Random Dot Cinematography
Global Motion Discrimination Threshold
USD Internet Psychology Laboratory

Introduction. The goal of this experiment is to examine the visual systems’ ability to detect global motion using a stimulus called a Random Dot Cinematogram, or RDC. The objective of this global motion detection experiment is to examine how the visual brain organizes and perceives motion. As we will see, our visual cortex can deploy a statistical averaging, or a cooperative process to detect motion. The authors of our text book discuss the neurological basis for the global motion phenomenon as follows: "The key idea is that neurons that respond to similar directions of motion cooperate, amplifying one another’s responses, while neurons that respond to different directions compete, tending to inhibit one another" (Sekuler and Blake, 1994). To truly explore this process, a stimulus like the RDC is required (see Figure 1 below).

Display Description. On the right hand side of the screen you will see a rectangular region containing several buttons. Notice the center button says Start. The four other buttons are to be used to enter your responses (Up, Down, Left, Right). Just below the response buttons, there are two output boxes that display the number of trials completed, as well as the percentage of correlated motion in the RDC for the current trial. To the left, the initial screen will say "Click START button". This is the area where the RDC will be displayed.

Experimental Protocol. The experiment begins when you press the Start button. To the left, you will see a brief, 11-frame animation of moving dots that comprises the RDC. The RDC contains a correlated motion level that is specified in the text box to the lower right. Your job is to respond by pressing one of the four direction buttons depending on which way you think the dots were trending. Typically with correlated motion levels above 50 percent, the answer will be immediately apparent. However, you have the option of repeating any trial before you enter a judgment by pressing the "Repeat the Stimulus" button. Do not be surprised if there are some RDCs that appear to have no detectable trend. When you are not sure, you must enter a guess.

1.  What is the independent variable?
2.  What is the dependent variable?
3.  What psychophysical method was used?
4.  Plot your psychophysical function and interpolate your 75 percent threshold.
5.  Plot the class psychophysical function and interpolate the group 75 percent threshold.
6.  Describe the relationship between motion discrimination and the actual physical stimulus. How does your 75 percent threshold compare to the class threshold?  What does this study tell you about the visual system’s ability to detect global motion?

Figure 1
Anatomy of a Random Dot Cinematogram

End of Global Motion/Random Dot Cinematography Experiment.