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Eye Movement Axes

 

About the vertical (X) axis the eye moves from side-to-side

eyeball motion, side to side.GIF (26900 bytes)

  • Temporal displacements =  Abduction

  • Nasal displacements =  Adduction

 

About the horizontal (Y) axis the eye moves up and down

eyeball motion, up and down.GIF (26568 bytes)

  • Downward displacements:  Depression

  • Upward displacements:  Elevation

 

About the anterior-posterior, or torsional (Z) axis the eye rotates

eyeball motion, torsional.GIF (27594 bytes)

  • Temporal rotations of the superior cornea:  Extorsions

  • Nasal rotations of the superior cornea:   Intorsions


 

A Brief Taxonomy of Eye Movements

Function of Eye Movement

Type of Eye Movement

"Version" (Conjugate)

"Vergence" (Disjunctive)

"Holding" (slow)

  • Smooth Pursuit
  • Optokinetic Nystagmus (slow phase)
  • Vestibular Nystagmus
  • Convergence
  • Divergence
  • Accommodative Vergence

"Catching" (fast)

  • Saccades
  • Optokinetic Nystagmus (quick phase)
 

"Sustaining" (miniature)

  • Microsaccades
  • Tremor
  • Drift

voluntary eye movement; involuntary eye movement


 

Types of Eye Movements

Much of the time the two eyes move as a unit, through equal angles in the same direction.  Such eye movements constitute conjugate, or version eye displacements.  Other eye movements involve coordinated, but unequal, degrees of movement of the eyes, and are variously referred to as non-conjugate, vergence, or disjunctive eye movements.

Conjugate

conjugate animation.GIF (55260 bytes)

Vergence

disjunctive animation.gif (19934 bytes)

Vergence movements result in pupils which move together or apart:

  • Interpupillary distance decreases:  congergence
  • Interpupillary distance increases:  divergence

 

 

 

Holding eye movements   (sitting still)

Catching eye movements  (saccades)

Sustaining eye movements (microsaccades)

Compared with either the "holding" or "catching" movements discussed above, the defining characteristic of "sustaining" eye movements is their amplitude:  they are extremely small.  As illustrated by inspecting the figure below, normal observers generally make, even while fixating a stationary object, small eye movements of which they are unaware, which are a few minutes of arc in extent.  Foveal cones are spaced approximately 0.5 minutes apart, so that these eye movements impose a temporal pattern on the photoreceptor's output.  Indeed, it turns out that vision is entirely dependent upon the small-scale irregular motion of the retinal image which is produced by these movements.  Hence, they actually "sustain" visual perception. 

micromovement grid.JPG (10703 bytes)

To demonstrate these micromovements of fixation, first fixate the center of the large white dot, then after sufficient time to generate a good afterimage (10 sec), fixate the small black dot.  Small displacememts of the afterimage relative to the figure are then made strikingly obvious -- you should be able to see the slow drifting movements as well as the corrective mcrosaccades.

 

troxler fading.JPG (36584 bytes)

color fill-in.JPG (32230 bytes)

To demonstrate what these micromovements of fixation normally accomplish, fixate on the crosshairs at the center of the figure above.  At first the reddish ring will be quite visible, but as you continue to hold your gaze steady on the crosshairs, you will notice that the ring disappears.  Because the edges are blurry, and because the reddish and greenish colors look the same to the more sensitive magnocellular (colorblind) cells, the ring becomes stationary enough on the retina to undergo Troxler fading.  All visual sensation would similarly disappear during prolonged steady gaze were it not for these micromovements.

This is a related demonstration.   Stare at the center of the reddish disk.  When the blurry border separating the reddish and greenish fields disappears due to Troxler fading, the greenish color proceeds to "fill-in" the formerly reddish region, until a small eye movement restores the visibility of the blurry edge.  This fill-in has been proposed to account for our lack of awareness of the blind spot.

 

 

eye-head.JPG (16678 bytes)

Head and eye movements made in looking at objects.  (a)  A saccade made in fixating an eccentric object with head fixed.  (b)  Head (h) and eye (e) movements made under exactly the same conditions, but with the head free.  Trace labelled (h+e) is the sum of the head and eye movements.


Tracking Eye Movements: Optokinetic Nystagmus and Smooth Pursuit

Tracking eye movements are those which are made under visual guidance for the purpose of holding an object of regard and/or following it when it moves.  They are rather less reflexive than vestibulo-ocular movements, for they are capable of being modified by acts of will.  However, few subjects can generate them in the absence of an appropriate stimulus, and so they are rather less voluntary than saccades or vergence eye movements.

Optokinetic Nystagmus (OKN):  Easiest to demonstrate by viewing a rotating, striped drum that fills a substantial portion of the visual field.  For moderate rotational velocities, the subject's eyes will follow a particular stripe as it moves (slow phase).  As the gaze is carried further from the primary position, a quick anticompensatory flick is made to bring the point of fixation back onto some new feature of the drum (fast phase).

OKN animation.GIF (13529 bytes)

Sinewave animation.GIF (52182 bytes)

Optokinetic Nystagmus

OKN depends upon subject's attitude, and instructions: "looking" and "gazing" at a rotating striped drum produce different kinds of OKN.  "Looking" instructions produce less frequent movements of larger amplitude; "gazing" instructions produce more frequent, lower amplitude eye movements (see figure below).  The slow phase of OKN can follow stimulus velocities up to approximately 100o/sec, beyond which the system breaks down, and inappropriate eye movements, or none at all, are made to moving targets. 

 

Smooth Pursuit:  Voluntary smooth eye movements which track the location of objects of interest.  Although voluntary, smooth pursuit requires a stimulus to track; they cannot be executed in the absence of some environmental stimulus (try it with a partner if you don't believe me).

smooth pursuit animation.GIF (45824 bytes)

Smooth Pursuit


Saccadic Eye Movements

saccade animation.GIF (18827 bytes)

Saccades

Saccades are the voluntary movements of the eyes which serve to bring a new part of the visual field into the foveal region.  While essentially voluntary, they share many characteristics with the quick phases of vestibular and optokinetic nystagmus and microsaccades. Saccades are stereotyped movements.  They are so fast that there is little time for visual guidance: saccadic eye movements are therefore said to be ballistic.   The figure below illustrates a number of saccades of different amplitudes.  A notable feature of such records is the velocities attained by the eye during large amplitude saccades:  in excess of 400o/sec.

saccades.JPG (26258 bytes)

Acceleration versus time for saccades of various extents.  The initial phase of the saccadic eye movement is independent of amplitude.

To execute a saccade, the visual system must convert retinal distance (for example, between current position, and a desired new position) into signals controlling the extraocular muscles.   The complexity of these calculations is reflected in the rather long latencies associated with the initiation of saccadic eye movements.  A typical stimulus arrangement is for the subject to fixate a small light.  This light is extinguished while another light, at a variable distance from the first, is turned on:  the saccade the subject makes to the new light typically does not begin before 200 ms. 

During saccades, visual sensitivity is suppressed (see figure below), though it is easily demonstrated that it is not completely eliminated.  In general, however, one cannot "see" ones own saccadic eye movements in a mirror. 

saccadic supression.JPG (29411 bytes)

Visibility of a test stimulus at various times before and after the execution of a saccadic eye movement.  

Saccadic eye movements can reveal global aspects of perception, such as the scan patterns and fixation locations of subjects inspecting human faces (see figure below).

Yarbus.JPG (31846 bytes)

Eye movements (saccades and fixations) were recorded while subject was inspecting face stimulus.  Note pattern of fixations along outline of face, and especially eyes.


 

 

 

Miniature Eye Movements

In humans there are three different types of miniature eye movements:  tremor, drift, and microsaccades.  The effect of these miniature movements is to move the retinal image about, over the fovea, in a pattern considerably larger than a foveal cone photoreceptor (see figure below).

micromovements.JPG (18805 bytes)

Simultaneous rcordings of micromovements in two eyes.  The small amplitude, high frequency component is tremor.   The larger, slower movements are drift.  At the arrow both eyes execute a microsaccade.  Tremor is independent in both eyes, microsaccades are conjugate.

Tremor:  The amplitude of tremor is smallest, about 5-10 seconds of arc. Binocular measures of tremor show that it is uncorrelated between the two eyes, suggesting a peripheral origin.

Drift:  Drift movements are relatively large and slow, possessing velocities of 1 min/sec, and median amplitudes of around 2-5 minutes.  Each drift movement is terminated by a microsaccade, and drift movements appear to be uncorrelated between the eyes.

Microsaccades:  Microsaccades serve the same function as their larger cousins, ordinary gross saccades, which is to bring a visual target into register with the fovea.   They probably share the same control mechanism as saccades.  Microsaccades are primarily corrective in nature.

 

 

 
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Last modified: December 09, 2001