Magnetic Fields on the Brain
Chris - Also this week, scientists homed in on the parts of a pigeon's brain that enable it to use the Earth's magnetic field to navigate and Louise Anthony has been finding out a bit more...
Louise - Pigeons, as well as a host of other species including bats and fish, are known to be sensitive to magnetic fields and they appear to be able to use them to find their way around. But exactly how these animals detect and then process this information neurologically has always remained a grey area. Now, working with homing pigeons, David Dickman from the Baylor School of Medicine in Texas, together with his colleague Le-Qing Wu, has identified the parts of the brain that give these animals their mental compasses. Dickman and Wu reasoned that nerve cells involved in decoding magnetic signals should be activated by changes in the surrounding magnetic field. So to track down these cells, as David Dickman explains, they began by exposing a group of birds to a changing artificial magnetic field and then looked in the animals' brains for nerve cells that had switched on a gene called c-Fos, which is an indicator of nerve cell activity.
David - We put birds inside a magnetic field, rotated the magnetic field around their heads, then reacted the brain tissue for the c-Fos antibody and we found four major locations that were strongly activated by magnetic stimulation. One of them turned out to be in the vestibular nuclei, another in the anterior thalamus, an area that processes spatial information and the third was the hippocampus which is well-known to be a spatial memory centre. The fourth was an area of association, visual cortex.
Louise - These are all regions that are known to be involved in navigation functions and spacial orientation. In other words, knowing where you are, what position you're in, and in which direction you're pointing. Next, to find out how these nerve cells might be processing magnetic information, Dickman and Wu placed 7 pigeons in changing magnetic fields of a similar strength to the Earth's own magnetic field, and then used electrodes to record the nerve activity in one of the brain regions they'd already identified - the vestibular nuclei - which also helped to control balance.
David - We placed birds in the dark because there's a competing idea that the retina has a photo pigment contained inside the retina itself which could be reactive under certain wavelengths of light to a magnetic field. We didn't want to activate those receptors if they exist, so we put the birds in total darkness and they were motionless because we didn't want to activate the vestibular system since we're recording from vestibular cells. So while they sat there quiet, we rotated this magnetic field around them in different planes and we found that the neurons are all tuned to a specific direction in space and the tuning is really interesting because the neurons respond when the magnetic field is pointed basically in all directions except one plane where they're silent, and they build up the response so that when the magnetic field is pointing in one direction, the cell likes it the most and when it's pointing opposite that, the cell likes it the least.
Louise - And the responses of those cells effectively signal the strength, direction and the polarity of the magnetic field; In other words, whether the bird is pointing north to south or south to north, and this means that the birds can most likely use this information to work out where they are.
David - If you look at the Earth's magnetic field, what you see is that field lines come out of the south magnetic pole, they circle the Earth and they go back in, in the north magnetic pole. And they come out of the Earth at different angles, depending upon whether it's the south, north or the equator. It's 90 degrees at the poles and it's zero at the equator and then it varies systematically between the equator and the poles. That's called the inclination angle. These neurons theoretically could use that inclination angle to tell you your latitude.
Louise - But what they still don't know is how the birds are actually detecting the magnetic field in the first place. So whether it's a magnetically sensitive chemical in the eye or deposits of an ion containing mineral like magnetite - that's still a mystery, but there are several possibilities.
David - There are three candidates out there - the retina, using these photo pigments. We've never looked at retinal cells before. There's also the inner ear, where there are these iron particles that were found by another group; and then the beak, although the beak looks like the magnetite is macrophages, it could be that there is still a mechanism there but is yet to be discovered. There are behavioural studies that suggest that the beak is involved, so we really don't know. We'll probably go after the receptor in the inner ear first because we're familiar with that territory and we've done some preliminary experiments about that already.
Louise - So, for the moment at least, the jury is still out, but in the meantime, there are some very real potential spinoffs from this work that could benefit us too.
David - Humans often have disruptions in their spatial orientation ability, particularly people with dementia or people that have inner ear disease such as Meniere's disease. They find it very difficult to sometimes even find their way in the kitchen if the lights are turned off and they lose their way when they're driving from home to the grocery store. So, we have an idea now about how the brain is taking some signals and feeding in to this, what we call the navigation network. So we're hoping that will lend us some clues that we might be able to use in the future to help people with spatial orientation in spatial memory loss.
Chris - David Dickman speaking with Louise Anthony and that work was published this week in the journal Science.