As a special ed teacher, here's my little rant on TV:
It's a receptive activity. The brain sits there and takes it all in. The brain doesn't get a chance to actively participate in an expressive way.
Humans think in pictures. When the pictures are constantly supplied to us, as in TV, we're denied the opportunity to make the pictures in our heads ourselves (as with books). When we don't make pictures inside our head, we don't comprehend well. So, yes, TV dumbs us down, but not in the way that people usually think.
I guess I shouldn't knock TV; it's probably my largest financial supporter. I tutor many kids in strategies that get them visualizing on their own. If they don't visualize, they won't be able to reason abstractly. It's a critical skill for academics and the ubiquity of TV is denying them the development of these skills.
Yes, audiences at performing arts events are using only the receptive centers of their brains. But they're not doing this day after day, night after night (no matter how much we WISH we could!) as with TV-watchers.
In the old days, when folks DID regularly attend performances of various kinds, they were also actively engaging their brains in creative arts. People sang together, people danced together - great ways to feed the expressive centers of our brains. Nowadays people watch TV together.
So we're becoming - pardon the expression - Dumb and Dumber.