The stereograms were created (Mathematica; Wolfram Research, Champaign, IL) and presented on computer in a slide show program (PowerPoint; Microsoft, Redmond WA). The hemistereogram intended for the right eye was projected via the red output channel of the computer, which drove the green primary of one video projector (DLA 2000; JVC, Wayne, NJ) and was projected via a right-handed circular polarizer onto a rear projection screen (ST-professional-W; ScreenTech, Myaree, Western Australia), which preserved polarization (crosstalk, 3.5%). The hemistereogram intended for the left eye was projected via the green output channel, which drove the green primary of a second, identical projector and was projected via a left-handed polarizer. The subject viewed the stereograms via a pair of pediatric spectacle frames, which were fitted with a right-handed circular polarizer over the right eye and a left-handed circular polarizer over the left eye. The use of circular polarizers ensured that the separation of the hemistereograms to the two eyes did not depend critically on the subject’s head remaining perfectly upright. The space-average luminance of the display presented to each eye was 35 cd/m2, as viewed through the polarizing glasses.
The stereogram textures were created from 440 × 440-pixel squares where each pixel was randomly assigned an initial value of either 1 or 0. The random pixels were filtered with a circular, symmetrical, tapered Bessell kernel with a peak spatial frequency of 0.95 cyc/deg. The filtered image was then thresholded at its median value, with half of its area being assigned to white and half to black, creating a stimulus with 100% Michelson contrast. The resultant texture is illustrated in
Figure 1 , and its spatial frequency spectrum is the unfiltered spectrum in
Figure 2 .
The large, square, textured stereograms appeared on the right and left sides of the screen
(Fig. 1A) . The test and distracter stimuli were smaller square regions (11.5° visual angle [VA] on each side), one within each of the larger patches of texture. Within the square region on the right or left of the screen, the texture presented to one eye was shifted to the right or to the left relative to the texture presented to the other eye, to define a test stimulus using crossed or uncrossed horizontal binocular disparity. Within the square region on the opposite side of the screen, the texture presented to one eye was shifted vertically by an equal amount, to define a distracter stimulus using vertical binocular disparity. There were texture discontinuities at the boundaries of the test and distracter stimuli so the binocular disparities could be small compared to the gauge of the texture. Thus, the boundaries and the horizontal binocular disparity portrayed the square test stimulus as either a card suspended in space in front of the base plane of the stimulus, or as a square aperture opening like a window onto a plane appearing behind the base plane of the stimulus. The boundaries were adjusted in position to portray the card or the window correctly. When we viewed the stimulus without the stereo glasses, we could not readily determine which side of the stimulus contained the horizontal binocular disparity; although, when the disparity was small, we could see it by scrutinizing the edges of the texture elements. In a control experiment, a few infants were tested with a “flat” distracter stimulus with the boundaries of the test stimulus in place, but with a vertical disparity of zero.
Between stimulus presentations, a stimulus consisting of two “flat” distracter stimuli was presented. No fixation point was used, but the stimuli were large enough and at this age the fovea is immature enough that we presumed that visual performance was largely mediated via the extrafoveal retina.
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