---

rds_cd_mod

---

Summary

This stimulus is a dynamic sparse random dot stereogram (RDS) that modulates in depth using changing disparity (CD) cues without frontoparallel (FP) motion cues. The dots lie on either one or two disparity planes and the disparity of the plane is randomly moved toward or away from the observer over time.

Currently, the range of disparities is constrained to be centered around zero.

One each video frame, a new set of random dots is created, thus there is no intentional coherent motion of dots, and thus no intentional IOVD cues.

Examples

• rds_cd_bip.stm - 2016 Jun 3 - Movie (.mov) - Bipolar dots.
• rds_cd_bip_sz4.stm - 2016 Jun 3 - Movie (.mov) - Bipolar dots, dot size 4 pixels.
• rds_cd_bip_2p.stm - 2016 Jun 3 - Movie (.mov) - Bipolar dots, two depth planes.
• rds_cd_bip_2p_env.stm - 2016 Jun 3 - Movie (.mov) - Bipolar dots, two depth planes, triangular contrast envelope.

Imperfections

The stimulus is created by generating a random dot movie for the left eye (LE), and then copying and shifting the dots for placement in the right eye (RE). The amount of shift (disparity) varies over video frames (time). Thus, the shifted dots in the RE may fall outside the field of view, lowering the density of dots on either the left or right border of the RE image. This density artifact on the border has not been compensated for. This will not influence models that only process the center part of the stimulus. This imperfection would not be difficult to fix.

Parameters

• n_planes - 1-one plane of dots, 2-two planes of dots. If there are two planes of dots, then the first plane begins at zero disparity and the other begins at maximum disparity. Both planes move in the same direction (toward or away) on each time step, wrapping around at the max_shift boundaries.

• 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 the plane of dots nears the wrap-around boundaries. This may reduce the visibility of the sudden change in depth when the dot plane jumps (wraps-around) between the front and back depth limits.

• max_shift - (deg) As the dot plane move in depth, the dots shift their disparity between the left and right stimulus images. This parameter determines the maximal shift (disparity between the images) allowed before the dot is wrapped around.

• 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.

• size - future.

• aptype - [0] - future.

• anticorr - [0] - future.

---