Biomedical Engineering Reference
In-Depth Information
10.2.2.4.1 Methods
Saccades are tested with the same parameters used for pursuit: the possibility of per-
forming them without involving the head, the quality of the metrics, the automaticity,
the consistency of the performance over time, and the integration with a hand movement.
Saccades can be reflected, induced by changes in the peripheral visual space, or voluntary.
During the evaluation, the presence of the reflex skill is first tested using single stimuli
presented behind masking grids as well as stimuli with alternating illumination or stimuli
in tachistoscopic presentation on a computer.
10.2.2.5 Binocular Vision
During fixations, pursuits, and saccades, both eyes must point at the same object. In this
way, the images, slightly different from eye to eye, are mixed together into one visual
perception, providing for a tridimensional perception of space (stereopsis). Any alignment
error prevents the development of such skill. The absence of eye alignment is called het-
erotropy or strabismus, which can be triggered by various causes. Muscular paralysis,
innervation deficiencies, structural alteration of the orbit, and focus deficiencies can trig-
ger various kinds of strabismus of various levels. Strabismus implies a fixating eye, except
in the cases in which there is an alternating fixation between the right and left eye; in this
case, the functional tests on fixation and movement must be performed in the function of
the fixating eye.
There are forms of inaccurate binocular alignment that can be kept under control by the
central system using a fusional process. These kinds of latent strabismus, the correct term
for which is heterophoria or phoria, do not manifest themselves with the deviation of an
eye, if not in particular stress/decompensation, usually because of weariness. However,
even if not revealed, heterophorias produce effects on posture, effectiveness, correctness,
and the rapidity of other visual skills.
10.2.2.5.1 Methods
A preliminary test is performed on steroacuity. To carry out such a test, both a stereotest and
the Titmus can be used; in this case polarizing glasses are required (Figure 10.15), as with
tables such as Lang, which do not require any glasses. As in younger children, it is possible
to obtain a direct verbal response instead of a motor response with attempts at grasping.
Detecting a stereoacuity informs us of the presence of a sufficient binocular integration
and of an accurate perception of depth, especially at a proximal distance. With cases of
strabismus it is important to detect the fixating eye, even observing corneal reflexes. It is
also important that the stability of the fixating eye is tested in different gaze positions and
width of the eye movement.
As far as the phorias are concerned, they are physiologically present in all of the population
at a very slight level. High levels of phorias can produce rapid weariness, especially when
working at close distances, and if not appropriately compensated for they can manifest them-
selves into heterotropias. The methods that allow for the assessment of phorias are varied and
can also be used with noncooperative patients (e.g., the covertest) or to obtain more accurate
minimal values the tests often require a high level of cooperation from the patient.
10.2.2.6 Convergence
Convergence is the simultaneous movement of both eyes when objects at a close distance
are observed (Figure 10.3). This movement is also triggered to maintain single binocular
