorsolateral pontine nucleus concerning the direction and speed of the object. The dorsolateral pontine nucleus then matches the speed and displacement of the eye movement to follow the target (3). The information is then sent to the cerebellum (which includes the flocculus, paraflocculus and the vermis). (4) In the cerebellum are neurons who have a firing rate that is tightly coupled to the velocity of the eye rotation especially during smooth pursuit. These neurons in turn are projected to the vestibular nuclei where motor error detection information becomes included in the vestibulopontine occulomotor system(2) . This information finds the static signal needed to keep the eye in a set position should the eye stop. This information is found in prepositus nucleus of the hypoglossal nerve. From there, the information goes to the cranial nerve (9).
Two different hypotheses attempt to explain the source of the dynamic movements. One hypothesis is muscle driven, believing that speed and direction are the only inputs that the smooth pursuit system needs. The other hypothesis states that knowledge of position, acceleration and speed are a dynamic force used to enhnce the properties of the visual stimulus. This hypothesis is sensory driven (9).

There are two decussations also involved in the smooth pursuit pathway. The DLPN axons and those of the vestibular nuclei provided excitatory input to the abducens nucleus. Because of this, the command signals that are produced by the MST and MT cortical neurons allow for an execution of the eye movement. The movement is performed in a ipsilateral direction to these neurons (2).



Normal Functioning of Smooth Pursuit

ch20f5.jpg

Taken from http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=neurosci&part=A1357
This shows how the smooth pursuit eye movement occurs. Three different eye movements attempt to follow a stimulis moving at three different speeds. After a saccade which quickly tracks the movement, the velocity of the eye movement is the same as the target movement (7).

The smooth pursuit movement is used to continually track a moving object. During a smooth pursuit, the VOR is inhibited. They allow for stable viewing of constantly moving objects (1).

A study done by the University of Toronto tested smooth pursuit in children. The researchers utilized an infrared eye tracker to test children between the ages of 8-19 years old. They found that children generally have lower gains of smooth pursuit than adults do, especially vertical gains. However, these gains improve with age and generally are the same as adults in mid adolescence. This suggests that smooth pursuit eye movement needs to be trained and as children continue in their development, they will be able to track objects better (8).


Signs of dysfunction

In monkeys, damage to the middle temporal area causes abnormal movements. In humans, the parietal and occipital lobes damage causes abnormal smooth pursuits. The deficits in eye movements are on the same side as the lesion. If a lesion takes place on the right region, then there will be an inability to track an object from left to right (7).

Studies have been performed addressing the idea that schizophrenia patients have abnormal smooth pursuit eye movement. The researchers used an MRI to compare brain response when performing a smooth pursuit eye movement in patients who were healthy and those who had schizophrenia. The MRI revealed schizophrenic patients had deficits in the frontal and occipital regions of the brain, the areas of the brain directly involved in smooth pursuit. They also had lower gains when performing smooth pursuit as well as more eye movement due to anticipatory saccades (6).

There has also been studies concerning whether autism is affected by smooth pursuit of eyemovement. Children with autism often show a reduced velocity of smooth pursuit during ongoing tracking. However, this correlation is still being investigated as patients with autism normally show a number of visual deficits (10).

Another correlation between smooth pursuit deficits can be post traumatic stress syndrome with secondary psychotic symptoms (2). These patients can barely reach a velocity of above 30 degrees per second. There have also been studies done in which poor performance on visual tasks can be associated with a history of physical or emotional abuse (5).

Glossary of Terms
Cerebellum- three layered cortex and deep nuclei. Responsible for coordinating movement.
Decussate- crossing sides at the midline
Floccolus-oldest portion of the cerebellum
Fovea- area of the retina containing rod and cone cells
Frontal eye field- located in the premotor cortex and helps to activate movements during smooth pursuit
Paraflocculus- smallest lobe of the cerebellum
PPRF- paramedian pontine reticular formation- facilate the neurons in the abducens and oculomotor nuclei to generate eye movement
Saccade- rapid movements which quickly change the point of fixation
Smooth pursuit- slow movements that keep the stimulus on the fovea
Temporal eye field- neurons in nonhuman primate. Guide initiation of smooth pursuit movements
Vermis- midportion of the cerebellum

http://www.physpharm.fmd.uwo.ca/undergrad/sensesweb/L11EyeMovements/L11EyeMovements.swfThis site helps to furthur explain The various types of eye movement with some helpful animations.

http://www.journalofvision.org/content/8/14/3.full
This research article discusses whether smooth pursuit and attention are coupled together. Pursuit may be started at little cost when attention is diverted from the pursuit target, unlike the first catch-up saccade. A strong coupling may not emerge until the first steps of steady-state pursuit.


Krauzlis, RJ The control of voluntary eye movement: New perspectives. The Neuroscientist. 2005 Apr;11(2):124-37. http://nro.sagepub.com/cgi/content/abstract/11/2/124
This article explains the similarities between smooth pursuit and saccadic eyemovement.

Quiz questions
1. The velocity of smooth pursuit movements is _
a) 700 degrees per second
b) 20 degrees per second
c) 100 degrees per second
d) between c and a

2. Smooth pursuit is controlled by neurons in the
a) PPRF
b) Cerebellum
c) MT
d) Eyes
e) None of the above

3. How many decussations take place in the smooth pursuit pathway?
a)0
b) 10
c) 1
d)2

4. What are the neurons in the cerebellum responsible for?
a) Find the position, acceleration and direction for the visual stimulus
b) Have a firing rate that is tighly coupled to the velocity of eye rotation
c) Keep the eye in the proper position
d) Directing the information to other cortical centers

5. Smooth pursuit focuses on stimulus of the
a) Fovea
b) Retina
c) Optic nerve
d) Rods
e) All of the above

True and False
6. True/False During a smooth pursuit, the VOR is inhibited.
7. True/ False There is a temporal eye field in humans, not primates which is responsible for initiating the movement.
8. True/ False The vestibular nuclei help the antagonist muscles in smooth pursuit.
9. True/False Lesions take place on the contralateral side.
10. True/False A quick saccade occurs in the smooth pursuit in order to track the movemnt.

Essay- For this scenario, please describe the neurological mechanism associated with the eye movement.

You are casually enjoying the Yankees Red Sox game from your seat high in the upper deck of Yankee Stadium. Your favorite player, Derek Jeter comes to plate and the score is tied 1-1. You intently watch his at bat. Josh Beckett pitches the ball in and you watch Derek's bat make contact with the baseball. From there you follow out it out towards dead center field as it goes over the fence. You continue to follow it until it descends into the stands and the Yankees pull ahead 2-1.

Answers:

1. C
2. A
3.D
4. B
5. A
6. True
7. False, only in primates
8. True
9. False, ipsilateral side
10. True
References
1. Blumenfeld, H. Neuroanatomy Through Clinical Cases. Sunderland, MA: Sinauer Associates, Inc, 2002, 59-60, 548, 553.
2. Cerbone A, Sautter FJ, Manguno-Mire G, Evans WE, Tomlin H, Schwartz B, Myers L. Differences in smooth pursuit eye movement between posttraumatic stress disorder with secondary psychotic symptoms and schizophrenia. Schizophr Research 2003 Sep 1;63(1-2):59-62 http://www.ncbi.nlm.nih.gov/pubmed/12892858
2. Dragoi, Valentin. Gaze Shifting: Eye movements to focus the image on the fovea. Neuroscience Online: A Textbook for the Neurosciences. Houston: University of Texas Health Science Medical Center, 1997, Section 3, Chapter 8, Part 2, page 2.
3. Gardner, E. Fundamentals of Neurology. Philadelphia, PA: W.B. Saunders Company, 1993, 208-210.
4. Greenstein, B., & Greenstein, A. Color Atlas of Neuroscience Neuroanatomy and Neurophysiology. Stuttgart, NY: Thieme, 2000, 236.
5. Irwin HJ, Green MJ, Marsh PJ.Dysfunction in smooth-pursuit eye movement and history of childhood trauma.Percept Mot Skills. 1999 Dec;89(3 Pt 2):1230-6. http://www.ncbi.nlm.nih.gov/pubmed/10710773
6. Kathmann, Norbert Ph.D., Andrea Hochrein, Dipl. Psych., Ruth Uwer, Ph.D., and Brigitta Bondy, M.D. Deficits in Gain of Smooth Pursuit Eye Movements in Schizophrenia and Affective Disorder Patients. Am J Psychiatry 160:696-702, April 2003 http://ajp.psychiatryonline.org/cgi/content/full/160/4/696
7.Purves, D. et al. (Eds.). Neuroscience. Sunderland, MA: Sinauer Associates, Inc, 2004, p. 457-466
8. Salman, MS. Smooth pursuit eye movements in children. University of Houston, 1995.
9. Squire. et al. Eye Movement. Fundamental Neuroscience. New York: Academic Press, 1998, Ch.36, pgs, 147-149.
10. Takarae Y, Minshew NJ, Luna B, Krisky CM, Sweeney JA. Pursuit eye movement deficits in autism. Brain. 127(12):2584-2594 http://brain.oxfordjournals.org/cgi/content/full/127/12/2584