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P. Sampat, H.E. Bedell; Effect of Vertical Fixation Disparity on Stereothresholds . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3140.
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Stereothresholds vary systematically with the magnitude of horizontal fixation disparity, i.e. with an increase in distance from the horopter. We studied how stereothresholds vary with vertical fixation disparity (VFD) by comparing the effect of various vertical vergence demands on VFD and on stereothresholds.
Subjects (N = 2) viewed random dot stimuli (RDSs) through ferro–electric goggles on a 120–Hz frame–rate monitor from a distance of 1m. Each RDS consisted of a 1.1–deg test square centered in a 3.3–deg reference square. Pairs of white outline squares at eccentricities of 2.75 and 5 deg provided a peripheral fusion lock. Crossed or uncrossed horizontal position disparities were produced in each eye’s test square by introducing proportional phase shifts in all spatial frequency components. Vergence demands between 2 pd base up and 2 pd base down were introduced by shifting the stimuli seen by each eye equally in opposite vertical directions. After each 2–sec presentation of the stereotarget, the subject indicated if the test square was nearer or farther than the reference square. Estimated stereothresholds were a change from 50–84% on the psychometric function, fit to data obtained using the method of constant stimuli. To assess VFD, the subject adjusted horizontal nonius lines inside zero–disparity RDSs to appear aligned. Stereothresholds and VFD were estimated during interleaved series of trials for each vertical vergence demand.
Stereothresholds increased by approximately 80% across the range of vergence demands tested. In both subjects, VFD changed by approximately 1 min arc/pd of vertical vergence demand. The average correlation between stereothresholds and the VFD was 0.80.
The elevation of stereothresholds that occurs in the presence of VFD is qualitatively similar to the previously reported increase of stereothresholds with horizontal FD and may be attributable to the difficulty of binocular neurons to match vertically displaced images within their receptive fields.
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