How Two Ears Tell Left From Right
Sound is not heard in one place. It arrives at your two ears, and the brain compares them. A sound coming from your right reaches your right ear slightly before your left ear. High-pitched sounds also get partly blocked by your head, so the far ear hears them a bit more quietly. These clues are called the interaural time difference and the interaural level difference, which simply means “the difference in timing” and “the difference in loudness” between the ears [5].
That works well for left and right. But the clues share a weakness. Picture a sound straight ahead, straight above, or straight behind you. For positions like these, the timing and loudness differences between your ears can be very similar. So those two clues alone usually cannot tell front from back, or up from down. Yet people do much better than that, and some of the extra information comes from the outer ear itself [5].
How the Folds of the Outer Ear Change a Sound
The visible part of your ear is not a plain funnel. It has ridges, a bowl and a curl, and sound bounces off them on its way into the ear canal. Sounds from different directions are shaped differently by the folds of the outer ear before they reach the ear canal. The result is that the same sound, played from two directions, reaches your eardrum with a slightly different mix of high and low pitches [5].
Scientists call these changes spectral cues, because a “spectrum” is just the mix of pitches in a sound. Your brain does not need to compare two ears to use them, which is why they can work even with one ear. Your head and shoulders also shape the sound a little, and every person’s ear is a little different, so each person’s pattern is personal [5].
Here is a way to picture it: think of a sound as a ball bouncing around inside a small, oddly shaped room. Where the ball is thrown from changes how it bounces, so the person inside can guess where it came from. This is only a picture, and a real ear is far more complicated.
What Happened When Scientists Reshaped People’s Ears
If the folds really matter, changing them should change what people hear. A team at a university in Nijmegen, a city in the Netherlands, tested this with adult volunteers. They changed the shape of the volunteers’ pinnae using molds. Right after the change, the volunteers’ judgments of how high a sound was were “dramatically degraded” [1].
A later study from the University of Sydney used eight listeners who wore small molds in both ears. The molds changed the pinna clues a lot, though some clues remained. At first the listeners confused front and back about seven times as often as before, and their errors in judging up and down more than doubled. The authors describe the molds as distorting the spectral cues, the ones that come from the pinna [2].
So the molds did not make people deaf. They took away a clue that people had been using without knowing it.
Can the Brain Learn to Use a New Ear Shape?
It can, and this is the most surprising part. In the Nijmegen experiment, accurate judging of height was “steadily reacquired” while the volunteers kept the molds in [1]. In the Sydney study, listeners wore the molds all the time for up to 60 days, with a median of 38 days. Their performance improved over that time, though it was still worse than normal at the end [2]. So learning takes weeks, not minutes, and it may not be complete.
The Nijmegen team found something else interesting. Learning the new clues did not erase the old ones. The volunteers could localize sounds both with their normal ears and with the modified ears, so the brain kept two sets of clues and used the right one for each case [1].
You might guess that the eyes teach the ears: when a sound seems to come from the wrong place, you look and see the real source. The Sydney team tested that idea by comparing sounds in front of the listener, where vision could help, with sounds behind, where it could not. Learning was about the same in both places. The authors suggest that, in this small study, learning did not depend on vision as a teacher [2]. That is one study of eight people, so it is a hint, not a final answer.
How Barn Owls Find Mice in the Dark
Barn owls can catch prey in the dark. The neuroscientists Masakazu Konishi and Eric Knudsen studied how, and found a map of auditory space in the owl’s midbrain: a part of the brain where neurons are arranged according to where a sound is coming from. The Peter Gruber Foundation gave them its 2005 Neuroscience Prize for this work and for showing how the owl’s brain calibrates that sound map with its visual map [3]. Hofman’s team noted that this kind of ongoing calibration had been shown in barn owls but not yet in humans, before their experiment [1].
The owl’s ears are shaped to help. Its facial ruff, the ring of stiff feathers around the face, and its asymmetrical ears (set at different heights on the head) were thought to create a two-dimensional grid of clues: timing for horizontal direction and loudness for vertical direction. A 2017 study showed that this picture was too simple. The owls also use loudness differences to judge the horizontal direction when the timing clue is ambiguous [4].
The takeaway is the same as for people: the shape of a head and ears turns direction into clues, and the brain learns to read them.
A Gentle Way to Explore Your Own Hearing
- Sit on a chair with your eyes closed. Ask a family member to stand a few steps away and tap a spoon on a table, softly, from the right, left, front and behind. Point to where each sound comes from. Skip sounds from above, and open your eyes before anyone moves closer. Never ask anyone to make a noise close to your ear.
- Cup a hand behind one ear, bending the ear forward a little, and try again. Does anything change? It may or may not, and it can differ from person to person.
- Do not put anything inside your ears. The scientists in these studies used carefully made molds under supervision, and that is not something to copy at home.
If you want to read more, the two experiments are listed in the sources below, and both have short summaries you can read online.