Without intending any diversion into brain=computer debate there are some similar mechanisms involved and so a limited analogy might be useful.My knowledge of brain/mind/memory and the like is reasonable considering it's merely an interest but I'm still a hobbyist not an expert.Apparently though, if Microsoft and my clients can be believed I can offer info on computers with a degree of authority. I hope its of interest.
The clock is always running and with each tick an event occurs, no matter what. Even if there's nothing to do or even if the explicit instruction is to no nothing, there is always something that gets done- which is to monitor any change in that instruction. Tell it to be idle for 10 cycles and it can't - it stubbornly works at counting them. The instructions to do stuff come from various sources, which are cycled round to check for new input. These input points are not equally important but prioritised. audio and vision data is dealt with at the brain at the same rate, but due to the different frequency ranges our ears and eyes accept data at different rates. Consequently the eyes sends all its stuff, the ears, being slower sends what its got, and the scan for input goes round. By the time the ears are done the eyes might have been asked for input 5 more times. So, we end up looking out of the window. This fact, commonly taught to speakers, teachers, presenters etc, is why pointers, slides, arm-waving and so on, is used to keep attention.
But, ints not all input. There are background tasks, and often these are fitted in as and when, Sleeping, for example, boredom, or a quiet road at night stopped at red, especially lacking sleep since by then some dream-time tasks are overdue.
These tasks will be automatically attended so long as no other higher priority task is required and we cant' prevent it. We can delay it though, by introducing something that must be done first, such as listen to the radio etc.
My point is that having no stimulus present is not the same as disregard for simulus present.
Or, daydreaming at a stop light is not minor-narcolepsy. One is boredom and the other an affliction.
Although if you are the person in the lime green uno in front of me last night, you really should get to a doctor, cos there's something seriously wrong with you.