The human visual threshold depends on direction and strength of a weak magnetic 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.