Built-in Compass
© GARY S CHAPMAN/GETTY IMAGES
Compasses are very useful, but, researchers suggest, the best one might reside somewhere in your brain.
The Earth's magnetic field is faint, yet creatures from birds and bees to lobsters and bacteria have been shown to detect its dull pull. Now, after half a century of looking, scientists have reported the most convincing evidence yet to suggest humans, too, share this ability.

The mysteries surrounding magnetoreception, as it is called, abound. It makes sense for globetrotting migratory birds and turtles to have an in-built compass, but it is far less obvious why cows might need one to orient their bodies along the magnetic field lines when grazing, or dogs to point north or south when defecating.

The first inklings that humans might have an internal compass came from studies by Robin Baker at the University of Manchester in the UK. In 1980, he reported that if he blindfolded students and transported them out of town, they could almost always point towards the quadrant of their starting point, but they lost this ability if a bar magnet was strapped to their heads. Subsequent attempts to replicate the findings failed, however.

Biophysicist Joe Kirschvink, then at Princeton University in the US, is one person whose replication experiments fizzled in the 1980s. But three decades later, and now at the California Institute of Technology, he and colleagues came up with a better way of testing whether humans have an internal compass.

Instead of asking his subjects for a conscious, behavioural response to changes in magnetic field, he decided to ask their brains directly.

To do that, his team rigged up a high-tech metal cell. The dark chamber is covered in thin aluminium sheets to shield the person sitting inside from the Earth's magnetic field. An array of electrical coils lining the cell exposes the person to custom magnetic fields, and a cap studded with 64 electrodes connected to an electroencephalogram (EEG) machine records brain waves.

What the team was looking for were the tell-tale signs that the brain was busy "processing" something when the magnetic fields were altered.

Away from the pings and flashes of smartphones, or even the gentle huff of a summer breeze on one's neck, the brain settles into a pattern of synchronous activity known as alpha waves.

"When the brain receives a stimulus, like sound or smell, parts of the brain will hop out of this hum and start to worry about what's going on," Kirschvink says.

"It's a response that's across a lot of different senses, so we said, 'all right, let's try that to see if the magnetic sense is being perceived'."

Sure enough, when people were subjected to a magnetic field that rotated, as though they had been swivelled in their chair by 90 degrees, the alpha waves in their brain dipped. Neurons were being recruited to "sense" the change in magnetic field.

Curiously, the effect of the rotation only occurred when the vertical magnetic field - which tells us where we are in relation to magnetic north - was directed downwards, as it is in the northern hemisphere. If the field was flipped to mimic a southern hemisphere magnetic field, the brain responses disappeared.

According to Kirschvink, that's because all 34 of the participants in the study grew up in the northern hemisphere. If the input makes no sense to what the brain knows of its surroundings, it disregards it.

The experiments also reveal how the body perceives magnetic fields.

One theory suggests they spark quantum chemical reactions in crytochromes - proteins found in the retina of the eye. Another, which Kirschvink subscribes to, holds that receptor cells containing tiny molecular bar magnets - most likely made of an iron mineral called magnetite - trigger brain cells to react.

The work rules out a quantum compass.

"A quantum compass can't tell a field down to the north from up to the south. So it's not a quantum compass," he says.

But the location of the magnetite receptors - if they exist - is a mystery. Kirschvink thinks it's probably somewhere in the large trigeminal nerve that splays out around the head, but further research is needed to pin down that hunch.

"If it's right then I think it's really important," says Peter Hore from the University of Oxford, UK. But, he adds, independent verification is necessary.

"My impression is [the experiments] have been done very, very carefully indeed, yet this area of research on magnetic sensing in animals has been fraught with results that no-one else has been able to repeat, so I won't get too excited about this until someone else has independently replicated these measurements and found the same thing."

Kirschvink and his colleagues welcome the scrutiny.

"This result is something that needs to be reproduced, it needs to be replicated by other lab groups," says co-author Isaac Hilburn from Caltech.

The team is also keen to see how people from the southern hemisphere or equatorial regions respond to the changing fields. This could help to unravel whether everyone is capable of sensing the Earth's magnetic field, and whether anyone is able to do so on a conscious level.