  By using a piezoelectric sensor, it was demonstrated that the visual evoked
potential of a rat brain was accompanied by mechanical movements of the brain
when it was excited. A phase of upward movement was found to be followed by a
phase of downward movement. The largest upward movement was a rise in swelling
pressure on the order of 100 micro gram, which was about 40 times larger than
that of the bullfrog sympathetic ganglion. The waves of mechanical movements
were more complicated than those of the evoked potentials. These findings are
thought to be due to the fact that the evoked potentials are propagated from
the immediate surroundings of the sensor, while the mechanical signals are
produced from anywhere beneath the piezo sensor. The mechanisms of mechanical
movements propagated in the brain by electrical stimulation are discussed.
