Tests for Light Dependence

There are a number of tests that are derived from the necessary conditions of the radical pair mechanism. For example, if a radical pair is created through a light induced electron transfer, then the magnetic compass will be light dependent. Behavioral tests show that birds cannot orient under red and yellow ambient light conditions, while they can orient under monochromatic blue and green light. Newts show 90 degree shifts in magnetic orientation under long wavelength light. The light sensitive organ has been localized to be the right eye in birds and the pineal gland in newts. However, these light dependent effects could be due to an indirect influence of light on the magnetic compass and thus offer no conclusive evidence for the involvement of radical pairs in magnetoreception.

Animal Response Detected Magnetic Resonance

The sensitivity of radical pair reactions to the geomagnetic field is based on the influence of static magnetic fields on the spin states of the radical pair. However, there is a different way to influence spin states of a radical pair, namely through resonant electromagnetic radiation, i.e. through electromagnetic fields that provide precisely the energy that is needed to bridge the energy gap between two different spin states. The external static magnetic field as well as the static magnetic fields of nuclear spins in the radical pair system will induce an energetic splitting of spin states in the region of about 0.5 to 20 MHz. Therefore, weak electromagnetic fields at appropriate frequencies in the radio frequency 
(RF) range should disrupt or change magnetic orientation behavior if the magnetic compass were 
based on radical pair reactions.

In essence, a magnetic resonance experiment can provide a test for the radical pair hypothesis. However, the two main differences to standard magnetic resonance tests are that we detect the resonance very indirectly, namely through the magnetic orientation response of an animal in the presence of various RF fields. Secondly, the static field needs to be either the geomagnetic field or a field of similar strength, so that animals show their natural magnetic orientation response. The static field is therefore much weaker than the fields used in standard magnetic resonance techniques.

Our Research Program

In order to identify for which frequency and amplitude a RF field will disturb a radical-pair based magnetic compass, a quantitative understanding of oscillating and static magnetic field effects is necessary which requires solving the stochastic Liouville equation for an appropriate spin Hamiltonian. The physics of weak magnetic field effects is not yet fully understood because former research focussed mostly on effects of large fields, such as are employed in standard magnetic resonance techniques. 

Using the outcome of these calculations as input, the effect of weak RF magnetic fields on magnetic orientation behavior of various animals will be investigated at predetermined frequencies. We collaborate with several groups that have developed behavioral assays to study magnetic compass orientation. With the John Phillips lab, Virginia Tech, we investigate the effects of RF fields on newts (Notophthalmus viridescens) and fruit flies (Drosophila melanogaster, see picture for the fruit fly testing arena). With the Wiltschko lab, Frankfurt, Germany, we investigate RF effects on migratory birds.

Effects of RF fields on orientation behavior at the frequencies predicted and an 
absence of RF effects at other frequencies would provide compelling evidence that magnetic 
sensors are based on radical-pair processes and thus solve a fundamental problem of sensory 
biology.