As Neuroskeptic points out, evidence from brain damage supports the idea that everything the mind does involves corresponding brain activity. Therefore, we should expect, at some level, that everything we think about, do, or perceive will change the brain in some way. There are a lot of philosophical positions you could take that are compatible with this notion--that "the mind is what the brain does," that "the mind is the brain," that "the brain causes the mind to do stuff," that "the mind and the brain just happen to perfectly parallel each other," and so on. Regardless, if something is happening in the mind, then something must be happening in the brain, too. Therefore, the mere existence of a change in the brain isn't necessarily headline news. Neuroskeptic has a great example of trivial brain changes we experience every day:
"Every time you open your eyes, for example, widespread changes in your brain activity result. But every time you close your eyes, these changes are reversed."
Above: "Sleep changes the brain?! I'd better not sleep any more, then!"
So if some brain changes are trivial, how are we supposed to evaluate whether a change reported in a news story should matter to us?
It may not always be possible to tell, given either the limitations of the study or the article reporting on it. But here are some questions one can ask:
1) At what time scale does the brain change?
Brain plasticity acts at two major time scales. One, on the order of milliseconds, involves changes in the pattern of neural firing, and the biochemical processes that affect it--neurotransmitter release and the like. Different neurons in your visual cortex fire when you look at a perfectly vertical line versus when you look at a perfectly horizontal line, for example. Different assemblies of neurons will fire when you look at (or think about) a cat and when you look at (or think about) a dog; or when you smell Lysol vs. your grandmother's cookies. And each time you think about something new, the pattern of neurons that fires changes again. These changes are so fast that fMRI is too slow to catch them. On the other hand, over years, memories form and solidify through changes in the strength and structure of synapses.
2) How long does the change last?
Brain plasticity at the millisecond scale lasts as long as the thought or behavior it signifies. Long term potentiation--the brain changes that enable memory formation--can potentially last as long as an individual lives.
3) What sort of change is it? How big is it?
The birth of new neurons and the creation of new synapses is a big deal. For example, a major factor in developmental changes is a proliferation of synapses in a particular brain region, followed by a pruning of the synapses that turn out not to be useful. Some regions, such as primary visual cortex at the very back of the brain, both proliferate and prune earlier than others, such as the frontal lobe, and this has real behavioral consequences. Long-term changes in the pattern of short-range and long-range white matter connections in the brain also seem important, as do changes in the functional communication between brain regions that they enable. Look for evidence of large, long-lasting, long-time-scale changes of this sort.
4) How and why does the change happen?
This is really a question about the mechanisms of the brain change, which, for a neuroscientist, means understanding its effects at all levels, from the molecular up to the whole brain level. But a layperson can approach the question in a less technical way. Suppose the brain change is caused by a drug or therapy. Do these changes simply reflect the transient action of the drug or therapy, and end once the patient is no longer in treatment? Or does the drug or therapy change the brain in some sort of meaningful way that persists even when the patient is no longer receiving treatment? For example, people helped by cognitive-behavioral therapy for anxiety may develop habits of examining and correcting their thinking that become so effective that they no longer needs the therapy, because they have been trained to provide their own. One might expect to see a long-lasting, long-time scale brain change of one of the sorts described in Question 3.
5) What is the real-world, behavioral consequence of this brain change? Is there one at all?
As Dorothy Bishop points out, teachers don't really care if a dyslexia intervention changes the structure or function of the brain in some way; they care if it teaches dyslexic children to read faster and more accurately, and if this improvement lasts. A depressed person evaluating possible therapies wants to know if changes in the brain reflect an actual reduction in depression symptoms. In cases like this, brain changes are interesting because they may inform us about how the behavioral change occurs, but the real measure of interest is behavioral, not neurological.
Next time you read a headline about how something changes the brain, keep these questions in mind and ask yourself what sort of brain changes are actually involved. The reality may be less exciting, or terrifying, than it first appears.