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Heimo Steffen, Mark Walker, David S. Zee; Changes in Listing’s Plane after Sustained Vertical Fusion. Invest. Ophthalmol. Vis. Sci. 2002;43(3):668-672.
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purpose. To determine whether prolonged fusion of an imposed vertical disparity
leads to a change in the orientation of Listing’s plane, even when
measured during monocular viewing.
methods. Four normal subjects (age range, 24–37 years) wore Fresnel prisms of
increasing power for 72 hours to produce a final left-over-right
disparity (range, 7–11 prism diopters [∼3.9–6.2°]) that was
still fusible. Eye movements were measured binocularly, using
three-axis search coils, as subjects fixed on an array of
light-emitting diodes (LEDs) arranged on a flat screen, 124 cm away. A
regression was used to fit the data points to a plane (Listing’s
plane) during monocular and binocular viewing. From each planar fit,
the horizontal and vertical components of primary position (the
direction of gaze that is perpendicular to Listing’s plane) were
calculated. Baseline data were collected in the unadapted state, either
just before or at least 4 days after wearing the prisms.
results. After the period of viewing through the prisms, there was a change in
vertical phoria (prism adaptation) ranging from 1.6° to 3.3°. There
was a significant (P < 0.01) shift of the relative
orientation of the vertical component of primary position between the
two eyes of 6.3 ± 1.7° (right eye value minus left eye, up
being positive, each measured during monocular viewing). There was no
consistent pattern of change in the horizontal component of primary
conclusions. Prolonged fusion of a vertical disparity is associated with a change in
the orientation of Listing’s plane that persists under monocular
viewing. Possible mechanisms include phoria adaptation, the prolonged
fusional effort itself, and the residual disparity that must be
overcome by sensory mechanisms.
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