Pigeons could tell when the magnetic field was flipped, so they avoid flying in the wrong direction (Image: Carl de Souza/AFP/Getty Images)
David Dickman and Le-Qing Wu of Baylor College in Houston, Texas, collected seven homing pigeons (Columba livia) and inserted electrodes into their brains to record the activity of individual neurons.
They then placed the birds inside an artificial magnetic field, which masked the Earth's usual magnetism. The room was completely dark to prevent the birds using visual signals to orient themselves. As the researchers adjusted the intensity and angle of the magnets, they monitored how the pigeons' brain activity changed.
"We know birds and many other animals can sense the magnetic force;
behavioral studies show that birds fly along magnetic routes during
seasonal changes," said Dickman, who conducted much of the research at
Washington University in St. Louis. "It is still unknown what exactly
acts as a receptor within the bird; however, in our current study we are
able to show how neurons in the pigeon’s brain encode magnetic field
direction and intensity. This is how we believe birds know their
position on the surface of the Earth."
Dickman said certain areas of the brain are activated when a
particular area of the inner ear, known as the lagena, is exposed to a
magnetic field. Without it, several of these corresponding areas in the
brain show no activity.
Dickman and Wu used electrodes in one brain area, known as the
vestibular nuclei, to record activity when the bird was exposed to a
changing magnetic field.
"The cells responded to the angle and intensity of the magnetic
field. Some cells were more sensitive depending on what direction we
aimed the magnetic field around the bird’s head," Dickman said.
In the new study, published online today in Science, Dickman and BCM biologist Le-Qing Wu placed seven homing pigeons (Columba livia)
in a dark room in the center of a cube-shaped set of magnetic
coils. As the cube was rotated, the intensity of the magnetic field
felt by the pigeon in the center varied. The scientists turned
it in every direction, testing out the effect of various magnetic
fields found on Earth. As they did this, Dickman followed the
activity of 329 neurons in one of the areas of the brain he'd
previously implicated. Fifty-three of the brain cells showed significant changes in activity as the coils rotated,
reacting to field strength and polarity. The properties of the neurons
allow them to have a unique activity pattern for every different spot
on Earth, the scientists realized. Not only can the neurons
allow the pigeons to pinpoint their longitude and latitude, says
Dickman, but they can differentiate the Northern Hemisphere
from the Southern Hemisphere and tell the pigeons which direction