The current used in electroconvulsive therapy (ECT) is usually in the region of 800 milliamps (mA). Although it is strong enough to cause instance unconsciousness (if someone isn’t anaesthetised) and a seizure, the current is sometimes described as “small” or even “very small”. So when it was announced a couple of weeks ago that a team of scientists at Oxford University had found that passing a current of 1mA (yes, that is one milliamp as opposed to the 800 milliamps used in ECT) through people’s heads could improve their scores on a couple of numerical tests, I was curious to know how the current would be described. The answer is “tiny”, according to the BBC. You can read the study here. When current flowed in the opposite direction the effect was reversed.
There is nothing new about transcranial direct current stimulation. Experiments were done with it as a possible psychiatric treatment in the 1960s and 1970s. In the past decade there has been a renewed interest in it.
For information on current strength that is a bit more specific than small and tiny, there is a table on page 49 of the Handbook of electrical hazards and accidents, edited by Leslie A. Geddes (CRC Press, 1995). Here is a slightly abbreviated version. The figures in brackets are for women.
Effect Direct current (mA) 60-Hz current (mA rms)
No sensation on hand 1 (0.6) 0.4 (0.3)
Slight tingling 5.2 (3.5) 1.1 (0.7)
Shock – not painful 9 (6) 1.8 (1.2)
Painful shock 62 (41) 9 (6)
Painful and severe shock* 90 (60) 23 (15)
*muscular contractions, breathing difficult
The latest machines from Ectron Ltd deliver a current of 750 or 900 mA.