Biomedical Engineering Reference
In-Depth Information
FIgUre 10.15
Titmus stereo test.
vision with foveation of both eyes on the same object. Convergence becomes critical
in the actions performed at a proximal distance, such as manipulation, reading, and
writing. Difficulties in convergence, with loss of alignment and subsequent adjustment,
slow down all of the other pursuit and saccadic eye movements. A particularly critical
situation is when there is a rapid request of shifting the fixation from the blackboard to
the exercise topic.
Convergence, activated to maintain binocular vision of close objects, is strictly linked to
adjustment. It is often clinically observed how the two processes can mutually interfere,
sometimes being an obstacle for each other, but also involving mutual strengthening.
10.2.2.6.1 Methods
Convergence movement can be assessed in terms of the most proximal point that can be
singularly seen (Figure 10.16) and in terms of shifting the speed of binocular vision. This
parameter becomes particularly critical when the patient is asked to shift the gaze from
points at different distances in space.
An excessive slow-down will indicate the use of systems for which the stimuli and
instruments are coplanar. No particular expedients are needed to assess convergence. It
is therefore important to create a double testing channel with stimuli requiring accom-
modation, such as a stick presenting writings or images, and stimuli that do not require
accommodation, such as a ball or a toy. This is because accommodation can favor better
convergence efficiency.
10.2.2.7 Accommodation
The optical system of an emmetropic eye, that is to say without optic deficiencies such as
myopia, hypermetropia, and astigmatism, in a relaxed state allows for perfect focus of
the images placed at the optic infinite, approximated at 5 m (Figure 10.2). The rays of light
from any object coming closer to the eye focus on the back of the retina, creating a blurred
image. This problem has been solved for cameras, shifting the lenses in the objective; the
