Recent work due to the proliferation of cell phones has indicated that the human
body does indeed react to small geomagnetic fields. This appear to occur because
some chemical reactions are extremely sensitive to small EM changes. These
reactions increase or decrease greatly when a person is exposed to power lines
or other EM radiation such as emitted by cell phones.
Studies of the brain of people using cell phones indicate that a change in the
brain changes and lasts for several hours after use of cell phones. Probably
some relation to a chemically reactive substance.
At any rate, there is now ample evidence that the brain does respond to changes
(small) in EM field conditions.
The magnetite nodules have been found in a number of species and appear to
be a way to orient ourselves with the magnetic field. I am not sure if they have
been discovered in human brains, however.