New model for the avian magnetic compass.

It is proposed that the avian magnetic compass depends on the angle between the horizontal component B(h) of the geomagnetic field (GMF) and E(r), the radial electric field distribution generated by gamma-oscillations within the optic tectum (TeO). We hypothesize that the orientation of the brain relative to B(h) is perceived as a set of electric field ion cyclotron resonance (ICR) frequencies that are distributed in spatially recognizeable regions within the TeO. For typical GMF intensities, the expected ICR frequencies fall within the 20-50 Hz range of gamma-oscillation frequencies observed during visual stimulation. The model builds on the fact that the superficial lamina of the TeO receive signals from the retina that spatially map the visual field. The ICR frequencies are recruited from the local wide-band gamma-oscillations and are superposed on the tectum for interpretation along with other sensory data. As a first approximation, our analysis is restricted to the medial horizontal plane of the TeO. For the bird to fly in a preferred, previously mapped direction relative to B(h), it hunts for that orientation that positions the frequency maxima at appropriate locations on the TeO. This condition can be maintained even as B(h) varies with geomagnetic latitude during the course of long-distance flights. The magnetovisual coordinate system (straight phi, omega) overlaying the two halves of the tectal surface in a nonsymmetric way may imply an additional orienting function for the TeO over and above that of a simple compass (e.g., homing navigation as distinct from migrational navigation). Copyright 2000 Wiley-Liss, Inc.