Magnetoreception – a Quantum Sixth Sense

Posted by Seraphina Solders on May 20, 2021 in Animals, Neuroscience, Perception, Physics and Neuroscience, Sensory Systems | Leave a comment

Imagine you are dropped off hundreds of miles away from your home, deep in some unknown  forest. Would you be able to find your way home using only your five basic senses – sight, smell, sound, taste, and touch? If you’re anything like me, you may struggle to navigate around your own city without help from a GPS. However, some animals are actually able to accomplish this impressive navigational feat, and scientists believe they may be using a special sixth sense called magnetoreception. This is the ability to sense magnetic fields, usually that of the Earth, in order to orient and navigate.

Most of the evidence we have for magnetoreception in animals comes from studies of migratory birds, such as homing pigeons, which are able to find their way home after being taken hundreds of miles away from their nests. However, lots of other animals have this magnetic compass as well. For instance, sea turtles, which migrate extensively throughout their lives, use a combination of wave direction and the Earth’s magnetic field to make their way across oceans as they travel between feeding and nesting sites [1]. There is also evidence that Pacific salmon, which spend years out in the ocean before finding their way back to their home river to lay eggs, use a magnetic sense to orient themselves in the waters [2]. Magnetoreception doesn’t only exist in migratory animals, however. There are also observations that suggest magnetoreception exists in mammals, and that in addition to helping with navigation, this sense may help animals carry out daily spatial orientation tasks. For example, cows grazing in pastures tend to orient themselves to be aligned with the magnetic north-south poles, even after ruling out other explanations such as shielding from the wind or basking in the sun [3]. Off-leash dogs tend to align themselves with the magnetic poles when defecating or urinating [4], and a recent study suggests that dogs also use magnetoreception while scouting [5]. Red foxes, who jump high into the air and then nosedive into the snow to catch prey (a technique adorably called “mousing”), are thought to use magnetic alignment to help them determine the precise distance and direction of attack [6]. How exactly any of these animals are able to detect magnetic fields has long been a mystery. Recently, thanks to researchers studying magnetoreception, we are beginning to fill in the picture.

There are currently two leading hypotheses about how animals are able to detect magnetic fields: one is a chemical reaction that involves a type of protein called a cryptochrome, and the other is a mechanical reaction that involves a form of iron called magnetite.

Cryptochrome hypothesis

The basis for the cryptochrome hypothesis for magnetoreception stems from the discovery that the magnetic compass of some animals depends on light (meaning the magnetic sense only works at certain wavelengths or intensities of light), suggesting some kind of light-dependent chemical reaction must be taking place. In 1978, Klaus Schulten, a German-American biophysicist studying the impact of magnetic fields on biochemical reactions, proposed such a mechanism – a radical pair reaction [7]. Radicals are molecules with unpaired electrons (negatively charged subatomic particles that exist in all atoms), and they are produced when light excitation causes one molecule to release one of its electrons and give it to another molecule, creating a pair of radicals (the radical pair). These unpaired electrons are affected by magnetic fields in a manner that depends on the precise orientation of these fields. In this way, the reaction could serve as an internal magnetic compass. Interestingly, there is only one known type of protein in vertebrates that is capable of forming these light-induced radical pairs – cryptochrome. Even more interestingly, scientists have found high levels of cryptochromes in the retinas of migratory birds, suggesting that these animals may be able to see magnetic fields! 

Two research groups published articles in 2018 giving evidence that a specific cryptochrome in the bird eye’s retina, Cry4, may play a role in magnetoreception. In one study, scientists measured the gene expression of three different types of cryptochrome proteins, Cry1, Cry2 and Cry4, in the eyes of zebra finches throughout the day [8]. They were looking to see whether the expression levels of these proteins changed throughout the day, as cryptochromes are also known to be involved in the day-night biological clock (the circadian rhythm). They hypothesized that if any of these cryptochromes were involved in magnetoreception, which is used not only for migration but also for spatial orientation tasks required throughout the day, the expression levels should be relatively constant. On the other hand, if the expression levels of these cryptochromes fluctuated or showed any circadian rhythmicity, that would be evidence that they are primarily involved in the night-day cycle rather than magnetoreception. They found that Cry1 and Cry2 varied throughout the day as expected for circadian clock genes, while Cry4 was expressed at constant levels over time. 

Around the same time, the other research group measured cryptochrome expression levels in birds during migratory and non-migratory seasons [9]. They found that Cry4 was expressed at much higher levels during migratory seasons compared to non-migratory seasons, while Cry1 and Cry2 levels were relatively unchanged. Together, these studies provide evidence that the Cry4 cryptochrome is a prime candidate magnetoreceptor, at least in migratory birds. However, this leaves the question of how birds use magnetic fields to migrate at night, when there is no light to trigger the radical-pair reaction.

Magnetite hypothesis

The other leading hypothesis for a magnetoreception mechanism involves crystals of a form of iron called magnetite, which is the most strongly magnetic naturally occurring mineral. Magnetite crystals are widely found in organisms, from magnetotactic bacteria to various animals. The basic idea here is that external magnetic fields exert a force on the crystals (just like two magnets attracting each other) and cause them to rotate, and this movement could be detected by nearby cells designed to notice such movement, potentially serving as a magnetic sensor.

As mentioned before, much of our scientific research on magnetoreception has been done in birds. Clusters of magnetite have been found in the upper beak of migratory birds, including homing pigeons [10], which is innervated by the trigeminal nerve (an important nerve for sensations in the face). Further, scientists demonstrated that changing magnetic fields lead to high neuronal activation in and near the brain areas receiving input from this nerve [11]. This is solid evidence that birds use at least two different strategies to detect magnetic fields – both cryptochromes and magnetite – which may be particularly helpful since the cryptochrome mechanism requires daylight, and birds do not migrate strictly during the day.

While the magnetite hypothesis seems to make sense in light of the evidence we have from birds, scientists are hesitant to assume a similar mechanism is occurring in some other animals in which we have found magnetite. Iron is required for proper biological functioning in most organisms, so the presence of magnetic iron molecules does not necessarily imply a meaningful interaction with magnetic fields, unless it can be shown to occur at specific and consistent locations in the body and is linked to the nervous system (such as the magnetite in the bird beak linked to the facial nerve) [12]. Because so many animals have naturally occurring magnetite in their bodies, researchers are still studying whether and how it may contribute to magnetoreception.

Do humans have a magnetic sense?

As magnetoreception is a fascinating topic, it may come as no surprise that scientists have been trying to determine whether humans have this sense in any capacity. Cryptochromes, which are not unique to birds and often function as circadian proteins in animals, also exist in the human eye. In 2011, researchers performed a study with fruit flies, which can naturally detect magnetic fields with their cryptochromes. They showed that removing the flies’ Cry genes abolished their ability to respond to magnetic fields, and that inserting the human Cry2 gene into the flies not only recovered this ability, but did so in a light-dependent manner [13]. This suggests that the human Cry2 protein can perform a similar magnetoreceptive role as the cryptochromes in the bird eye. However, the authors concede that this is not evidence that this gene carries out the same function in humans – indeed, we have no evidence to believe humans can see magnetic fields. Additionally, magnetite crystals have been found in the human brain [14], but there has been no evidence that they allow us to detect magnetic fields… until recently.

In 2019, a research team led by Dr. Connie Wang at California Institute of Technology measured electrical activity on the human scalp with electroencephalography (EEG) while varying the external magnetic field to mimic what a person might experience while moving around in the world [15]. They found that varying the field consistently led to a drop in amplitude of alpha waves, which are typically associated with a brain at rest (and a reduction of alpha wave amplitude is associated with sensory stimulation like vision or cognitive processing – basically anything other than rest). Further, this consistent drop in alpha amplitude only occurred when the magnetic field was rotated horizontally and the vertical field held static and directed downwards, which mimics the rotation of the Earth in the Northern Hemisphere. Fascinatingly, they were able to rule out any free-radical/cryptochrome based mechanism (while the explanation is beyond this article, briefly: the neural response was sensitive to the polarity of the magnetic field, which cryptochromes are not). Therefore, while this is not direct evidence per se for the magnetite hypothesis, such a process remains the only plausible mechanism we currently know of to explain the results. 

Nonetheless, at the end of the day these findings are still not proof that humans can actually detect magnetic fields, consciously or subconsciously. Indeed, because magnetite is distributed widely throughout our brain tissue [14] and is not (as far as we know) connected to our nervous system, it is unlikely that we can use its interaction with external magnetic fields in any meaningful way. But the fact that we can detect neural responses to changes in magnetic fields is still fascinating, as is the fact that many animals today still actively sense and respond to magnetic fields.

References

[1] Goff, M., Salmon, M., & Lohmann, K. J. (1998). Hatchling sea turtles use surface waves to establish a magnetic compass direction. Animal behaviour, 55(1), 69–77. https://doi.org/10.1006/anbe.1997.0577

[2] Naisbett-Jones, L. C., Putman, N. F., Scanlan, M. M., Noakes, D., & Lohmann, K. J. (2020). Magnetoreception in fishes: the effect of magnetic pulses on orientation of juvenile Pacific salmon. The Journal of experimental biology, 223(Pt 10), jeb222091. https://doi.org/10.1242/jeb.222091

[3] Begall, S., Cerveny, J., Neef, J., Vojtech, O., & Burda, H. (2008). Magnetic alignment in grazing and resting cattle and deer. Proceedings of the National Academy of Sciences of the United States of America, 105(36), 13451–13455. https://doi.org/10.1073/pnas.0803650105

[4] Hart, V., Nováková, P., Malkemper, E. P., Begall, S., Hanzal, V., Ježek, M., Kušta, T., Němcová, V., Adámková, J., Benediktová, K., Červený, J., & Burda, H. (2013). Dogs are sensitive to small variations of the Earth’s magnetic field. Frontiers in zoology, 10(1), 80. https://doi.org/10.1186/1742-9994-10-80

[5] Benediktová, K., Adámková, J., Svoboda, J., Painter, M. S., Bartoš, L., Nováková, P., Vynikalová, L., Hart, V., Phillips, J., & Burda, H. (2020). Magnetic alignment enhances homing efficiency of hunting dogs. eLife, 9, e55080. https://doi.org/10.7554/eLife.55080

[6] Červený, J., Begall, S., Koubek, P., Nováková, P., & Burda, H. (2011). Directional preference may enhance hunting accuracy in foraging foxes. Biology letters, 7(3), 355–357. https://doi.org/10.1098/rsbl.2010.1145

[7] Schulten, K., Swenberg, C., & Weller, A. (1978). A Biomagnetic Sensory Mechanism Based on Magnetic Field Modulated Coherent Electron Spin Motion. Zeitschrift für Physikalische Chemie, 111, 1 – 5.

[8] Pinzon-Rodriguez Atticus, Bensch Staffan and Muheim Rachel (2018). Expression patterns of cryptochrome genes in avian retina suggest involvement of Cry4 in light-dependent magnetoreceptionJ. R. Soc. Interface.152018005820180058

[9] Günther A, Einwich A, Sjulstok E, et al. Double-Cone Localization and Seasonal Expression Pattern Suggest a Role in Magnetoreception for European Robin Cryptochrome 4. Curr Biol. 2018;28(2):211-223.e4. doi:10.1016/j.cub.2017.12.003

[10] Fleissner, G., Holtkamp‐Rötzler, E., Hanzlik, M., Winklhofer, M., Fleissner, G., Petersen, N. and Wiltschko, W. (2003), Ultrastructural analysis of a putative magnetoreceptor in the beak of homing pigeons. J. Comp. Neurol., 458: 350-360. https://doi.org/10.1002/cne.10579

[11] Heyers D, Zapka M, Hoffmeister M, Wild JM, Mouritsen H. Magnetic field changes activate the trigeminal brainstem complex in a migratory bird. Proc Natl Acad Sci U S A 2010,107:9394-9399.

[12] Hore PJ, Mouritsen H. The Radical-Pair Mechanism of Magnetoreception. Annu Rev Biophys. 2016;45:299-344. doi:10.1146/annurev-biophys-032116-094545

[13] Foley, L., Gegear, R. & Reppert, S. Human cryptochrome exhibits light-dependent magnetosensitivity. Nat Commun 2, 356 (2011). https://doi.org/10.1038/ncomms1364

[14] Kirschvink JL, Kobayashi-Kirschvink A, Woodford BJ. Magnetite biomineralization in the human brain. Proc Natl Acad Sci U S A. 1992;89(16):7683-7687. doi:10.1073/pnas.89.16.7683

[15] Wang CX, Hilburn IA, Wu DA, et al. Transduction of the Geomagnetic Field as Evidenced from alpha-Band Activity in the Human Brain. eNeuro. 2019;6(2):ENEURO.0483-18.2019. Published 2019 Apr 26. doi:10.1523/ENEURO.0483-18.2019

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