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Chapter 2
TAXONOMY:
THE CLASSIFICATION OF EYE MOVEMENTS

Key words: saccades, pursuits, vergence, VOR, OKN, conjugacy

Outline
I.  Functions of Eye Movements

·         Anatomical considerations
Retinal organization (fovea) requires tracking
Location of orbits (forward eye placement) requires vergence

 

·         Tasks performed
Place images on the Fovea
Enhance visual acuity and visibility of interesting objects
Reduce retinal image smear
Focus images of interest
Control light level
Expand field of view
Support stereoscopic depth perception

II. Classification of 5 types of Eye Movements: Saccades, Pursuits, Vergence, OKN, VOR

·         Functional
Image stabilization during body movements (OKN, VOR)
Image tracking of moving objects: saccades, pursuits and vergence
Suppression of stabilization by tracking

 

·         Speed
Saccades (fast movements) - do not require continuous visual feedback
Pursuits (slow movements) - require continuous visual feedback

 

·         Conjugacy of two eyes
Conjugate (yoked)-version
Disconjugate- vergence
Hering's law- adapted calibration

 

·         Direction for version and vergence
Vertical version- elevation and depression
Vertical vergence- hyper, hypo, supra, infra, sur and deor sumvergence
Horizontal version- ab duction and ad duction (nose ref), dextro, levo (body ref)
Horizontal vergence- convergence and divergence
Torsional- dextro (clockwise) & levo cyclo (counter cw) version-(body ref)
Torsion cyclovergence- in and ex cyclo vergence (nose ref)
 

·         Review Questions
 

Functions of eye movements
In order to appreciate problems your patients might encounter with a faulty eye movement system let's go over some of the functional benefits of a normal oculomotor system (reference: Leigh and Zee chapter 1 and Adler chapter 5). 

Anatomical
First of all there are the anatomical considerations.  1) Our retina is specialized for enhanced resolution in the central visual field at the fovea.  This isn't the case in many other mammals, such as the rabbit or even the cat to some extent.  We need to be able to direct the fovea in any direction in space and to follow moving objects that interest us. Our foveas give us very high spatial resolution that assists us in stereo tasks, object recognition, and our ability to manipulate small objects with our hands. Our large heads make this difficult to do using the neck by itself with any degree of speed or precision.  2) The second anatomical consideration is the forward placement of the eyes in the head allows us to have large overlap of the visual fields to permit stereopsis.  Binocular fields must be precisely matched to have good stereo, and vergence eye movements allow us to align the two eyes with targets over a large range of distances.  Vergence development is rare in non-primates.  There is some ability in cats but not much.  It serves us in our manipulation of tools with our hands at near viewing distances.

Tasks
We can also look at the benefits provided by eye movements in terms of vital tasks we perform.  1) I have already mentioned object recognition and inspection which is aided by using our foveas.  Eye movements expand the regions of space that we have high resolution.  2) Accommodation expands the range of distances that we can have high acuity and clear vision from 5 cm to optical infinity.  When we become presbyopic small pupil size serves a similar function at the expense of night vision sensitivity.  3) Pupil size is normally adjusted to give us an ability to regulate light level by a factor of 10.   4) Having forward placed eyes has the benefit of allowing stereopsis but it has the disadvantage of reducing our visual field from 360 degrees to about 200 degrees.  Many animals with laterally placed eyes can literally see behind their heads and have panoramic vision.  Eye movements let us expand our field of view.   5) As mentioned earlier, vergence eye movements give us the ability to fuse targets on corresponding retinal points such as the fovea and have stereopsis.  6) Eye movements also allow us to stabilize our visual field as we move about and to stabilize images of moving objects as they move about.  The stabilization of the visual field during our motion is a task performed by any animal that has the ability to move.  Image tracking of small moving objects is only seen in animals with foveas that need to inspect small moving objects.  The function of tracking is to bring targets to the fovea and keep them there.

These latter two tasks, image stabilization during locomotion and image tracking, each have several types of eye movements that support these activities.  In all, there are five types of eye movements (VOR, OKN, saccades, pursuits, and vergence) and they can be classified in many different ways.  They were first classified by Raymond Dodge in 1902.


Classification of movements
Classification by Function
The first and most general classification scheme is by function.  Either the eye movements stabilize the motion of the retinal image generated by our own body and head movements or they stabilize the image of a moving object in space.  In the former case the whole retinal image is moving in unison whereas in the latter case only the objects image is moving and when you track that object you cause the previously stable background retinal image to move.  This puts these two tasks in conflict with one another.  You can't track the moving object at the same time you stabilize the background field motion.  Fortunately image tracking efforts are able to suppress the background stabilization reflexes.  Animals like the rabbit don't have this ability.  Their eyes mainly serve to stabilize background motion and they are unable to follow small moving objects with their eyes.

Figure 2.1   Illustration of head and eye velocity and eye position in space that result from the vestibular and visual stimulation during head rotation.

 

 

 

 

Figure 2.2   Illustration of the placement of the vestibular canals in the head that sense angular velocity and cause the eyes to maintain a steady direction of gaze while the body moves about in space.

Two classes of reflex eye movements perform the image stabilization task.  They are called reflex because they occur automatically without any conscious effort. Phylogenetically, these are the first eye movements to appear.  Actually there are three if you count head movements.  Head movements are apparent in birds, like the chicken or pigeon, that pull their head back slowly when they walk forward and then make a quick thrust forward.  This combination of slow motion followed by a fast movement in the opposite direction is also seen in field stabilizing eye movements.  This type of eye movement is referred to as nystagmus, which is Greek for nod and means a rhythmic oscillatory pattern.

The first class of stabilizing eye movement compensates for brief head and body rotation and is called the vestibulo-ocular reflex (VOR).  During head movements in any direction, the semicircular canals of the vestibular labyrinth signal how fast the head is rotating and the oculomotor system responds to this signal by rotating the eyes in an equal and opposite velocity. This stabilizes the eyes relative to the external world and keeps visual images fixed on the retinal.  This reflex is almost always active and without it we would be unable to see much of anything due to constant smear of the retinal image.

Optokinetic reflex (OKR) also responds when we move about in a visual scene.  Unlike the VOR, this reflex requires a visible retinal image whereas the VOR works in total darkness.  You may wonder why we need both and OKR and VOR when they both respond to the same condition of body movements.  In fact the OKR supplements the VOR in several ways.  The VOR responds to acceleration and deceleration but not to constant velocity.  In contrast, OKR responds to constant retinal image velocity caused by constant body rotation or translation.  Basically the VOR controls initial image stabilization and OKR maintains the stabilization.  OKR also compensates for a damaged vestibular apparatus.  People who have had infections of their inner ear often complain of motion sickness in cars and boats.  The best thing for them to do is look out the window at the horizon and allow the OKR a chance to keep their eyes stable with respect to gravity.

Eye movements are also classified as object trackers.  These eye movements place the retinal image of the object of regard on the fovea and keep it there, even if the object moves in space.  The class of eye movements that places the image on the fovea is called a saccade.  This is a very fast eye movement that shifts the image in a step-like motion.  Saccades can be made voluntarily, in response to visual, tactile and auditory stimuli, and even in darkness to willed directions of gaze. Their chief characteristic is they are fast, reaching velocities of nearly 1000 deg/sec.  Rapid vergence movements can also shift the image of a near target on the two foveas after viewing a more distant target.  These vergences can be very fast when accompanied by saccades and cause the saccades to be unequal in the two eyes.  It is not clear whether the saccades are truly unequal or simply summed with a slower vergence eye movement.  The vergences respond to the same broad range of visual and non-visual stimuli as do saccades.

Once the target is on the fovea, slow following eye movements called pursuits maintain the image of the target on the fovea.  They can keep images stabilized that are formed of objects moving as fast as 30 deg/sec in humans and 90 deg/sec in monkeys.  Generally pursuits can not be made voluntarily.  However they do respond to a moving sound or tactile sensation or in darkness.  A few people have trained themselves to make smooth non-visually stimulated pursuits but these are rare occurrences.  Binocular vergence can also track slow variations of target distance and maintain bifoveal fixation when a visual stimulus is present.

Fig 2.3 illustrates the pattern of searching eye movements composed mainly of fast fixation changing saccades used in visual search.  (This pattern has been used to objectively measure how we process complex images like faces.)

 

 

Classification by Speed developed by Raymond Dodge (1902)
A second way to classify eye movements is by their velocity.  As just mentioned, movements that shift different targets images onto the fovea are fast and the include both saccades and quick vergence eye movements.  Slower movements tend to maintain the image on the fovea and they include pursuits and continuous tracking vergence.  Sometimes fast and slow movements are mixed such as in the OKR.  Here we have what is called Jerk nystagmus which is a mixture of a fast phase that is saccadic and a slow phase which is like a pursuit movement.

Classification by Conjugacy
If you observe the motion of the two eyes, you will notice that they either move in the same or opposite direction.  When they move in the same direction they are said to by yoked, or conjugate, like a team of horses.   These are also called version movements.  At other times the eyes move in opposite directions, especially when we converge from far to near.  These are called disjunctive or vergence movements.  Both conjugate and disjunctive movements can be either fast or slow depending if the eyes are shifting fixation or maintaining foveation of a target.  A century ago, Ewald Hering described these movements and his name has been affixed to a Law which states that the movements of the two eyes are equal and symmetrical.  Hering's law refers to interactions between contralateral agonists or synergists.  The law implies that a single innervation from the CNS moves both eyes, however different muscles in the two eyes move them in the same direction.  The muscles have very different mass and depending on the starting eye position, different mechanical advantages.  Clearly, they must have unequal innervation, however the outcome is equal rotation of the two eyes.  Hering noticed this yoking in newborn infants and concluded that it was innate.  He had quite an argument with Helmholtz over this.  Helmholtz believed that conjugacy was learned.  Helmholtz believed that both eyes were trying to foveate the same object and eventually this common task taught them to move in synchrony.  This is the old nature vs. nurture argument.  Today we know that both were correct.  Our eyes are yoked when we are born but the yoking can be modified if we decide to wear anisometropic spectacles that produce unequal image sizes and require us to make unequal eye movements to see bifoveally.  This image inequality leads to the binocular anomalies of aniseikonia (unequal image size) and anisophoria (noncomitant phoria).  Interestingly, during some phases of sleep the eyes undergo large (>10 degrees) independent movements in both the horizontal and vertical directions which suggests that yoking is under active control and is not strictly a low level reflex.

Fig 2.4 Schematic drawing of version (A) and vergence (B) eye movement.

Classification by direction
Finally you can classify eye movements by the direction of the rotation vector that moves the eye.  Directions are classified differently for conjugate and disconjugate eye movements.  Conjugate vertical movements are called elevation and depression.  Disconjugate vertical vergence movements are called right-hyper or left-hypo vergence if the right eye is above the left eye.  Disconjugate vertical vergence movements are called sursumvergence or supravergence if one eye is elevated and deorsumvergence or infravergence if one eye is depressed.  Disconjugate vertical phorias are called right-hyper (or left-hypo) if the right eye is above the left eye and right-hypo (or left-hyper) if the left eye is above the right eye.

Lateral conjugate movements have several names.  A levo-version is to the left and a dextro-version is to the right with respect to the patient.  An adduction is a lateral conjugate movement towards the nose and an abduction is away from the nose.  Horizontal disjunctive movements are called convergence when fixating from a far to a near target, and divergence when moving from a near to a far target.

Finally, we have torsional or cyclo movements.  Motion is described in reference to the top part of the iris.  For conjugate torsion, levo cyclotorsion is a rotation of the top of the eyes to the left with respect to the patient, and dextro cylotorsion is a rotation of the top of the eyes to the right with respect to the patient.  For disconjugate torsion, incyclo-torsion is a rotation of the tops of the eyes towards the nose, and excyclo-torsion is a rotation of the tops of the eyes away from the nose.  As with the vertical and horizontal movements described above, these torsional movements can be either fast or slow.

Review Questions:

    1.   Name the two types of gaze stabilizing eye movements.
      2.   List three types of foveal tracking eye movements.
      3.   Define Hering’s Law of ocular movements.

 

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