Hearing is a vastly underrated sense. Studies have shown that conscious thought takes place at about the same rate as visual recognition, requiring a significant fraction of a second per event .
If your home is like mine, you hear the humming sound of a printer, the low throbbing of traffic from the nearby highway and the clatter of plastic followed by the muffled impact of paws landing on linoleum – meaning that the cat has once again tried to open the catnip container atop the fridge and succeeded only in knocking it to the kitchen floor.
The slight trick in the question is that, by asking you what you were hearing, I prompted your brain to take control of the sensory experience – and made you listen rather than just hear. That, in effect, is what happens when an event jumps out of the background enough to be perceived consciously rather than just being part of your auditory surroundings.
The difference between the sense of hearing and the skill of listening is attention.
Hearing is a vastly underrated sense. We tend to think of the world as a place that we see, interacting with things and people based on how they look.
Studies have shown that conscious thought takes place at about the same rate as visual recognition, requiring a significant fraction of a second per event. But hearing is a quantitatively faster sense. While it might take you a full second to notice something out of the corner of your eye, turn your head toward it, recognise it and respond to it, the same reaction to a new or sudden sound happens at least 10 times as fast.
This is because hearing has evolved as our alarm system – it operates out of line of sight and works even while you are asleep. And because there is no place in the universe that is totally silent, your auditory system has evolved a complex and automatic "volume control," fine-tuned by development and experience, to keep most sounds off your cognitive radar unless they might be of use as a signal that something dangerous or wonderful is somewhere within the kilometre or so that your ears can detect. This is where attention kicks in.
Attention is not some monolithic brain process. There are different types of attention, and they use different parts of the brain. The sudden loud noise that makes you jump activates the simplest type: the startle. A chain of five neurons from your ears to your spine takes that noise and converts it into a defensive response in a mere tenth of a second – elevating your heart rate, hunching your shoulders and making you cast around to see if whatever you heard is going to pounce and eat you. This simplest form of attention requires almost no brains at all and has been observed in every studied vertebrate.
More complex attention kicks in when you hear your name called from across a room or hear an unexpected birdcall from inside a subway station. This stimulus-directed attention is controlled by pathways through the temporoparietal and inferior frontal cortex regions, mostly in the right hemisphere – areas that process the raw, sensory input, but don't concern themselves with what you should make of that sound.
But when you actually pay attention to something you're listening to, whether it is your favourite song or the cat meowing at dinnertime, a separate "top-down" pathway comes into play.
Here, the signals are conveyed through a dorsal pathway in your cortex, part of the brain that does more computation, which lets you actively focus on what you're hearing and tune out sights and sounds that aren't as immediately important.
In this case, your brain works like a set of noise-suppressing headphones, with the bottom-up pathways acting as a switch to interrupt if something more urgent grabs your attention. Hearing, in short, is easy.
You and every other vertebrate that hasn't suffered some genetic, developmental or environmental accident have been doing it for hundreds of millions of years.
Listening is a skill that we're in danger of losing in a world of digital distraction and information overload. And yet we dare not lose it. Because listening tunes our brain to the patterns of our environment faster than any other sense, and paying attention to the nonvisual parts of our world feeds into everything from our intellectual sharpness to our dance skills.
Luckily, we can train our listening just as with any other skill. Listen to new music when jogging rather than familiar tunes. Listen to your dog's whines and barks: he is trying to tell you something isn't right. Listen to your significant other's voice – not only to the words, which after a few years may repeat, but to the sounds under them, the emotions carried in the harmonics.