it is called a stroop test invented by psychologist john ridley stroop in the 1930s. it sounds simple: all i had to do is identify the color i saw in the screen. >> but you noticed right away how you slow down. >> yeah. i don't know why this... this shouldn't be that hard. why is that? >> that was stroop's big discovery. >> reporter: keilp had two groups of people complete stroop tests while their brains were in an m.r.i. >> lie still and relax, okay? >> reporter: one group was depressed; some had attempted suicide and the other was healthy. he noticed a surprising difference in their brain scans. these are healthy brains doing a stroop test when the color and the word don't match. the red areas denote increased blood flow, and thus brain activity, in the frontal cortex region, the cingulate, which resolves conflicting perceptions and the visual regions as well. now look at the brains of depressed and suicidal people doing the same test. that's very dramatic. keilp says their brains seem inclined to focus on one thing, in this case the word, not the color, and are les