The exposed surface of the brain is shown in the Figure in an anesthetized experimental preparation. The arrow points to an area where electrical stimulus was applied through a spherical Platinum electrode placed on the brain surface. Serum Albumin labeled with the dye Evan’s Blue, injected into the veins prior to stimulation can be seen leaking out into the tissue in the stimulation zone.
Around 1885, Paul Ehrlich found that an intra-venous injection of Evan's Blue into an animal stained all the tissues blue except for the brain. The specialized ‘blood-brain barrier’ regulates transport of substances to and from the fluid compartment bathing the brain tissues. The electrical stimulation applied during this experiment resulted in breakdown of the ‘Blood-brain barrier” and allowed the dye to leak and stain the local tissue. This breakdown is believed to be an early sign of neural injury.

Two frames from a video during a surface stimulation experiment are shown in the Figure. A spherical Platinum electrode can be seen on the surface of the brain,
Within 60 seconds of application of a Cathodic, monophasic stimulus train, the small vessels in the zone surrounding the electrode are seen to close down, vasoconstriction.