I can do it, but it's a struggle. When I'm tired, or have had one too many pints of beer, I find that my ability to hear what my friend is saying suddenly disappears, as if someone had flipped off a switch. His voice is swallowed up by a cacophony of random noise in which I literally can't isolate a single word.
Usually my indication that it's time to call it a night.
[Image is in the Public Domain]
Yet most of us can isolate that one voice and hear it distinctly, and tune out all of the other voices and ambient noise. So how do you do this?
Scientists at Columbia University got a glimpse of how our brains might accomplish this amazing task in a set of experiments described in a paper that appeared in the journal Neuron this week. In "Hierarchical Encoding of Attended Auditory Objects in Multi-talker Speech Perception," by James O’Sullivan, Jose Herrero, Elliot Smith, Catherine Schevon, Guy M. McKhann, Sameer A. Sheth, Ashesh D. Mehta, and Nima Mesgarani, we find out that one part of the brain -- the superior temporal gyrus (STG) -- seems to be capable of boosting the gain of a sound we want to pay attention to, and to do so virtually instantaneously.
The auditory input we receive is a complex combination of acoustic vibrations in the air received all at the same time, so sorting them out is no mean feat. (Witness how long it's taken to develop good vocal transcription software -- which, even now, is fairly slow and inaccurate.) Yet your brain can do it flawlessly (well, for most of us, most of the time). What O'Sullivan et al. found was that once received by the auditory cortex, the neural signals are passed through two regions -- first the Heschl's gyrus (HG), and then the STG. The HG seems to create a multi-dimensional neural representation of what you're hearing, but doesn't really pick out one set of sounds as being more important than another. The STG, though, is able to sort through that tapestry of electrical signals and amplify the ones it decides are more important.
"We’ve long known that areas of auditory cortex are arranged in a hierarchy, with increasingly complex decoding occurring at each stage, but we haven’t observed how the voice of a particular speaker is processed along this path," said study lead author James O’Sullivan in a press release. "To understand this process, we needed to record the neural activity from the brain directly... We found that that it’s possible to amplify one speaker’s voice or the other by correctly weighting the output signal coming from HG. Based on our recordings, it’s plausible that the STG region performs that weighting."
The research has a lot of potential applications, not only for computerized vocal recognition, but for guiding the creation of devices to help the hearing impaired. It's long been an issue that traditional hearing aids amplify everything equally, so a hearing-impaired individual in a noisy environment has to turn up the volume to hear what (s)he wants to listen to, but this can make the ambient background noise deafeningly loud. If software can be developed that emulates what the STG does, it might create a much more natural-sounding and comfortable experience.
All of which is fascinating, isn't it? The more we learn about our own brains, the more astonishing they seem. Abilities we take entirely for granted are being accomplished by incredibly complex arrays and responses in that 1.3-kilogram "meat machine" sitting inside our skulls, often using mechanisms that still amaze me even after thirty-odd years of studying neuroscience.
And it leaves me wondering what we'll find out about our own nervous systems in the next thirty years.
In keeping with Monday's post, this week's Skeptophilia book recommendation is about one of the most enigmatic figures in mathematics; the Indian prodigy Srinivasa Ramanujan. Ramanujan was remarkable not only for his adeptness in handling numbers, but for his insight; one of his most famous moments was the discovery of "taxicab numbers" (I'll leave you to read the book to find out why they're called that), which are numbers that are expressible as the sum of two cubes, two different ways.
For example, 1,729 is the sum of 1 cubed and 12 cubed; it's also the sum of 9 cubed and 10 cubed.
What's fascinating about Ramanujan is that when he discovered this, it just leapt out at him. He looked at 1,729 and immediately recognized that it had this odd property. When he shared it with a friend, he was kind of amazed that the friend didn't jump to the same realization.
"How did you know that?" the friend asked.
Ramanujan shrugged. "It was obvious."
The Man Who Knew Infinity by Robert Kanigel is the story of Ramanujan, whose life ended from tuberculosis at the young age of 32. It's a brilliant, intriguing, and deeply perplexing book, looking at the mind of a savant -- someone who is so much better than most of us at a particular subject that it's hard even to conceive. But Kanigel doesn't just hold up Ramanujan as some kind of odd specimen; he looks at the human side of a man whose phenomenal abilities put him in a class by himself.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]