Technical Write-up
When a pair of Bagolini lenses are mounted so
that you can rotate them, it is then possible in addition to the known
examination methods to produce and test the perception of stereopsis.
The Bagolini engages in each case at the 450 and 900 settings. The stops are
marked in red on the rotating parts and in white on the rim of the frame. The
two white dots on the striated lenses indicate to the examiner the axle
direction of the striations in the lens. Two additional white dots on the rims
indicate 100 upward and downward deviation from the zero position. The three
dimensional impression arises on rotating the streaks slightly outwards in the
region of 10 to 80. The normal sighted person now no longer has the impression
that the streaks cross. The small tilt on either side of the vertical creates
lateral disparity, which gives rise to sensory fusion of the streaks in space.
The person with uncorrected or corrected normal binocular vision has the
impression that a beam is lifted towards or away from him in space. When on end
of the image is fixated the other end appears in physiological DIPLOPIA. During
the examination observe, particularly in the case of children, whether a
spontaneous reaction follows. Children often try to reach for the beam. When the
stimulus light source is at a near examination distance, both young and adult
patients may be asked to run their fingers along the beam thus saving time
necessary for the explanation of this unusual effect. However one should take
care the hand or arm does not obstruct the passage of light from the source to
either striated lens. In order to double check the response the spectacles
should be reversed round their horizontal axes, when the spatial position of the
beam should also be seen to reverse. Another possible cross-check is to observe
the eyes which should show increased convergence when the nearest points of the
beam are fixated.
Furthermore, to the person who possesses a normal binocular response, the fused
stereoscopic precept appears tilted in his median plane provided the streaks are
rotated outwards or inwards by the same amount from the vertical. A patient who
suppresses sees the beam inclined to the right or left due to the left or right
images being suppressed respectively. The stereo experience is reinforced if the
beams observed from the end to the end. At near the whole extent of the
stereoscopic tilted beam will usually appear fused, whereas at distance the
lines can no longer be fused on fixation of their extremities. Therefore at
distance only the near end of the beam can be expected to be seen singly by the
patient.
The inclination of the beam in space is maintained, by a person who possesses
normal binocular vision, up to a turn of about 200-s of the Bagolini's against
each other. The Bagolini Lenses are not only suitable for the examination of
simultaneous perception of vertical, horizontal and inclined presentation of the
streaks but also for the assessment of stereopsis and stereoscopic fusion. In
order to measure the amplitude of fusion the patient should fixate the light,
while the angle the two streaks is increased until fusion is broken and the
streaks appear double. As a rule the lines are perceived in stereopsis and
without physiological diplopia up to an angle of inclination of the streaks of
60 both in crossed and uncrossed field disparity.
The increased separation of the two streaks at their extremities can still be
fused due to the increased size of Panum's fusional areas at the periphery.
Broader functions can be covered with this stereo Bagolini test than with the
customary stereo tests. Retinal zones further away from the fovea are simulated
than, for instance, with the TNO-, Randot or fly test, moreover no demands are
placed on resolving power.
Some patients indicated binocular vision with the Stereo Bagolini lenses, while
they showed suppression with their usual glasses. Observations on the polar
projector also point in this direction: for normal sighted persons too, the
stereo sensation is much more constant than the simultaneous perception of
monocular images. The three dimensional impression for concentric circles
produced by lateral disparity remains always present, while the simultaneous
controls of the two diagnosis are subject to constant alternate suppression due
to natural rivalry.
It is thus possible with amblyopes and with patients whose visual axes are
almost parallel, after prismatic correction or with harmonious anomalous
correspondence, to verify the unstable, low grade binocular vision with the
Stereo Bagolini test, owing to the reduced tendency of suppression when this
test is used.
Special reprint: Dr. Bagolini, Examination with the striated lenses.
Bagolini Striated Lens
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