Looking at Sound

post from Beta Rhythm (Music of the mind) on 19 November 2005 08:10:00 AM. © Beta Rhythm (Music of the mind)

Lately I've been listening a lot to Kate Bush's album Aerial - beautiful, wonderful stuff. The album cover is interesting too - the 'islands' that are reflected in the water are actually the amplitude envelope of a recording of some birds singing.

This idea of 'looking at sound' in different ways has been something I've really enjoyed exploring over the last several years. To help visualize the harmonics in a piece of music, I wrote a program a while back that analyses the frequency content of a sound waveform and creates a spectrogram (spectrum over time) of it, colour coding the intensity levels of each frequency.

I think I've found the bird song shown on the cover - it's 2:25 from the start of the song 'Aerial'. Here's what its spectrogram looks like:

The parallel contour lines that are stacked one on top of each other are the harmonics of the bird song. (A synthesizer's been added to the recording, which has changed the amplitued envelope somewhat and contributed the 'white noise' vertical smears and horizontal tones seen in this spectrogram.)

Here's what a bird singing solo looks like (from the song AerialTal at the 7 second mark):

Once you've learned what to look for, you can look at sound in the frequency domain and sort of recognize individual 'voices' by looking at their harmonic patterns. You can pick out harmonic 'signatures' like this even if there's background noise or other sound sources. It's much tougher to look at a spectrogram and figure out what's going on than it is simply to listen to the sound and figure it out, however. There's must be some pretty awesome signal processing going on in the ear+brain combo...

When I first started writing this, I thought I had a pretty good grasp of how hearing works - you know, vibrations in the air moving the ear drum and getting picked up by little hairs in the inner ear. But this only goes so far... How does the movement of these tiny hairs get turned into something the brain can make sense of? (Especially since there are only around 16,000 of these hair cells in the human cochlea.) This is where it gets totally fascinating. I stumbled across this awesome MIT website that delves into the micromechanics of the inner ear, and has some cool photos and videos of how these tiny hair cells convert sound energy into a form of chemical energy that the brain understands. From the website:
The inner ear performs some very remarkable signal processing. For example, the inner ear can detect motions of the eardrum on the order of a PICOMETER -- i.e., much smaller than the diameter of a hydrogen atom. ... Hair cells are small. But hair cells are themselves complex micromechanical systems whose function relies on an array of even smaller mechanical parts. Displacements of hair bundles generate electrical responses in hair cells via mechanically sensitive ion channels in the cell membrane.

video (165K)
The tip links are tiny filaments only 2nm in diameter. In the video you can see them pulling open little 'trap doors' - opening the 'mechanically sensitive ion channels in the cell membrane' mentioned earlier. (Aside: It's pretty easy to visualize these filaments getting snapped when listening to music at high volume. No more 'turning the volume up to 11' for me...)

These ion channels are basically pores in the cell membrane that allow charged Potassium (K+) ions to move into the cell, which causes the cell to lose polarization. "In order to be able to process sounds at the highest frequency range of human hearing, hair cells must be able to turn current on and off 20,000 times per second. They are capable of even more astonishing speeds in bats and whales, which can distinguish sounds at frequencies as high as 200,000 cycles per second"(ref.)

From The Neurobiology of Harmony by David Benner:
Once frequency and amplitude are converted into action potentials, the biochemical pathway leads sounds from the inner ear along the auditory nerve which is part of cranial nerve VIII through parts of the medulla, pons, midbrain, thalamus, and finally to the auditory cortex of the temporal lobe. The parts of the brain involved in the perception of sound locate its origin and involve the limbic system in the recognition of a given input.

From Music in Your Head by Eckart O. Altenmüller:
After sound is registered in the ear, the auditory nerve transmits the data to the brain stem. There the information passes through at least four switching stations, which filter the signals, recognize patterns and help to calculate the differences in the sound?s duration between the ears to determine the location from which the noise originates. For example, in the first switching area, called the cochlear nucleus, the nerve cells in the ventral, or more forward, section react mainly to individual sounds and generally pass on incoming signals unchanged; the dorsal, or rear, section processes acoustic patterns, such as the beginning and ending points of a stimulus or changes in frequency. After the switching stations, the thalamus?a structure in the brain that is often referred to as the gateway to the cerebral cortex?either directs information on to the cortex or suppresses it. This gating effect enables us to control our attention selectively so that we can, for instance, pick out one particular instrument from among all the sounds being produced by an orchestra. The auditory nerve pathway terminates at the primary auditory cortex, or Heschl?s gyrus, on the top of the temporal lobe. The auditory cortex is split on both sides of the brain. It seems that the way the music is handled in the brain from this point on differs greatly between non-musicians and musicians, and in fact even between individuals. In imaging studies the same music is represented in multiple ways in the brain of a professional musician: as a sound, as movement (for example, on a piano keyboard), as a symbol (notes on a score) and so on. Not so in the brain of an unpracticed listener. Generally, however, rhythm is handled by the left side of the brain and pitch and melody are handled by the right side of the brain.

Harmonics are a set of frequencies that are integer multiples of a common 'fundamental' root frequency. My guess is that, when enough pulses from the frequency detectors for a particular harmonic series fire at around the same time, a 'harmonic detector' neuron is pushed over a trigger threshold. And then, the outputs of these harmonic detector neurons and frequency detector neurons somehow get compared to harmonic profiles stored in memory (e.g. the sound of a voice or of a musical instrument).

By focusing on the harmonic structure that is present in the sound, we are able to focus in on one voice in a crowd, one instrument in a band, isolate signals from noise, spatially locate a sound in a 3D sound field - lots of things our present-day technology has difficulty doing. It provides key information to the brain that allows it to recognize voices, pick out rhymes and rhythms, melodies and harmonies, associate everything with feelings and meaning.

U2's lead singer Bono has noted that "songs are not like movies where you can see them once, twice, three times - they become part of your life. They're more like smells." Music doesn't seem to get registered into memory the same way that visual images do. What you remember is the way the music makes you feel, and the stuff that is repeated several times (chorus, guitar riff, killer bass line). It takes a while to learn the rest, to hang onto it long enough for you to anticipate it fully. Perhaps it's because the brain needs a certain amount of repetition to convert a short term memory into a long term memory (see "Making Memories Stick" by R. Douglas Fields for more info). For whatever reason, music is very much 'in and of the moment'. And it's deeply rooted - it can trigger an emotional and/or physical response, make you want to dance and sing - such a joyous thing. The essence of now, of life.

See also:
Music and the Brain (Scientific American)
Getting a Leg Up on Land - the evolution of four-limbed animals from fish (includes info on how hearing evolved)
Eaton-Peabody Lab (one of the world's largest basic research facilities dedicated to the study of hearing and deafness)
Research into regeneration of damaged inner ear hair cells
Gene therapy stimulates new hair growth in the cochlea

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Epidemic Pandemic Why Should I Care

post from Science In Action on 19 November 2005 02:00:00 PM. © Science In Action

Should You Be Scared About "Bird Flu"?

We read a lot in the news recently about the threat of a bird-flu pandemic. Is this just media scare, or is it something we should really be worried about? A severe pandemic of a novel, virulent avian influenza would kill millions, stunt economic growth, and maybe even topple governments. However, some of the current scare is overblown. Here are some facts:

A "pandemic" is an epidemic that covers a large geographical area. A "global epidemic".

An "epidemic" is a significant outbreak of an infectious disease, more cases than expected. So malaria in most tropical poor countries is not "epidemic", even though it kills millions of people every year, because this is the "expected" rate. If malaria broke out in Washington, D.C., and killed even ten people, it might be considered a malaria "epidemic". Similarly, about 36,000 people die every year from influenza in the U.S. So an "epidemic" would have to infect a much larger number.

Influenza

The "flu" is caused by a virus. It affects the upper respiratory tract (nose, throat and lungs), and is spread by transfer of virus particles in saliva or mucus droplets, usually expelled in coughs or sneezes. The infection causes fever, body aches, headache, sore throat, fatigue, and coughing and sneezing. Most people will recover in one to two weeks. The disease is life-threatening particularly for the elderly and the young, and for people with underlying medical conditions such as heart or lung disease.

Bird Flu

"Bird flu" or "avian influenza" is a disease of aquatic birds. Sometimes people catch it (if they have been in very close contact with infected birds, usually involved in raising domestic fowl) and if they catch a virulent strain like H5N1 it is very serious. More than half the infected individuals die. Around 100 people have died of bird flu world wide in recent months.

The reason for global concern about bird flu is that influenza viruses can mutate to become more infectious (more easily transmitted). If one of the dangerous (pathogenic = disease-causing) strains of the virus were to mutate to become "human-adapted", so that it could be easily transmitted from one person to another, the stage would be set for a very serious epidemic, or even a pandemic.

The Great Influenza Pandemic of 1918-1919

This global disaster was caused by a novel and deadly strain of avian influenza virus. More than 25 million people died, most of them in poorer countries. In the United States about 28% of the population became ill and more than half a million people died. For comparison, HIV/AIDS has killed about 25 million people over 25 years, while the 1918 pandemic killed the same number in a few months. This is why public health officials are so concerned.

Samples of the 1918 pathogen have been recovered and analyzed. Recently its complete genetic makeup has been published.

What would a flu pandemic be like today?

In contrast to 1918, today we know what causes influenza (a virus) and how it is transmitted. We have some anti-viral drugs (but not many, and they would not be widely available, especially to the poor). We know how to produce flu vaccines, though it takes time to manufacture and administer them. Would these advances enable us to prevent or control a flu pandemic?

Current models of the possible impact of a flu epidemic in the U.S. suggest that between 15% and 35% of the population would be affected, and 100,000 to 200,000 would die ("medium-level" case). Rates of infection and mortality would probably be similar in other developed economies. In poorer countries the impact would be greater. The World Health Organization base case predicts 2 million to 7.5 million deaths world wide.

Political and economic effects could be severe. Restrictions on travel and trade, and reduced business activity due to closed businesses and reduced productivity, would be like a recession. Political instability could develop in places where governments do not appear to be responding effectively or fairly to the crisis. Reduced agricultural productivity and restrictions on food trade could create localized food crises.

Recent disasters have hurt the government in power if their responses are perceived as ineffective (Hurricane Katrina). On the other hand, crises can be used to consolidate political power (September 11th).

Managing A Pandemic Today

Flu epidemics in 1957-1958 and in 1968 killed about 70,000 and 34,000 Americans, respectively. The primary public health tools used to minimize the impact of these outbreaks were vaccination, information, regulation, and more effective treatment.

Vaccination -- After a new strain of influenza virus emerges and is determined to present a threat of widespread human disease, it takes several months for vaccine targeted at that strain to be developed and manufactured. As the vaccine first becomes available it will be used to protect health care workers and others who are both at high risk of being exposed to the disease and in a position to spread it to others. As larger quantities of vaccine are available they will be allocated by public health services to stop the spread of the disease in particular areas, such as specific cities, military bases, or the like. The standard "flu vaccine" available now is not designed to prevent avian influenza, but is targeted at the normal influenza strains identified earlier this year as most likely to be dominant during the current "flu season".

Information -- Public health agencies will try to teach people behaviors that will protect them from catching and spreading the disease.
- "Hygiene" and "sneeze etiquette" will be strongly recommended. Covering your nose and mouth when you sneeze can reduce the dispersal of airborne droplets of mucus which can potentially carry the virus to others.
- Washing your hands frequently can prevent infecting yourself with virus you have picked up, and can help prevent you from spreading the virus to others. Regular soap and water or alcohol hand cleaning solutions work fine. Antibacterial soaps provide zero additional effect.
- Masks may be suggested, or even required in some places. One key benefit of a mask is to remind you not to touch your face, thus reducing transmission of virus on the hands. Masks will not filter out viruses, but may prevent dispersal of mucus droplets when you sneeze. Proper disposal of contaminated masks is important. Wearing of masks by the non-infected public may not actually do much to slow the spread of the disease, but it may make people feel more secure.
- People with flu symptoms will be encouraged, or indeed in some cases required, to stay at home.
Regulation -- To reduce the rate of spread of any new, contagious, virulent flu virus several public health measures are likely to be put in place:
- Travel from regions where the new strain has broken out will be discouraged or forbidden.
- More aggressive monitoring of flu cases will be required, and cases or clusters of cases may have to be isolated (quarantined).
- Schools will be closed when the disease breaks out, and some other activities where people gather may be curtailed (e.g. entertainment and sporting events). Some businesses will close or be required to close.
Treatment -- There are some antiviral drugs which may be used to reduce the severity of the disease, and even some which appear to prevent getting it. Unfortunately, existing flu strains are already evolving resistance to some of these drugs. The drugs would effectively be rationed to be used to protect health-care workers and other essential workers, and to treat the elderly, the young, and others at high risk of complications or death.

Antibiotics do not affect viruses, but many flu deaths are due to secondary bacterial infections such as pneumonia. Antibiotics would be used to treat these cases.

Summary

Flu epidemics will happen in the future, but nobody knows when. Preparations are under way to minimize the social, economic, and public-health impact of the next big one. The best protections against any influenza virus are washing your hands and avoiding getting sneezed on by an infected person. Those most at risk of death in a flu epidemic are those without access to an effective health care system, which includes people in poor countries, people in regions of conflict, and elderly people living alone in developed countries.