Stimuli were generated using an Apple Mac Pro (Apple, Cupertino, CA) running MATLAB (Mathworks Inc., Natick, MA) and Psychophysics Toolbox 3 (Brainard,
1997; Kleiner, Brainard, Pelli, Ingling et al.,
2007; Pelli,
1997). All stimulus elements were antialiased to achieve subpixel resolution. The motion stimuli were presented within one of two circular apertures with a solid background at mean luminance (8° diameter) directly above and below the fixation point (
Figure 1a). Each motion stimulus consisted of 16 black and white dots (8-pixel diameter) presented in a 4° diameter region centered on the aperture. The dots were randomly distributed and moved laterally in opposite directions for each eye at the following speeds: 0.14°, 0.27°, 0.55°, 1.10°, 2.19°, 3.29°, 4.39°, 6.58°, 8.78°, and 13.16°/s. Faster speeds initially were included but were removed from analysis because of potential motion artifacts at the monocular frame rate (60 Hz/eye). 3D motion direction sensitivity was measured in three conditions, each at a different pedestal: middle, near, and far. In each condition the binocular disparity range was limited, creating a perception of dots continuously wrapping through a cylindrical volume (
Figure 1b). Individual dots that reached the limit of the stimulus volume were extinguished and redrawn in a random position at the opposite end of the volume. In the middle condition, this volume contained disparities ±0.55° from the fixation plane. In the near and far conditions, this volume was placed on a disparity pedestal and contained disparities between 0° and 1.10° either crossed or uncrossed, respectively. Thus, in the middle condition the volume appeared to straddle the display plane, and in the near and far conditions the entire volume was in front of or behind the display, respectively. Lateral motion sensitivity was measured by covering the left eye and presenting the middle stimulus.