epilepsy and other neurological disorders that are affected by the electrical potential difference between intracellular fluid and extra - cellular fluid and therefore the cell membrane potentials , and therefore the thresholds for the communication between brain cells can be controlled by re - circulating extra - cellular brain fluid after the fluid has been treated to alter its ion concentrations . a computer - controlled pump can precisely control the extraction and delivery of brain fluid after the ion concentration of the fluid is appropriately adjusted , e . g . guided by the goldmann equation . well - known techniques for modifying ion concentrations can be used to raise or lower ion concentrations as needed .