I read this on a news site, "Neuroscience tells us that humans are flooded with over 11 million bits of information per second, yet we only actually process about 50 bits per second on a good day.
This means we filter out most of the incoming information. The Autonomic Nervous System or “ANS” filters out whatever you have been trained to ignore, have not been trained to pay attention to, or don’t find important at the time.
In short, we tend to fixate on those things we think are important at the exclusion of most other things. "

It makes it easy to understand how those of us with dysfunctional 'brain filtering' can become so overwhelmed. When our brain fixates on a single issue, it also can become overwhelming, like the various ways we can detect our own heart beat.