Biomedical Engineering Reference
In-Depth Information
Edelman 2008). In particular, erratic looks and nystagmus can be found. The first case
is characterized by a fluid and continuous movement, not aimed at a particular object
and that never completely blocks. This picture can be found in patients affected by severe
vision impairment. Nystagmus is a continuous alternating of a slow sliding phase and a
rapid saccadic recuperation (Traccis 1992). Nystagmus can have a physiologic origin, as
when we look at the landscape out of the window of a moving train or during vestibu-
lar stimulations, or a pathologic origin. Various forms of nystagmus have been identified
and are differentiated by their metrics, duration, age of onset, and various other causes.
Nystagmus produces a reduction of visual acuity, sometimes rather severe, and a difficulty
in performing all eye movements, such as pursuit eye movements and saccadic move-
ments. Often, but not always, nystagmus can decrease and be reduced to particular gaze
positions. There is no effective therapy against nystagmus; a few optical, surgical, or pos-
tural expedients can be used to limit its effects.
Some subjects cannot keep a stable fixation on a particular point because they cannot
inhibit the orientation reflex that is activated as soon as the peripheral field changes. These
subjects do not show altered metrics, but rather reduced fixation timings, which are usu-
ally mistaken for attention deficiencies.
10.2.2.2.1 Methods
Testing the fixation is apparently the easiest thing to be done. A target is placed in front
of the subject, and the position of the eyes is observed. If the eyes cannot remain focused
on the target, the contrast is deepened, lowering the environmental light and setting light
on the target until an absolute contrast of 100% is reached. If stable fixation still does not
occur, red targets can be used because they can be better perceived by foveal receptors in
comparison with peripheral receptors. In particularly severe cases, the target must be pas-
sively put in front of the eyes, and operators will check whether fixation occurs.
Another element that proved to be of major importance was posture, which may vari-
ably affect the stability of this visual component. Some children with no apparent fixation
ability managed to keep an acceptably stable gaze in the supine position. This shows that
it is often recommended to try the less proficuous ways to solve situations of impasse.
10.2.2.3 Slow Pursuit
Slow pursuit movements are the movements performed in an attempt to maintain fixation
on a moving object. Pursuit movement requires a high level of attention on the task and ade-
quate oculomotor skill (Contreras, Ghajar, Bahar, Suh 2011). These movements are performed
to keep the image of the object impressed on the fovea. In little children, pursuit movements
are aided and integrated with head movement. As the children grow up, they progressively
learn how to only move their eyes and not their head. In situations of retardation in develop-
ment, the head movement is still present. Pursuit movements are the movements that the
eyes use to follow the mouse cursor, for instance, and performing them properly leads to
continuous loss of fixation and consequent saccadic leaps to recuperate.
10.2.2.3.1 Methods
In testing pursuit movement, parameters must be considered such as the ability to perform
them without involving the head, fixation stability, automaticity (i.e., the possibility of
performing a second interfering task), the consistency of the performance over time, and
the integration with hand movement. There are various tests through which the perfor-
mance can be quantified under these parameters. They are also useful to facilitate a rapid
