Neurons in a pigeon’s brain respond to magnetic fields

By Ed Yong | April 26, 2012 2:00 pm


In retrospect, the helmet should have been a clue…

Of all the super-senses that animals possess, the ability to sense the Earth’s magnetic field must be the most puzzling. We’ve known that birds can do it since the 1960s, but every new attempt to understand this ability – known as magnetoreception – just seems to complicate matters even further.

Take the latest discovery. Le-Qing Wu and David Dickman from the Baylor College of Medicine have found neurons in a pigeon’s brain that encode the properties of a magnetic field. They buzz in different ways depending on how strong the field is, and which direction it’s pointing in.

This is a big step. Scientists have identified parts of the brain that are important for magnetoreception, but no one has managed to nail down the actual neurons responsible for the sense. Miriam Liedvogel, who studies magnetic senses, calls it “a milestone in the field”.  It’s a key puzzle piece that has been unavailable for a very long time.

But Wu and Dickman’s discovery doesn’t solve the magnetoreception puzzle. If anything, it makes it more complex. Until recently, scientists thought that birds had two separate magnetic detectors – one in the eye and one in the beak. And it looks like the new magnetic neurons don’t hook up to either of these. “We can’t say where the signals come from,” says Dickman.

If these neurons are responding to magnetic fields, which part of the bird is feeding them their information? Is there a third sensor?

Finding the neurons

Wu and Dickman found their neurons by placing pigeons in a set of coils that can produce bespoke magnetic fields. First, they programmed the coils to cancel out the Earth’s magnetic field around the pigeon’s head. Next, they created fields of their own, and gradually altered their strength and direction.

As the fields shifted, Wu and Dickman recorded the activity of individual neurons in the pigeons’ vestibular brainstem – an area that connects the brain and spine, and is involved in balance. Based on earlier experiments, they knew that neurons in this area fire when pigeons are using their magnetic sense.

The duo found 53 neurons that fire at different strengths depending on how strong the magnetic fields around them are. They’re most sensitive to a range of intensity that’s naturally produced by the Earth’s actual magnetic field. They also fire differently depending on where the field is pointing along the horizon (the azimuth), where it points above or below the horizon (the elevation), and the direction it points in (the polarity).

The last bit was a surprise. Earlier experiments from the 1970s showed that birds aren’t sensitive to the polarity of the Earth’s magnetic field.  But Wu and Dickman’s experiments suggest otherwise. As “north” moves around the bird’s head, the neurons fire at their fullest in one direction, and at their weakest in the opposite one.

Now, the duo wants to understand how birds use this information to navigate. It’s easy to take a guess. Sensing the azimuth tells the bird where to head, just like a compass. Sensing the elevation provides information about latitude. Sensing intensity could tell the bird where exactly it is, because the strength of the Earth’s magnetic field varies from place to place, often at a very fine scale. This is all plausible in theory, but how it works in practice is another matter. And there is an even bigger mystery…

Where’s the sensor?

If these neurons are processing magnetic fields, what’s feeding them with information? Where’s the compass? We know that birds have a compass in their eyes, which relies on a protein called cryptochrome and a dash of quantum physics. When the protein is struck by blue light, it shunts one of its electrons to a partner molecule called FAD. Electrons normally waltz in pairs, and molecules with lone electrons – as cryptochrome and FAD now do – are called radicals.

Electrons have a property called ‘spin’. The solo electrons of cryptochrome and FAD are linked, so that they either spin together or in opposite directions. These states have different chemical properties, and the radical pair can flip between them. Here’s where the Earth’s magnetic field comes in – it can influence these flips. In doing so, it can affect the outcome or the speed of the chemical reactions involving the radical pair.

This explains how living cells can be sensitive to something as weak as the Earth’s magnetic field. It also explains why birds need to see for their internal compasses to work. A robin in a shuttered cage will try to escape in a fixed direction, even if it can’t see any landmarks. If you cover the robin’s right eye, it loses its bearings. (Read my feature from New Scientist for the story of the cryptochrome discovery.)

Cryptochrome is found in the retina, and it seems to connect to a part of the brain called Cluster N. To this date, we still don’t know how information about magnetic fields gets from the eye to the brain, or how it’s processed there. Thorsten Ritz, who helped to uncover the role of cryptochrome, once told me: “We need to find that magnetic neuron.”

Wu and Dickman’s study might look like it provides the answer, but it doesn’t. The brainstem neurons they identified aren’t anywhere near Cluster N, and they responded to the polarity of magnetic fields. “The cryptochrome idea can’t give you polarity information,” says Dickman.

The duo thinks that the properties of their magnetic neurons are more consistent with a sensor that uses magnetite – a magnetic iron mineral that could act as tiny compass needles. Until recently, scientists thought that birds have such as sensor in their beaks. Neurons in their bills supposedly contained magnetite crystals, and send information to the brain via the trigeminal nerve. Cutting the nerve robs pigeons of their ability to detect strong magnetic fields. Everything added up.

But a study published earlier this month blew that idea apart. Christoph Treiber showed that the iron deposits in the beak are actually are found in immune cells called macrophages, not neurons, and they don’t contain magnetite. Instead, these cells are simply recycling iron from red blood cells, rather than using it to sense magnetic fields. There might still be a magnetic sensor in the beak (how else could you explain the effects of cutting the trigeminal nerve?) but it’ll be a very different to the one that people had assumed.

So if the magnetic neurons in the brainstem aren’t getting their signals from the eye or the beak, what’s the alternative?  Dickman thinks that the answer lies in the inner ear, and that’s where he is currently looking. “Maybe there are two or even three receptors that come together in the brain to work together,” he says. For example, as you move about, your brain combines what you see with your eyes with information form the inner ear. Perhaps the magnetic sense works in the same way.

Reference: Wu & Dickman. 2012. Neural Correlates of a Magnetic Sense. Science http://dx.doi.org/10.1126/science.1216567

Image via Wikipedia

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Comments (10)

  1. Alex

    Very interesting. Have there been similar tests in other animals? Perhaps the response to changes in the magnetic field occurs in the neurons of many animals and is not functionally relevant for navigation.

  2. Davide Castelvecchi

    Actually, none of the experts I talked to when I wrote about this (Scientific American, January 2012) said that the cryptochrome theory was fait accompli. Not even its staunchest proponents thought it had been proven true yet. And whether FAD is the molecule involved is highly speculative. The results published in Nature were presented at a meeting last year. I couldn’t go into detail in my piece but I did quote people saying that that the beak magnetite model had been discredited. Also, no one seemed to buy the eyepatch results either. What people did tell me is that the trigeminal nerve may be involved because it also carries information from the snout… that is where people are betting that the magnetite sensors are located.

  3. Alex: magnetoperception has been pretty clearly implicated in navigation. For example, when robins are placed in a magnetic field polarized opposite to the Earth’s they migrate in the wrong direction (eg, heading south instead of north).

  4. Ali Cat

    Do ocean mammals have similar abilities?
    Maybe if we can locate it in one animal it will help us locate it in other animals.
    When we sense EM fields from things like crystals and unearthed electrical devices, what part of us is perceiving these sensations?
    Is it at all similar to magneto bird?
    BTW, Love the picture! :)

  5. “Do ocean mammals have similar abilities?”

    Not to anyone’s knowledge. One dolphin can sense electric fields, but no magnetic sense yet. Turtles (not mammals, but ocean animals) can sense magnetic fields. But the mechanism is unclear. See one of the links at the end.

    “When we sense EM fields from things like crystals and unearthed electrical devices, what part of us is perceiving these sensations?”

    Your imagination.

  6. Ali Cat

    Also, fish seem very sensitive to magnetism, and other vibrations like microwaves, my very friendly fish freak out if I use my iPhone too close to their tank. And calm down when I take it away.
    Do some migrating fish use the earth’s magnetism to navigate? Is it possible they may hold a clue to how it works?

  7. “Turtles (not mammals, but ocean animals)”.

    Besides pigeon and turtles there is some arthropods with “magnetic sensors”, right?

  8. Yep. Some beetles. Ants. I *think* spiny lobsters…

  9. Ali Cat

    Hi Ed,
    Thanks for your reply.
    I was wondering more about whales, dolphins and porpoises. Do they use the Earth’s magnetism?
    With pigeons, is it possible that the position of the trigeminal nerve allows it to detect a polarity within the iron in the beak even if it is not magnetite? Can the iron be doing two jobs?

    I question the dead end thinking that something is just imagination. Surely there is a spectrum of sensations between nothing and electrocution? Depending upon the sensitivity of the individual. Ever ‘tasted’ a 9volt battery?

    Two other examples, I can tell you if hematite has been magnetized or not, by how it feels in my hand, not everyone is able to discern this subtle difference but it’s probably just practice. Magnetized hematite makes my hand ache, it feels like resistance, I’m only speculating but perhaps it effects the flow in my own magnetic field, effecting they way localized nerves talk to each other.
    Also, When my child is holding their iPod and it is being charged using only a 2 prong plug (Australian, unearthed) I can feel the current when I touch them. It’s the same fuzzy buzz sensation I get from my lap top when it is also charging with only the 2 prong plug.
    When I switch to the 3 prong plug the unpleasant sensation is not present because it is earthed.

    So what part of me is detecting the electrical current and/or magnetism and what part is perceiving the electrical current and/or magnetism?

    It seems pointless to dismiss these phenomena as imagination, if there could be some clues to how electrical and magnetic fields might effect us, considering we are bio-electrical, we have iron in our hemoglobin, we evolved on a planet with magnetic fields, and we are now surrounded by man made electrical fields. Don’t we owe it to ourselves to find out how these things effect us?
    Just because we rely on GPS now doesn’t mean that ancient cultures didn’t use the earth’s magnetism to navigate, perhaps our bombarded modern lives have cheated us out of some amazing abilities that only ancient or seemingly primitive cultures could use.

    It’s exciting to speculate, and even more exciting when that speculation bares fruit.

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