The assumed centrally symmetric activation pattern on the retina illustrated by Ritz and colleagues thus does not seem unrealistic. If the birds turn their heads, this magnetic field-induced activation pattern would shift as a whole across the retina, so that the direction of the magnetic field can be detected independent of the birds�� position. The association of magnetoreception and ultraviolet/violet vision suggests an interrelationship between the two and raises the question whether magnetoreception is affected by the visually induced activity of the UV/V cones. Behavioral tests with migratory European robins under dim monochromatic light have shown that the birds were well oriented in their migratory direction under light from the entire short-wavelength range of the spectrum, indicating that their magnetic compass worked properly from below 372 nm UV to 565 nm green. The spectral emission of the green diodes used in these studies, does not contain any light in the range to which the UV/V cone could have responded. This indicates that magnetic directional information is mediated regardless of whether the UV cone opsin is light-activated or not �C in the initial stage, magnetoreception seems to occur independent of UV vision. Another observation supports this idea: When the ABH hydrochloride intensity of monochromatic lights was increased above certain levels in the behavioral tests, the robins failed to orient properly, indicating an interference with their magnetic compass. This was observed under bright UV, blue and green light alike. That is, a strong activation of the UV cones, but also a strong activation of the green cones with the UV cones not activated, disrupts the function of the magnetic compass in a similar way. These ABT-089 dihydrochloride effects, restricted to monochromatic lights as rather unnatural stimuli, suggest that the primary magnetoreception processes themselves are largely independent of the visual activation of the cones, and indicate interferences at higher processing levels. This leads to the question of how the radical pair mechanism of Cry1a generates the signal that mediates magnetic information. Vertebrate photoreceptors have ion channels for Na + and Ca2+ that are kept open by cyclic GMP, and the resulting dark current leads to slight depolarization.