GLYPH<145>treatmentGLYPH<146>and go trials called GLYPH<145>control.GLYPH<146> During an experimental session, brain activity was recorded using fMRI at two second intervals. The experiment sought to determine how the brain reacted di⁄erently to go and stop trials, where stop trials call for inhibition of a previously requested motor response from the subject. Figure 1 shows one session for one subject. The vertical grey lines are go trials. The vertical black lines are stop trials. Based on fMRI, Figure 1 shows activity in the subthalamic nucleus (STN). Aron and Poldrack (2006) and Li et al. (2008) suggested that the STN plays an important role in response inhibition. In the lower portion of Figure 1, the STN activity is GLYPH<133>ltered without use of the stop/go distinction. The GLYPH<133>lter is a high-pass GLYPH<133>lter of 128s: it removes slow, low frequency drifts, leaving behind the high frequency ups and downs thought to reGLYPH<135>ect brain activity. The e⁄ects of the GLYPH<133>lter are somewhat visible in Figure 1. For the STN, we analyze unGLYPH<133>ltered and GLYPH<133>ltered data in parallel, obtaining similar conclusions. In e⁄ect, the experiment produces 58 GLYPH<2> 4 = 232 GLYPH<133>gures analogous to Figure 1, one for each subject in each session, and does this for many regions of the brain. The assumption of GLYPH<147>no interference between unitsGLYPH<148> is not at all plausible in Figure 1. A typical session has about a hundred trials or experimental units in about 600 seconds. There is interference if the response of a subject at a given trial is a⁄ected by treatments at other trials. Interference is likely for at least two reasons. First, the brain has a measurable response to a stimulus for many seconds after the stimulus has been withdrawn, so a subject is still responding to one trial when the next trial begins. Second, the response to a stop trial preceded by a long string of go trials may be di⁄erent from the response to a stop trial preceded by another stop trial. In addition, as time goes by, subjects are experiencing the normal responses people have when performing a repetitive task: they become familiar with the task, or bored by the task, or less distracted by the recording equipment and more focused on the task, or distracted by something else, and each of these is a response to their 7