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rds_mid_mod

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Summary

This stimulus is a sparse random dot stereogram (RDS) that approximates motion in depth (MID) and that can be modulated randomly (or moved smoothly) along the toward-away axis.

The stimulus, when fused binocularly, appears to be a field of dots with random placement in x, y and z (depth). The dots move coherently in depth (z) over time. When dots move too far or too close, based on a disparity limit, they disappear and eventually wrap around (between near and far).

The dots in the right eye can be made to have opposite sign compared to those in the left eye, or to be spatially unrelated to those in the left eye (see the dotcorr parameter below).

Currently, the random dot field is centered at zero disparity.

Examples

• rds_3dm_bip.stm - 2016 May 26 - Movie (.mov) - Bipolar dots.
• rds_3dm_uni.stm - 2016 May 26 - Movie (.mov) - Unipolar dots.
• rds_3dm_bip_anti.stm - 2016 Jun 3 - Movie (.mov) - Bipolar dots, anti-correlated luminance.
• rds_3dm_bip_unc.stm - 2016 Jun 3 - Movie (.mov) - Bipolar dots, spatially uncorrelated dot positions across eyes.

Parameters

• depth_envelope - [0,1,2,3,4] 0-square, 1-tent (triangle), 2-sqrt(tent), 3-cosine, 4-circle. When this parameter is set to a non-zero value, the contrast of the dots is reduced when dots near the wrap-around boundaries. This prevents the twinkling appearance when dots enter and leave the visible field. See Figure 1 (right panel).

• max_shift - (deg) As dots move in depth, they shift their position in the left and right stimulus images. This parameter determines the maximal shift (disparity between the images) allowed before the dot is rendered invisible. See Figure 1.

• dotsize - (pix) Dots are rendered as Gaussians to allow sub-pixel movements. This parameter specifies the SD of the Gaussian.

• dotdens - (dots/deg²/frame) Dot density. This parameter can be used to change the number of dots on the display.

• bipolar - [0,1] If set to 1, the dots should be white and black on a gray background. If set to 0, dots are unipolar, e.g., white on black.

• dot_seed - This integer randomization seed determines the (x,y) positions of the dots.

• distrib - [7] 7-m-sequence. Only one type of random sequence has been implemented: the m-sequence. This is a controlled pseudorandom binary sequence.

• mseq_ord - [8-16] The order of the m-sequence. An m-sequece of order n has 2ⁿ random values.

• seed - This is the starting tap register value for the m-sequence; it determines the particular sequence of binary values that will be used to modulate the motion of the stimulus. Valid values are listed in Wyeth Bair's rand.h C source code header file, e.g., "sstap11" lists forty values for an order-11 m-sequence.

• step_pref - [1.0] This determines the relative step size of motion toward the observer. Typically this remains at 1.0, but if it is changed to -1.0, then the stimulus will always move in the anti-preferred direction (assuming that step_anti is also -1.0). In fact, this parameter was added for the purpose of over-riding the random walk and allowing the stimulus to move smoothly. However, if the value was set to 0.5, then presumably the stimulus would move more slowly in the preferred direction than in the anti-preferred direction.

• step_null - [-1.0] This determines the relative step size of motion away from the observer. Typically this remains at -1.0, but if it is changed to 1.0, then the stimulus will always move in the preferred direction (assuming that step_pref is also 1.0).

• dt - [1,2,3,...] This integer parameter determines the number of frames over which each random motion step will be applied. For example, 1 indicates that the stimulus may change direction with every frame, whereas 5 indicates that the stimulus will move smoothly for 5 frames before it may change direction. The value 5 is often used when the time scale (tscale in most model files) is set to 0.002 (2 ms), because 5 x 2 ms = 10 ms, which approximates (very roughly) stimuli that are shown on 100 Hz monitors.

• speed - (deg/s) This floating point parameter determines how fast the dots will move in depth.

• ampl - [0..1] This determines the amplitude (luminance) of the dots. For white dots on a black background, set this to 1.0.

• bgval - This determines the luminance of the background. In a [0..1] range, 0 is black, and 0.5 is gray.

• dotcorr - [1] 1: dots are correlated across the eyes (their positions and amplitudes are matched); 0: dots have uncorrelated spatial positions across the two eyes; -1: dots have opposite sign (white vs. black) but have spatially matched positions across the two eyes.

• size - future.

• aptype - [0] - future.

Figures

FIGURE 1. Conceptual model underlying the generation of images for the left and right eyes. A fixed number of dots (black circles) are descibed by an (x,y,z) position, where z is depth. The dots may change their z-coordinate according to a random walk. The left and right stimulus images are created by drawing the dots with a horizontal (x) offset determined by their depth (z). The diagram shows the positions of four example dots in the left (green) and right (red) stimulus images.

The inset plot at the right shows the contrast of dots as a function of z for three example envelope shapes: square (black), tent (red) and circle (green).

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