Imaging of the brain response to magnetoreception
Biophysics
Final Report Abstract
Many animals are able to sense steady magnetic fields even weaker than the earth field and exploit this extraordinary capacity for orientation, navigation and homing. The primary goal of this project was to find out how magnetoreception works physically and neuro-physiologically. Our approach was to develop imaging techniques capable to localize brain areas stimulated by weak magnetic fields which could be the clue to location and working principle of the magneto-receptor. Given than functional magnetic resonance imaging, fMRI, is precluded for its strong background magnetic field, we developed two new methods to visualize pathways of neural activity in small animals under controlled laboratory conditions: Near-Infrared Diffusing Wave Spectroscopy (NIR-DWS), a noninvasive optical speckle interference technique, and functional Doppler ultrasound imaging (fUS). Both monitor in real time stimulus related changes of blood flow and blood volume in the brain due to neurovascular coupling. After several years of methodological development to suppress artifacts, improve data reproducibility, signal to noise and resolution, both techniques compare as follows: While DWS is more sensitive (resolving % changes) it has much poorer spatial resolution because of multiple light scattering in tissues and bones. High frequency fUS, in contrast, has excellent 3D imaging performance, with spatial resolution of about 100µm and sensitivity in the % range. We have identified and localized primary neural pathways for visual and auditory stimulation in pigeons, auditory pathways in song birds, as well as mechanosensory response in rats, with stimulus related signal changes up to 20%, illustrating feasibility, sensitivity and potential of our methods. Our DWS data suggest that functional signals evoked by magnetic fields in the earth’s field range are about one order of magnitude smaller and hence more difficult to quantify. Magnetoreceptive stimulation patterns could not be detected by fUS and thus could not be localized so far. In an attempt to quantify respective contributions from flowing blood and electrical activity of neurons to the DWS signal we performed light scattering experiments on erythrocytes in suspensions and light scattering and birefringence measurements on electrically stimulated nerve cells. These experiments confirm that DWS from living brain tissue is dominated by vascularization but a small depolarized light scattering change due to nerve firing could be observed as well. In order to monitor overall navigational performances of our pigeons and to quantify their sensitivity to earth’s magnetic field gradients we performed homing experiments by GPS-tracking flight paths in Southern Germany and the Ukraine. We developed a simulation tool which predicts homing tracks by coupling the virtual pigeon to topographic and magnetic features taken from corresponding area maps. The surprising agreement between measured and simulated tracks suggests that pigeons use a combination of magnetic and topographic information for homing. Finally, in external collaborations we performed high resolution animal MRI as well as Magnetic Particle Imaging (MPI) and quantitative susceptibility mapping (QSM) of pigeon heads to search for magnetic partice based receptors noninvasively. Unfortunately, due to limited resolution and contrast of these techniques as implemented here, no particle-rich areas were detected and therefore no conclusions possible.
Publications
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Pulse wave analysis with diffusing-wave spectroscopy, Biomedical Optics Express 8, 3493-3500 (2017)
Markus Belau, Wolfgang Scheffer and Georg Maret
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3D Functional Ultrasound Imaging of Pigeons, NeuroImage, 183, 469-477 (2018)
Richard Rau, Pieter Kruizinga, Frits Mastik, Markus Belau, Nico de Jong, Johannes G. Bosch, Wolfgang Scheffer and Georg Maret
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Cerebral blood flow measured with diffusing wave spectroscopy during anesthesia, BioRxiv
Markus Belau, Wolfgang Scheffer and Georg Maret
