We're in Salt Lake City for the next couple of days for the 153rd meeting of the Acoustical Society of America. Because I couldn't fly with a pesky lingering ear infection, I drove my shiny red Prius through the southwestern desert for 10 hours, stopping overnight in Vegas to play a bit of poker. It was still enough of a change in elevation to cause my left ear to pop almost continuously whenever the altitude changed -- not painful, just irritating; a flight at 30,000 feet, on the other hand, probably would have been agony, possibly even ruptured the eardrum. Verdict: it's probably a good thing I decided to drive.
Speaking of strange popping sounds, I also managed to write my monthly "Random Walks" column for 3 Quarks Daily while in Vegas, on near-death experiences, via the sci-fi novel Passages by Connie Willis, and the blockbuster movie Titanic. It's fascinating stuff from a scientific standpoint, particularly the complex neurochemistry involved, which is only partially understood. One of the elements of your average NDE, apparently, is some kind of strange ringing or buzzing sound right before the individual feels him or her self entering the classic tunnel (after a 10-hour drive, I'd probably see the striking rocky tors lining the I-15 freeway as one passes from Nevada into Arizona). Except the sound apparently isn't coming from the outside world; it's generated from inside the brain. The brain is an amazing thing.
As luck would have it, one of the papers presented this morning at the ASA meeting dealt with how the human brain processes sound. We all learned about the ear's anatomy and how it senses sound vibrations and translates that into signals for the brain to interpret in grade school. But many of the specifics of how the brain does this remain shrouded in scientific mystery.
Take pitch as an example. When we listen to a piece of music -- say, Mozart's "Eine Kleine Nachtmusik" (which played on my iPod during my long drive) -- we can distinguish between two separate instruments, even though they're playing the same note, because of pitch. We can do this even when one's left ear is all blocked up from fluid in the middle ear, and popping like crazy in response to changing air pressure -- annoying, until one decides to simply consider the popping impromptu percussion.
Pitch, technically, is how "sounds can be ordered on a scale from low to high," according to Deborah Hall of the MRC Institute of Hearing Research in Nottingham, England. It's also how we can differentiate between sounds arising from separate sources, such as two people speaking at once. Subjects in laboratory auditory tests can match different synthesized sounds just on the basis of pitch, so it should be possible to identify neurons in the brain that correspond to the pitch property of sound. Hall and her colleagues have taken detailed MRIs showing the patterns of neural activity that arise as the subjects are listening to a wide range of these differently pitched, synthesized sounds.
The consensus among neuroscientists to date has been that the ability to discern pitch resides in a part of the brain's auditory region completely unfamiliar to me, called "lateral Heschl's gyrus." Hall's findings, though, seem to contradict that assumption. Apparently the LHG (as I have dubbed it) does kick in with a specific type of pitched sound. But the others produced a much bigger response in an area just behind the LHG. Hall concluded that "the generalized representation of pitch appears to be formed relatively late in the auditory processing stream, probably in the posterior part of the auditory brain." It's a new result, so I guess we'll have to wait for all the peer review and follow-up experiments and see if Hall's conclusions turn out to be right. But it's interesting!
Because of all the ear-popping, I've got hearing on the brain (ahem), which is probably why I felt compelled to dig up this nifty article in Physical Review Focus from March 2006 about why the inner ear (cochlea) might be snail-shaped. (It's at least tangentially related to the above work on how we distinguish pitch, only focusing on the anatomical structure rather than neuroscience.) Going back to our childhood science classes, once sound vibrations are transmitted to the fluid in the cochlea, they travel along a spiral-shaped tube, which changes along its length. The result is that different frequencies of sound reach their peak at different positions along the tube, so the cochlea can distinguish between them.
The spiral shape, however, doesn't seem to have any effect on this ability, so why the snail-shaped cochlea? What possible evolutionary advantage could it have? Researchers at Vanderbilt University and the NIH think it does serve a purpose: enhancing "the vibrational motions that translate into nerve signals." Prior mathematical models of the cochlea have used straight channels, because nobody thought the shape was significant, but the VU/NIH team's calculations showed that the spiral shape does have an impact on low frequencies. Specifically, it causes sound energy to accumulate near the outside edge of the spiral, instead of diffusing evenly across it. Per the PRF article: "Since the cells that detect the vibrations respond especially well to pressure differences between the inner and outer edge, this concentration of sound intensity translates into higher sensitivity."
It's similar to what happens as sound propagates in London's St. Paul's Cathedral, a so-called "whispering gallery." Even quiet sounds can travel very long distances in such a construction (a cylindrical wall, in this case) without losing energy. In the cochlea, the effect is even more pronounced: "The increasingly tighter turns ensure that the rays of sound will 'focus' steadily closer to the wall.'"
There are no scheduled talks specifically on whispering galleries at the ASA meeting (to my knowledge), but there was a session this morning on architectural acoustics, providing an acoustical analysis of concert hall spaces here in Salt Lake City. And there's lots of other nifty research being presented, such as papers on what sound is like on Mars or Titan; on dinosaur hearing; and digital morphing of unpleasant sounds into more preferable ones. But personally, my highlight will be this afternoon's mini-yodeling concert.