For animals to survive in nature—for predators to spot prey, or prey to avoid predators—they must be able to focus intently on well-camouflaged targets, screening out extraneous information. Because of this, animal brains automatically develop "search images," brain templates that help them zoom in on crucial survival information and ignore everything else.
You use search images, too. When you and a friend become separated in the mall, you scan the crowd, not scrutinizing every face but quickly filtering out everyone who's too tall, too short, wearing the wrong color of clothing. You're creating an "attention bottleneck," a narrow aperture that allows only certain information into conscious awareness, and going attentionally blind to anything that doesn't make it through the bottleneck.
To test your attentional targeting abilities, try simultaneously talking on the phone, watching TV, and conversing with someone standing next to you. You may be able to shuttle between these three stimuli, but as soon as you hear something particularly attention grabbing—"What? He locked his wife in a freezer?"—you'll lose the thread of the other two information streams.
When your brain assigns equal importance to several things at once, your attention bottleneck jams. You go attentionally blind to everything. This is the fuzzy, paralyzed feeling Sonya and Paula experience when they try to clear their space or time. It can be merely annoying, or catastrophic—for example, a driver engrossed in a cell phone conversation may go mind-blind to an obstacle right in front of his eyes.
It's unnerving, then, that humans have created an environment unnaturally jammed with attention-grabbing information. Take advertising (please). Our brains evolved to pay close attention to unusually bright colors, food, sex, babies, physical danger, and other information salient to survival. So marketers bombard us with such images, making them ever brighter, louder, gorier, geared to outcompete all other attention demands. Now consider the flood of information from new communication technology, being used by more humans than ever before. The result is rampant attentional blindness. It'll overwhelm pretty much all of us until we learn to outmaneuver our instincts.