Stimuli had a mean luminance of 6.0 cd/m
2, and they were presented within a square aperture (
Display Formula\({28^\circ }\) side) centered on the screen. Outside the aperture the screen was blank at mid luminance. Stimuli consisted of low-pass filtered horizontal 1D random line stimuli. Each stimulus was obtained by randomly assigning either a high or a low luminance value (symmetric around mean luminance) to each consecutive pair of pixel rows (
Display Formula\({0.06^\circ }\)); the resulting stimulus was then low-pass filtered in the Fourier domain (the gain of the filter was zero above 0.75 cpd and one below 0.375 cpd; the transition followed a raised-cosine function). Finally, the root mean square contrast of the stimulus was set to 24% (which kept the Michelson contrast below 100%, thus preventing saturations). We imposed a fixed value of root mean square contrast (as opposed to Michelson contrast) because with noise stimuli root mean square contrast has been shown to be a better indicator of stimulus strength.
11,12 Motion of the stimulus was simulated by shifting either up or down (by an integer number of rows at each frame), a pattern larger than the screen behind the fixed aperture (i.e., the stimulus did not “wrap around”). The drift speed was approximately 50°/s. The stimuli could be presented either to a single eye or to both eyes. During monocular presentations, a mid-luminance blank screen was presented to the other eye (
Fig. 1). During binocular presentations, the two monocular images drifted at the same speed and in the same direction. Three different types of binocular stimuli were used, each characterized by a different interocular correlation. In the first stimulus the two monocular images were identical. This is a binocularly correlated (interocular correlation = 1.0), zero-disparity, stimulus, but for simplicity in the text we indicate it as binocular-same. In the second stimulus, the two monocular images are generated independently. This is a binocularly uncorrelated (interocular correlation = 0.0 on average) stimulus, and we refer to it as binocular-different. In the third stimulus one monocular image is obtained by contrast-reversing the other. This is a binocularly anticorrelated (interocular correlation = −1.0) stimulus, with zero disparity, and we refer to it as binocular-opposite. Comparing responses to binocular-same and binocular-opposite stimuli is particularly interesting, as they are identical, both globally and locally, in terms of spatial frequency content, temporal frequency content, and contrast, and differ only in interocular correlation. With binocular-different stimuli this is true only on average.
We selected horizontal stimuli to avoid having to carefully position the stimulus as a function of each subject tropia, a common concern in studies of binocular function in strabismic participants. In four of five of our participants with stereodeficient vision, the eye misalignment was either very small or largely horizontal. With horizontal gratings drifting vertically, horizontal misalignments do not introduce any disparity (except at the stimulus aperture), ensuring that the interocular correlation of the stimuli was only marginally affected by tropias in our participants. The last stereodeficient participant had a large vertical deviation, making his results open to multiple interpretations.