>The results indicate a correlation, not necessarily causation

Just to drive that home, "Study participants also did not undergo a baseline MRI", which of course could mean that they already had some of these brain changes, which may have driven more TV watching.

>Sedentary activity is correlated with slightly faster degradation of multiple brain measures.

It seems to matter what you do while you're being sedentary, at least according to this study. From the article: "Strikingly, the sedentary element of TV watching didn’t appear to be the main driver for these changes."

Seems that the direction of causation is still an open question. Is TV watching causing smaller brains, or is it the other way around?

Intuitively both seem plausible. More research needed!