Can humans sense the Earth's magnetic field?

If you are lost on a farm, try using the cows to find your way home. Bizarre as it may seem, cattle apparently don't just stand around randomly chewing the cud - they in fact behave like huge compasses, aligning themselves with magnetic north. The German and Czech researchers who discovered the phenomenon by poring over Google Earth satellite images of pastures around the world also found the same phenomenon among herds of deer. Though they have no idea why the animals do it.

But can humans sense the Earth's magnetic field, and does it affect our behaviour or health? Put magnets and human health into Google (I don't recommend it) and you will be bombarded with products: magnetic pillows that supposedly cure insomnia, magnetically aged wine, bandages that use magnets to heal you quicker and so on. But according to the World Health Organisation, the evidence that humans are affected by low-level magnetic fields is weak to non-existent.

"Based on a recent in-depth review of the scientific literature, the WHO concluded that current evidence does not confirm the existence of any health consequences from exposure to low-level electromagnetic fields," it says. "However, some gaps in knowledge about biological effects exist and need further research."

But very precise electro- magnetic stimulation can have some interesting and useful effects on the brain. A relatively new technique used by neuro-scientists called Transcranial Magnetic Stimulation (TMS) involves placing a powerful and extremely short (200 millisecond) electrical current next to a person's head and inducing a much lower electrical current in part of the brain. This current disrupts activity there briefly, so allowing researchers to work out what individual bits of the brain do. "We temporarily modify the activity in this area artificially from the outside," says Dr Sven Bestmann at University College London. "It's a way to stimulate nerves in the cortex without having to open the skull and insert electrodes."

Magnetic fields may not help you find your way home then, but scientists have used them to investigate which parts of the brain are involved in a range of conditions, such as stroke, tinnitus, migraine and depression, says Bestmann. "It is really the only way we can make such an investigation in healthy volunteers, to say something about brain areas that are critically involved in a certain behaviour."