Each stimulus pair consists of a reference stimulus (visible only to the healthy eye) and a probe stimulus (visible only to the diseased eye). We ascertained the oculocentric direction of 16 retinal image locations by stimulating, selectively, corresponding retinal points using dichoptic stimulus pairs
(Fig. 1) . For each stimulus pair we measured the angular distance between the reference stimulus and the probe stimulus, the distance being quantified as the numerical sum of the vector components
Ax and
Ay. Subjects were seated at a slit-lamp table, leaning on a headrest and looking straight ahead at a flat white screen at 50 cm distance. A white incandescent background luminance was adjusted to 20 cd/m
2. Test spectacle frames mounted with appropriate lenses were used to correct for refraction and distance. A red filter glass was placed in front of one eye and a green filter glass in front of the other eye. The filters fully reject stimuli of the other color. A red diode laser (Pen-Pointer, 1 mW, 670 nm; Melles-Griot, Carlsbad, CA) and a green HeNe ion laser (05-SGR-810, 0.1 mW, 543.5 nm; Melles-Griot) provided the dichoptic stimuli. The lasers were placed on either side of the subject’s head on ball mounts that allowed full three-axis freedom of movement by manual adjustment. The size of both stimuli was 12 mm
2 and both stimuli subtended a visual angle of 0.46°. The stimulus luminances were adjusted using neutral density filters to correspond subjectively to Goldmann 4 e white, which approximately equals 318 cd/m
2. Since the eye is far more sensitive to the green 543.5 nm light than to the 670 nm red light, absolute photometric luminances were considered irrelevant.
The pattern of stimulation was a rectangular nine-square grid, with a black cross at the center for fixation
(Fig. 1) . The stimuli were applied successively at each of the 16 nodal points of the grid. The nodal points were at 4, 9, and 12° eccentricity. The central fixation target (cross) subtended 0.57° of visual angle and was fixated throughout the procedure. The subject’s response was plotted on a corresponding perimetry chart.
For display of the reference grid and its perceived shape, the perimetry plots were inverted, scaled to match the fundus photograph, and superimposed on the fundus photograph. The centers of the fovea and the optic nerve head were used as reference points for the scaling of the fundus photograph. The blind spot was mapped during the initial phase of the procedure. Normal reference intervals were defined as the 95% CI for the average visuospatial deviation score obtained in healthy subjects. Interocular correspondence was expressed using the sum of absolute vector component values (|Ax| and |Ay|) for the reference and probe deviations, which is defined as the interocular visuospatial deviation. This parameter is an error or deviation score that increases with increasing metamorphopsia. The interocular spatial deviation was expressed as the sum of absolute vector components rather than the vector itself. This measure augments deviations that both have a vertical and horizontal component and diminishes deviations due to one-dimensional eye misalignment (heterophorias and heterotropias).