Many animals have senses beyond the five found in humans: vision, hearing, smell, taste, and touch.  Magnetoreception, which plays a role in animal positioning, is one such sense and has been demonstrated in over 50 species, including representatives of all the major families of vertebrates.  In my search for a project that would combine my interests and skills in biology and electrical and computer engineering, the opportunity to investigate the biophysical basis of magnetoreception clearly provided me with an interesting venue to pursue.  While magnetite-based magnetoreception has received a lot of attention over the past 20 years, a second, light-dependent magnetic compass appears to exist in many animals, including amphibians, birds and flies [1].  However, the biophysical process underlying light-dependent magnetoreception has not been identified.  One of the most important clues to the identity of the underlying biophysical mechanism comes from remarkable new evidence for effects of very low level radiofrequency (RF) fields on the magnetic compass orientation of migratory birds [3] and amphibians [7].  Coming from an electrical engineering background, this research interests me, particularly because it has been assumed in the past that no biological system could respond to RF signals of such low magnitude. To show unambiguously that such RF effects are real, it is essential that they be demonstrated at the level of a change in spike activity in an individual neuron.

Confirming the existence of a novel sensory mechanism requires a combination of behavioral, neurophysiological, and biophysical approaches.  My interest is in developing a neurophysiological model system that can be used to characterize the neural mechanisms that underlie the light-dependent magnetic “compass.”  Several converging lines of evidence point to a role of the pineal complex of amphibians in magnetoreception [2], and I believe that the amphibian pineal is an excellent model system in which to test to investigate the possible effects of RF on a light-dependent magnetic sensor hypothesis. 

Behavioral evidence indicates that magnetic receptivity in both birds and amphibians is sensitive to the axis, rather than the polarity, of the geomagnetic field; in both groups, dip angle or inclination, rather than polarity, is used to distinguish between the two ends of the magnetic axis (a so-called “inclination” magnetic compass [5, 9]).  In addition, changes in the wavelength of light used in testing have been found to produce reproducible changes in magnetic orientation.  In newts, a 90° counterclockwise shift in the direction of the magnetic compass has been observed in animals trained to orient in a particular magnetic direction under full spectrum light and then tested under wavelengths greater than 500 nm.  In contrast, when newts were trained under long-wavelength light and tested under full spectrum light, they exhibited a 90° clockwise shift, indicating that changes in the wavelength of light were having a direct effect on the perception of the magnetic field.  The conclusion is that the underlying magnetoreception mechanism was sensitive to the wavelength of light.  The 90° shift can be explained by an antagonistic interaction between short wavelength (<450 nm) and longer wavelength (>500nm) light.  Intermediate wavelengths would cancel out patterns generated by the two activated systems.  Newts tested under 475 nm light indeed failed to show consistent orientation [6].  The wavelength dependence exhibited in the animal behavior is likely to be related to the biophysical process involved in magnetoreception (e.g. the two states of the radical-pair process or two separate receptors).

One biophysical mechanism that has been proposed to explain the light dependent magnetic compass orientation of amphibians and birds is a radical-pair process [1, 4].  In this process, a biochemical electron transfer reaction triggered by photoexcitation generates radical-pairs in which the alignment of an external magnetic field may induce a change in the spin states of the radical products.  The theoretical models indicate that these reactions are sensitive enough to be modulated by an earth-strength magnetic field.  The type of compass generated by such a process would be an inclination compass, as required by behavioral evidence, since changes in the magnetic field would influence the angle and magnitude of the effect.

In a magnetically-sensitive radical-pair process, RF fields should have an effect on the on the reaction.  These effects would be defined at specific frequencies that are in resonance with the hyperfine couplings of the radical-pair, i.e. 1-20 MHz.  On the other hand, magnetite particles would have a response over a larger range and would have maximum absorbance in the gigahertz range.  Additionally, the required strength of the RF signal in the case of a magnetite-based system is much higher than for a radical-pair-based system.  Testing under a very low level RF field (~10-100 nT) of specific frequencies in the 1 – 20 MHz range should cause a change in orientation if the studied magnetoreception is radical-pair based.  The first experiments validating the low frequency, low energy effects were recently published [3], and preliminary experiments have provided evidence for similar effects on magnetic compass.

Cryptochromes have been implicated as a possible class of photopigments that meet the requirements of the radical-pair mechanism.  These photopigments are involved in circadian rhythmicity in both plants and animals.  Cryptochromes belong to the same gene family as the photolyases, and the DNA repair function of photolyases involves a photoactivated radical-pair process.  The FAD-Trp radical-pair found in photolyases and cryptochromes has been suggested as a possible magnetoceptive candidate since it is affected by weak static magnetic fields [1].  Recently, cryptochromes have been associated with localized neural activity in the retinas of migratory birds showing magnetic orientation (in birds, the retina, rather than the pineal, appears to be the sight of the light-dependent magnetic compass), though more studies are needed to directly link cryptochromes to magnetoreception [8].

There are several reasons why the pineal organ in amphibians provides an excellent model system for studies of the biophysical basis of light-dependent magnetoreception.  Firstly, photoreceptors in the pineal complex of amphibians mediate celestial compass orientation involving the sun and polarized light patterns.  This signifies that at least one set of photoreceptors contains an ordered array of light absorbing molecules, which would also be required for a radical-pair based magnetoreception mechanism.  Secondly, so-called “chromatic units” in the frontal organ of frogs (an outgrowth of the pineal complex) exhibit properties similar to those implicated in the magnetic compass in newts.  Thirdly, the pineal is a site of the circadian clock, so cryptochromes are likely to be found there, thus providing a possible magnetoreceptor.  Finally, the amphibian pineal is much less complex than the eyes of birds, with much less peripheral processing of sensory inputs, simplifying the task of isolating magnetosensory signals.

Preliminary neurophysiological recordings from in the frontal organ (an outgrowth of the pineal complex) of frogs by John Phillips and Chris Borland has provided evidence for sensitivity to the alignment of an earth-strength magnetic field.  In these recordings, nerve signal spike counts were obtained from the frontal organ nerve during exposure to a 550 nm stimulus light.  These exposures alternated with a higher intensity 425 nm adapting light to suppress signals from achromatic units also found in the frontal organ nerve, as well as to maintain equilibrium between the short-wavelength inhibitory and long-wavelength excitory responses of the chromatic units.  The frogs were tested under different alignments of a horizontal, earth-strength magnetic field.  Bimodal patterns of response (i.e., independent of the polarity of the field) consistent with the behavioral responses in amphibians (see earlier discussion) were observed under particular wavelengths and intensities of light [7]. 

Coming into a biology lab with a computer and electrical engineering background has given me a different perspective on the experiments as well as giving me the necessary expertise to understand and maintain the complex array of magnetic field generation and electrical measurement equipment.  In addition the electrical signal analysis will be critical in determining if the experimental results match the hypothesis.  I have prior experience combining engineering with biology in the form of bioinformatics work I performed under Dr. Cynthia Gibas.  While not directly related, my earlier research showed me many parallels between computer and biological systems.

I have worked over the summer and current semester to upgrade the computer system involved in controlling the experimental apparatus and recording and analyzing the nerve signals in one of John Phillips’s laboratories at Virginia Tech.  Dr. Phillips is one of the leading researchers in the area of light-dependent magnetic reception, and several related experiments are currently ongoing in genetically engineered mice, newts, and fruit flies.  I am working closely with him to complete the set up of the magnetic field generation equipment and am preparing to perform the surgeries on the frogs necessary to perform the measurements.  Once completed, I will be performing the recording and analysis of the data.  If the pineal’s involvement in magnetoreception can be established, this will provide a model system to use in characterizing the biophysical mechanism and, in particular, to investigate the effects of low-level RF on this system.  Neurophysiological experiments have the potential to provide a critical link between theoretical mechanisms and behavioral responses, and to greatly increase our understanding of a fascinating new sensory mechanism in animals. 




[1] Cintolesi, F., et. al. Anisotropic recombination of an immobilized photoinduced radical pair in a 50µT magnetic field: a model avian photomagnetoreceptor. Chem. Phys. 294, 384-99 (2003).

[2] Deutschlander, M.E., et. al. Extraocular magnetic compass in newts. Nature 400: 324-325 (1999).

[3] Ritz, T., et. al. Resonance effects indicate a radical-pair mechanism for avian magnetic compass. Nature 429: 177-180 (2004).

[4] Ritz, T., et. al. A photoreceptor-based model for magnetoreception in birds. Biophys. J. 78, 707-18 (2000).

[5] Phillips, J.B. Two magnetoreception pathways in a migratory salamander. Science 233: 765-767 (1986).

[6] Phillips, J.B. and S.C. Borland. Behavioral evidence for the use of a light-dependent magnetoreception mechanism by a vertebrate. Nature 359: 142-144 (1992).

[7] Phillips, J.B. and S.C. Borland. Unpublished data.

[8] Mouritsen, H., et. al. Cryptochromes and neuronal-activity markers colocalize in the retina of migratory birds during magnetic orientation. Proc. Nat. Acad. Sci. 101: 14294-14299 (2004).

[9] Wiltschko, W., et. al. Red light disrupts magnetic orientation of migratory birds. Nature 364: 525-27 (1993).