Orientation in the geomagnetic field is essential for many animal species. As yet, the interaction mechanisms of this weak field with the organisms are understood only incompletely. One mechanism in question is the interaction with the photochemical reaction in the retina. We show that the visual sensitivity of man is influenced by periodic sinusoidal inversion of the vertical component of the geomagnetic field. This effect indicates visual fixation in north-south direction and shows a pronounced resonance at a period duration of 110 s. These findings should be helpful in identifying in detail the mechanisms which are influenced by the geomagnetic field.

Completing our investigations concerning influences of weak magnetic fields on the visual discrimination threshold, we have investigated 30 subjects (20 female, 10 male) under rotation of the geomagnetic field without change in field strength. We related the thresholds to the values measured in the unchanged field. Comparison with a control experiment shows that correspondence between viewing and field direction results in a significant decrease of the threshold. The significance is lost if the angle between the directions is set to 20 degrees. On the basis of these and our earlier results an attempt is made to give a very simple description of the connection between field direction, field strength, and the change in the visual discrimination threshold.

The proximity of a mobile phone to the human eye raises the question as to whether radiofrequency (RF) electromagnetic fields (EMF) affect the visual system. A basic characteristic of the human eye is its light sensitivity, making the visual discrimination threshold (VDThr) a suitable parameter for the investigation of potential effects of RF exposure on the eye. The VDThr was measured for 33 subjects under standardized conditions. Each subject took part in two experiments (RF-exposure and sham-exposure experiment) on different days. In each experiment, the VDThr was measured continuously in time intervals of about 10 s for two periods of 30 min, having a break of 5 min in between. The sequence of the two experiments was randomized, and the study was single blinded. During the RF exposure, a GSM signal of 902.4 MHz (pulsed with 217 Hz) was applied to the subjects. The power flux density of the electromagnetic field at the subject location (in the absence of the subject) was 1 W/m(2), and numerical dosimetry calculations determined corresponding maximum local averaged specific absorption rate (SAR) values in the retina of SAR(1 g) = 0.007 W/kg and SAR(10 g) = 0.003 W/kg. No statistically significant differences in the VDThr were found in comparing the data obtained for RF exposure with those for sham exposure.

Experiments involving the orientation of animals, especially birds, have shown that the visual system and geomagnetic field are linked. On the basis of these findings, we have addressed the question of whether there is an influence of the geomagnetic field on light-sensitivity of the visual system in man. Here we report on measurements of the photopic sensitivity of the human visual system for a 0.5s test-stimulus without magnetic field and with the full local field of 48 microT rotated into the line of view. The experiments show that the geomagnetic field increases this sensitivity by 6-7%, depending on the stimulated area. Based on this increase the probable change in perception of the background luminance was calculated by the Weber-law.

These results suggest that attention devoted to a precision movement intensifies its phase coupling with breathing and enhances MRC-related improvement of tracking precision.