When you press the "Run" button you will see a salmon colored target disk -- the target disk -- surrounded by six grey disks -- the context. Your task is to control the size of the target disk; that is, keep the size of the target disk as constant as possible: try to keep it about 1/4 inch in diameter. You do this my moving the mouse so as to counteract disturbances to the size of the target disk that are being produced by the computer. This will take a little practice but keep at it until you are able to keep the target disk nearly the same size -- 1/4 inch in diameter -- during a test trial, which lasts about 45 seconds.
At the end of each trial a record of your performance is printed out in the form of graphs and root mean square (RMS) measures of how well you controlled the size of the target disk. RMS measures performance in terms of the deviation of the size of the target disk from its average size during a trial; the smaller the RMS value, the better your control of the size of the disk. Once you have gotten the RMS number in the upper left corner of the print out of the results (the one labeled "RMS Area 1") to be as small as you can get it you can come back and read the rest of this write up to find out what the demonstration shows.
You will have noticed that there were two phases of this demonstration. During the first phase (1) the size of the six context disks was constant; during the second phase (2) the size of these context disks varied from small to large. When you look at the results of the demonstration you should see that your control of the size of the target disk was much better during phase 1 than it was during phase 2 of the demonstration. An example of the results you might have gotten is shown in the figure below :
The values in the upper left corner of the display, labelled “Area RMS 1” and “Area RMS 2”, are measures of your performance in terms of the accuracy of control of the size of the target disk during phases 1 and 2 of the demonstration, respectively. Since the smaller the value of the RMS measure the more accurate the control of the disk, control of the disk was nearly 5 times more accurate in phase 1 than in phase 2 of the demonstration. The reason is clear from looking at the red lines in the graphs.
The red line shows the size of the target disk varying over time. The top graph, labeled “Constant Context”, shows the variation in the size of the target disk during phase 1; the bottom graph, labeled “Variable Context”, shows the variation in the size of the target disk during phase 2. The less the red line varies over time, the better the control of the size of the target disk. Clearly, the red line varies far less during phase 1 then during phase 2. The reason for this is clear from looking at the black line in each graph, which shows the variation in the size of the context disks during each phase. During phase 1 the black line is flat, indicating that the size of the context disks was constant throughout this phase; the only cause of the variation the size of the salmon disk during phase 1 was the computer’s disturbance (indicated by the green line in the graph) and the participant’s mouse movements that were made to compensate for this disturbance. During phase 2, however, the black line varies, indicating that the size of the context disks was varying throughout this phase and the size of the target disk varied in concert with these variations; the target disk got larger when the context disks got larger and smaller when the context disks got smaller. This relationship between the size of the context and target disks would be expected to occur if the participant is controlling the ratio of the size of target disk to the size of the context disks rather than the size of the target disk itself, as instructed.
The fact that the participant is controlling the ratio of the size of the target disk to the size of the context disk is shown by measures of how well this ratio was controlled in each phase of the experiment. These measures are labeled Ratio RMS 1 and Ratio RMS 2. These are measures of the average deviation of the ratio of target to context disk size from the average value of this ratio in phase 1 and phase 2 of the demonstration, respectively. Note that these values are both quite small and nearly the same, indicating that it was, indeed, the ratio of target to context disk size that was being controlled in both phases of the experiment. The fact that the ratio of target to context disk size is the controlled variable is also shown by the fact that the blue lines in the graphs of performance in both phase 1 and phase 2 of the demonstration are nearly perfectly flat, indicating that the ratio is being kept in the intended (reference) state – ¼ inch in diameter – in both phases of the experiment.