@serenesurface-- Well, the brain is responding to stimuli all the time, but it's not engaging in an attention task at all times. In fact, it can't. It would be tiring.
When one is at a park just sitting and watching the surroundings, he is not engaging in an attention task. But his brain is still receiving information from stimuli around him and processing it and determining what to focus on.
We can't process everything at once. So we are always prioritizing what we see. The difference with an attention task is that we have to give priority to that task for a considerable period of time. Usually the most we can manage with the attention span that we have. So it's more difficult.