Biomedical Engineering Reference
In-Depth Information
2.2
Fusion of Binocular Vision - Normal 3D Vision
Fusion is the neuropsychic mechanism that normal binocular vision is based on. It is
prerequisite for normal 3D binocular vision. All disorders of the binocular functions
disrupt this mechanism. The response is fusion adapting to the sub-normal mech-
anism (abnormal retinal correspondence) or it decays (suppression). This innate
ability of fusion is the base for pleoptic and orthoptic therapy. However, an objective
assessment of the efficacy of fusion is almost impossible in clinical conditions.
Dichoptic masking [
24
] is a complicated suppression process that removes a
monocular object with poor contrast from the binocular processing. A monocular
stimulus with better contrast resolution uses this physiological mechanism to
prevent the perception of a two-sided stimulus with lesser contrast. Contrast rivality
is the basis of the complicated suppression phenomenon. A basic condition for
dichoptic masking is normal binocular vision.
It is assumed that a more robust fusion retains an image with worse resolution
of contrasts in the binocular visual awareness, whilst a weaker fusion mechanism
will suppress this image. This methodology is based on discovering how big the
degradation in contrast of the retinal image must be for dichoptic masking. For
testing the dynamics of the masking mechanism, a variable burden was chosen in
the form of a forced unadaptable relative vergence.
A special computer haploscope was developed comprises two identical 17
00
LCD
monitors placed on two independent brackets. The testing images displayed on
both monitors are viewed separately by the left and right eye through
2.75 D
converging lenses that place the monitors at optical infinity. The person being tested
has their head stabilised in an adjustable headrest. Figure
19
shows the set up of this
computer haploscope.
The image sequence for measuring the dichoptic masking effect was set up from
a number of modified test images with a growing level of degradation in the image
information (see Fig.
20
).
The degradation of the initial image was carried as a low pass filtering of the
image. The degradation of the images was calibrated in line with the fusion cover
test (FCT) with 10
ı
scale (which is clinically applied). The cut-off values for the
frequency filter were set up empirically. Their logarithmic regression with regards
to the FCT scale was confirmation of their correct determination, which allowed
interpolation of the degradation in quarter of degrees. The measuring sequence of
the progressively degraded images according to previous interpolation (by the FCT
scale) was form 1 to 5 degraded by ¼ (1, 1.25, 1.5
:::
4.75) and from 5 to 6 by ½
(5, 5.5, 6). The duration of images was 2 s (0.5 fps). The indicator of the onset of the
dichoptic masking effect (suppression of the degraded image) was the eye deviating
from the forced vergence position to a phoric position. Subjectively this moment
was indicated by removing the red line from the red measuring box (see in Fig.
21
).
The measuring box had a width of 1
of the fixation disparity to the exodevia-
tion, i.e. to the esodeviation and was placed in the image for the dominant eye. The
measuring sequence with the measuring line was presented to the non-dominant eye
(ocular dominance was established using the Dolman method hole-in-the-card).
C
