The dot-defined square moved at one of four levels of speed in depth (6.27, 9.40, 12.54, and 15.67 cm s
−1), which were simulated by horizontal movements of stereo-pair images in opposite directions at four monocular speeds (0.69, 1.04, 1.39 or 1.73 deg s
−1). These disparity values were translated into depth values by the equation:
η ≈
Iδ /
D 2, where
η is the disparity,
I is the interocular separation,
δ is the simulated depth, and
D is the viewing distance. Speed in depth,
V z, was derived from the equation:
V z ≈
D 2 V δ /
I, where
V δ is the rate of change in disparity. Since the distance in depth traversed by the square was held constant (±4.18 cm), the stimulus duration was proportional to image speed. The stimulus durations were 1.33, 0.87, 0.67, or 0.53 s, which corresponded to the range of speed in depth from the slowest to the fastest. Two horizontal bars (1.52° × 0.07°) were continuously presented 3.85° above and below the center of the square to act as landmarks denoting zero disparity. At an intermediate point of every motion sequence, a Gaussian blob was presented in the central region of the square for 33 ms (two frames). The size of the Gaussian blob was held constant (
σ = 22.20 arcmin) (see
Movie 1). The luminance profile of the blob was defined by the equation:
L(
x, y) =
C 0 exp − ((
x −
x 0)
2 /
σ x 2 + (
y −
y 0)
2 /
σ y 2), where
x 0 and
y 0 denote the center position of the square,
x and
y denote the coordinates with respect to
x 0 and
y 0,
σ x and
σ y denote the horizontal and vertical standard deviations of the Gaussian, and
C 0 denotes the peak luminance of the profile, which was set to 132 cd m
−2 (Weber contrast, 340%).