#26 - Global Reference, Must Be True

[Poonam Agrawal]

Hi goingtosomewhere!

You're right in thinking that the areas not engaged in the task would not have a differential rate change - therefore, the fMRI would not show that part of the brain "lit up." However, answer choice (B) says that the areas of the brain not engaged in a task have a rate of oxygen use that is close to zero. This is not necessarily true. It is possible that those areas of the brain still are using oxygen at a rate above zero - all we know is that this rate of oxygen use did not change while doing the task, NOT that the rate itself was zero. Hope this helps!

[itstanaya]

Hi MN,

(C) says that the areas of the brain most metabolically active also show higher oxygen use, at least during certain cognitive tasks. The passage itself implies that this holds true at all times, and rate of oxygen use is essentially synonymous with how metabolically active that portion of the brain is. So if (C) is true all the time, or under all conditions, it would also be true under certain conditions. So (C) remains correct with or without the stipulation about dealing only with periods of performance of certain cognitive tasks.

Hope this helps!

I was in the same boat as the original poster, MN, and I still don't quite understand how (C) is the right answer. Just because one area of the brain shows a higher rate of oxygen use doesn't mean that this area's rate is "higher than the rest of the brain," right?

[Robert Carroll]

Tanaya,

"Most metabolically active" takes care of the problem. The answer choice is not just talking about any old active areas, but the areas that are most active. By definition, the most active areas would be more active than anywhere else. Because metabolic activity is directly tied to oxygen rate, the areas most active would have to have the highest rate of oxygen use.

I think your worry was that a particular area that might be more active when a cognitive task is performed doesn't have to be the MOST active metabolically. So an area could be using higher oxygen than normal, and higher than much, even most, of the brain, but not a higher rate than anything else. As I said in the first paragraph, though, the author is talking only about the areas most metabolically active, so areas of intermediate activity aren't relevant.

Robert Carroll