Transcranial direct current stimulation (TDCS) is the application of weak electrical stimulation to the brain.  It has been reported that application of TDCS can help people suffering from depression and other mental disorders.   A neuroscientist hypothesized that TDCS applied to specific regions of the brain could improve performance on cognitive tasks.   She obtained 100 undergraduate psychology majors from her university's subject pool and randomly assigned 50 of them to the experimental group (receiving TDCS) and 50 to the control group (no TDCS).    All subjects were connected to the equipment for delivering the weak current to the brain, however only those in the experimental group received actual stimulation.    During the experimental trials they received 2 milliamps of current applied to the right temporal lobe.

The subjects performed 500 trials of a mental rotation task.   In this task subjects are presented with two stimuli that are rotated images of one another.   By mentally rotating one image it is possible to align the images so that they are identical.  How on some trials, typically one-half, one image is also flipped front to back so that the two images can never be rotated to identical images.    The subject's task is to respond as quickly as possible either same or different by pressing one of two keys.  The amount of time (in milliseconds) it takes the subjects to respond is recorded.

A. What type of research design was used?

B. Was there a manipulated independent variable?   If so, what was it?

C. What was the dependent variable?

D. How were subjects assigned to the treatment conditions?

E. Why was the equipment hooked up to the control subjects if it wasn't being used?