For several years now I've taken part, as a guinea pig, in some Cambridge University research in the Department of Speech, Language and the Brain. It involves doing occasional audio and visual tests on a PC and the occasional MRI scan of my brain.
Today I did a 2.5 hour long test that included a fiendish test called an N-back test where you have to say if the letter on the screen is the same as the one before, or in further tests the one before the one before, or the one before the one before.
Now, whereas I could do almost every other test they gave me without difficulty, this one I found almost impossibly hard.
"Meta-analysis of 24 n-back neuroimaging studies have shown that during this exercise the following brain regions are consistently activated: lateral premotor cortex; dorsal cingulate and medial premotor cortex; dorsolateral and ventrolateral prefrontal cortex; frontal poles; and medial and lateral posterior parietal cortex." ...according to Wikipedia.
Now I am unsure what a poor performance in this test tells the researchers, but I hope my contribution helped.