What's the signal, and what's the noise?

As anyone who listens to the (non-satellite) radio knows, signal-to-noise ratio is an important consideration when analyzing a data set. If the ratio is too low, all you get is static. But what if that static actually contained its own signal?

The idea of useful information being "hidden" in apparent noise is nothing new--after all, cosmic background radiation was once thought to be just noise (and for many applications it still is). But in the May 22 issue of Science, Peter Bromirski outlines a rather unusual case of noise-becoming-signal: seismological evidence for climate change.

Geologists use seismographs to record the movements of the crust. Most of the time, the crust doesn't move much, aside from a background "hum" that results from Earth's natural oscillations. That hum can actually show up on seismograms; it has a period of 1-8 minutes or so. Occasionally, though, an earthquake--geologists also sometimes call it a "seism"--causes the crust to move much more emphatically.

During an earthquake, the movements of the crust cause the seismograph needle (or the digital analogue) to move in a specific way. The speed, amplitude, and duration of that motion are related to the motion that occurred to cause the earthquake, as well as to the composition and structure of the materials the resulting seismic waves passed through to get to the seismograph. By studying seismographs from around the world, geologists can infer where and when the earthquake occurred, what caused it, and how the waves it produced traveled. The background hum is just noise, and it's generally ignored.

The thing about seismographs is that, for the most part, they're terrifically sensitive. It's not unusual for them to detect trains and traffic. And, as Bromirski points out, under the right conditions they can also detect ocean waves, particularly those produced by big storms.

During a large storm over the ocean, high winds blow over the ocean's surface. The wind transfers energy to the water, which is where the big ocean waves come from. That energy can generate "microseisms" in the ocean crust. (A microseism is exactly what you'd guess from the name: a very low-amplitude vibration in the crust.) The vibrations produced by wave energy travel through Earth, just like those from an earthquake, and they can be detected on seismographs, too. Therefore, hypothetically, one could use seismogram records to determine the average storminess of the oceans over time.

The use of seismograms to study storminess has a few advantages over more common methods. For one thing, there are accurate seismograms that go back to the early 20th century--as far back as 1930, in some areas. These seismograms were all collected using pretty much the same technology and have similar precision, so they're readily comparable. This is unusual in climate science; many of the techniques commonly used to study recent climate change are fairly...well...recent, so the records don't go very far back or, if they do, they're much less precise.

Another advantage to using seismograms is that the global seismograph network (which has become more and more widespread over time) allows for comparisons between signals from different areas. This can allow scientists to infer the approximate paths and durations of storms in a region. In some cases, microseisms can give information about wave frequency and duration along specific coastline regions, data that may be hard to obtain otherwise.

Some studies using these long-term seismic records do suggest that Earth is becoming stormier: the ambient noise on the seismograms has increased over time.

Some researchers are also studying ways to use storm-driven microseisms to study more than storms. An important use of earthquake seismogram data is the study of Earth's interior. It's similar to the use of ultrasound to see inside the body: just as the path of a sound wave through your body depends on the density and structure of the organs below the skin, so the path of a seismic wave depends on the composition, temperature, and structure of the rock within Earth. Typically, seismologists use earthquake-generated seismic waves to study Earth's interior, because they're very high amplitude and generate strong signals. However, earthquakes are relatively rare and unpredictable. "Background" microseisms produced by storms and wave activity may provide a more long-term and consistent energy source for the study of Earth's interior.

Bromirski, Peter D., 2009. "Earth Vibrations." Science 324: 1026-1027. doi: 10.1126/science.1171839.

Using geometry to find a rainbow

Chad Orzel at Uncertain Principles has a neat little post up about rainbows.

And you thought geometry could never come in handy.

I think it's pretty neat that the angle made by two hands is nearly scale-invariant. I wonder what that angle is for chimpanzees or other apes whose arms are longer relative to their bodies than human arms are?

Scientia Pro Publica 6!

Scientia Pro Publica, the best in science/nature/medicine blogging for the general public, is now posted at Mauka to Makai. Yours truly has a post up!

Check it out.

Usage tip: complimentary vs. complementary

For me, the easiest way to remember the difference between these two is to remember the definitions of compliment (something nice you say about someone) and complement (something that completes a set or group).

Something that is complimentary is either a) free or b) flattering. (Maybe another way to remember it is to think "I like things that are complImentary". Or maybe that's just really corny.)

Something that is complementary completes a set, matches a pair, or fills out a group. Angles, base pairs, and wines can be complementary, but statements and newspapers generally aren't.

So, you can sip complimentary coffee while contemplating the complementary angles on the rafters above your head. But if you start encountering complimentary angles, you might want to get your eyes (ears?) checked...

Usage tip: enormity

Enormity does not refer to size. It means "horribleness" or "horrendousness."

If you talk about the enormity of a situation, make sure it's something terrible.

(My trusty Webster does allow the use of enormity to mean "enormous size or extent", but qualifies it by saying that it's considered "a loose use by some." Count me in that "some!")

If the Girl Scouts had badges like these...

I just discovered Science Scouts. I was a Girl Scout for more than 10 years, and earned my share of badges. But none as cool as these!

As far as I can tell, I've earned 13 of them:

The "I've named a child or pet for science" badge (Sandy's real name is Sanidine. She prefers Sandy because it's less pretentious.)
The "Works with acids" badge (Including both HF and aqua regia, plus the standard highly concentrated nastiness.)
The "I've set fire to stuff" (Levels I and II) badge (No self-respecting chemistry major--or Girl Scout!--has NOT earned these two.)
The "Somewhat confused as to what scientific field I belong to" badge (Probably pretty self-explanatory...)
The "Experienced with electrical shock (Level III)" badge (I grabbed an electric fence once. Actually, I think I've had contact with electric fences twice. Comes from growing up in the country...)
The "I know what a tadpole is" badge
The "I've done science with no conceivable practical application" badge (But don't tell the NSF.)
The "Has frozen stuff just to see what happens" (Levels I and III) badge (I haven't had much experience with dry ice.)
The "Arts and crafts" badge (I'm about to start a crocheted DNA molecule...and realized that the one in the sample photo twists the wrong way!)
The "Talking science" badge
The "I blog about science" badge

(h/t Chad Orzel)

It's all in your head

I think most of us are pretty willing to accept that the "will" or "urge" to move originates in the brain, and that the nerve stimulus that initiates the movement also originates in the brain.

What you might not know (I didn't) is that those two impulses--wanting to move, and initiating the movement--may actually happen in different parts of the brain.

I suppose it's not really surprising that this should be the case; the brain is, after all, a pretty big place (from a neuron's perspective), and obviously everything doesn't happen all in one spot. But in the May 8 issue of Science, Desmurget et al give pretty good evidence that the area that starts your body moving is distinct from the area that actually generates the urge to move.

The researchers studied seven human patients undergoing brain surgery for tumors. All seven were conscious during the surgery (possible because the brain, although the largest concentration of nervous tissue in the body, has no actual pain receptors on its surface), so they were able to answer questions. (Although it's not made clear in the article, presumably the patients were on several medications to relax them, but they were still conscious.)

In brain surgeries like this, doctors sometimes stimulate areas of the brain near the tumor to identify what parts of the body (or personality) may be affected by the surgery. In this case, the researchers used a similar technique to learn more about how the brain works.

During each surgery, several different regions of the patient's brain were stimulated with a small electrical probe. The shocks varied in intensity and duration. The researchers repeated the stimulations up to four times for each location, to check for reproducibility.

What they found out strikes me as pretty interesting. It turns out that, for several of the patients, when parts of the inferior posterior parietal cortex were stimulated, the patients felt an urge to move one or more body parts (arm, lips, chest, etc). If the stimulation was repeated with a higher intensity, the patients thought that they had actually moved that body part, even though no movement actually occurred. (The researchers report that one patient even said "I moved my mouth, I talked, what did I say?", although no mouth movement or speech was observed.)

Additionally, when portions of the premotor cortex were stimulated, the patients did actually move some of their body parts. When the stimulation was increased, the movement became more pronounced. However, and this was the part that I thought was kind of neat, the patients were completely unaware that they had moved at all. In fact, when they were specifically asked, the patients denied that they had moved, even when the movement was quite significant (e.g., raising an arm, or making a fist).

During the procedures, the researchers monitored the electrical signals in the patients' muscles as well. They saw no evidence of muscle movement when the parietal cortex was stimulated, even when patients were sure they had moved.

As an interesting side note, Desmurget et al report that stimulation of the right inferior parietal cortex caused patients to want to move their left limbs--hands, arms, feet, etc. However, stimulation of the left inferior parietal cortex seemed to prompt a desire to move the lips, or to talk.

References:
Desmurget, M., et al, 2009. "Movement Intention After Parietal Cortex Stimulation in Humans." Science 324: 811-813. doi 10.1126/science.1169896
Haggard, P., 2009. "The Sources of Human Volition." Science 324: 731-733. doi 10.1126/science.1173827

UPDATE: This post appears in the June 15 Scientia Pro Publica at Mauka to Makai.