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TAXONOMY: THE CLASSIFICATION OF EYE MOVEMENTS Key words: saccades, pursuits, vergence, VOR, OKN,
conjugacy
II. Classification of 5 types of Eye Movements: Saccades, Pursuits, Vergence, OKN, VOR
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. 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. |
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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 stablized 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. |
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Review Questions: Name the two types of gaze stabilizing eye movements. |
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