Magnetic Navigation of Animals, Contrast Sensitivity of Vision, and the Weber-Fechner Law

It is known that some animals can react to very small changes in the magnetic field – a thousand times smaller than the geomagnetic field – and use this to navigate the Earth’s magnetic landscape. However, the nature of the molecular magnetic sensor remains unclear, although it has been established that the magnetic sense is associated with vision. It is generally accepted that the operation of a magnetic sensor is based on a magnetochemical reaction. Cryptochromes of photoreceptors lining the retina contain photoinduced spin-correlated pairs of radicals involved in the formation of a nerve impulse and sensitive to a magnetic field. Therefore, the animal could sense the magnetic field as a change in the brightness of large visual fields and orient itself by their contrast. However, the sensitivity of individual sensors – of radical pairs – is known to be very low. Previously, it has been assumed that this difficulty is overcome by a statistical increase in contrast sensitivity due to the parallel processing by the brain of the primary signals of millions of photoreceptors. In the present work, this hypothesis is tested. It has been found that the threshold sensation of brightness contrast almost linearly depends on the logarithm of the angular size of contrasting stimulus, which is typical for the physiology of sensations that obey the Weber-Fechner law. Contrast sensitivity increases with the number of photoreceptors involved in stimulus recognition, however this increase is not quantitatively sufficient to reliably explain the magnetic navigation of animals.