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1 From the Departments of Neurology and 2 Ophthalmology, The Johns Hopkins Hospital, Pathology, Baltimore, Maryland.
PURPOSE. To determine whether prolonged fusion of an imposed vertical disparity leads to a change in the orientation of Listings plane, even when measured during monocular viewing.
METHODS. Four normal subjects (age range, 2437 years) wore Fresnel prisms of
increasing power for 72 hours to produce a final left-over-right
disparity (range, 711 prism diopters [
3.96.2°]) 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 (Listings
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 Listings 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 position.
CONCLUSIONS. Prolonged fusion of a vertical disparity is associated with a change in the orientation of Listings 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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