Oculomotor Functions

Vision is often thought to be a solely sensory function. Normal use of vision requires exact eye movements. When learning to read we especially need to have the ability to plan and execute small, accurate fast eye movements (saccades) in order to move our gaze from one word to the next. Therefore, motor functions are usually evaluated at the beginning of the functional assessment.

First, smooth pursuit movements are observed, i.e. we move an object slowly in front of the eyes and observe whether the child can move his/her eyes smoothly to follow the target horizontally, vertically, diagonally and circularly or whether the child uses compensatory head movements.

Next, we observe the saccadic eye movements, the quick movements from one fixation to the next. Hold two small objects at the eye level of the child on both sides of the midline. Ask the child to look at your nose, then at one of the two objects, back to your nose, then at the other object, repeat a few times and observe whether the quick movements are accurate. Next ask the child to move the gaze from object 1 to object 2 in one quick sweep from side to side and back. Observe, whether the movement is immediate, quick and accurate and whether there is any difficulty in crossing the midline. Some children have a small jerk in the movement at the midline, and some other children close their eyes to cross the midline. If a child has specific difficulty in using the eyes in the midline, (s)he should not be expected to hold reading materials in the midline.

Saccades used during reading are small and need to be exact to allow fluency in reading. When assessing the reading of a child, it is important to remember that there are numerous functions occurring at the same time. While the child is looking at a word reading it, the preattentive visual functions "measure" the distance to the next word and send the information to the centre planning the eye movements. Next the fixation is detached from the present focus, the quick eye movement, saccade, is executed, and fixation and attention land on the next word to be attached to it for reading. While these sensory and motor reading mechanisms are used, short and long term memory are involved in the reading process as are several higher analytic functions related to the content of the text. Problems in reading can be caused by the peripheral visual functions, (i.e. poor quality of the image or loss of visual field, basic motor problems of the eye movements) and/or changes in the numerous higher, cognitive brain functions related to reading.

Low vision causes typical motor problems. For example, when there is a central scotoma, a new area (=preferred retinal locus, PRL) is used for looking at fixation and becomes the new centre of the visual field. Planning of eye movements usually use this new centre as the centre of co-ordinates of the movements, but often there is competition between the anatomical and the new functional centre and the eye movements may be inaccurate requiring corrective movements.

Fixation outside the fovea or eccentric fixation should not be confused with "dragged macula". When scar tissue on the temporal side of the retina pulls the macula temporally, the fovea is displaced from its normal location at the optical axis of the eye. Then there seems to be eccentric fixation although the person uses his/her fovea to look at.

Sometimes fixation seems to be far from the fovea although it is foveal. In this case, the fovea of the right eye is dragged by scar tissue toward the temporal periphery so that the eye seems to look way past you. The fixation is at the camera. The left eye is blind. The most common cause of this retinal lesion is retinopathy of prematurity (ROP).

Nystagmus is a common motor problem for visually impaired people. While a normal eye has only microscopic movements, micronystagmus, during fixation, an eye with nystagmus has visible, constant or intermittent movements. These can be horizontal, vertical or rotatory. Quite often the child learns to bring the amplitude of the movement to minimum by turning the head to a "null point" or by using convergence (= turning eyes inwards for looking at close distance). The amplitude of the nystagmus may be decreased with base-in-prisms and can decrease with age and with training.

If there are small areas of missing vision (scotomas) in the central field to the right of the fixation point, one of them may happen to be located at a point where the next word to be read, begins (in cultures where we read from left to right). Since the beginning of the word disappears in the scotoma, the saccadic movement to the next word does not match the distance and makes the fixation to land on a part of the word further to the right (see the slide in the introductory lecture). This causes confusion, corrective movements and thus decreases reading speed.

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