Transcranial stimulation

1. A method comprising:

coupling a plurality of electrodes to a patient's head;

identifying whether two or more of the electrodes form a functional set, such that a desired therapeutic effect can be achieved when the two or more electrodes deliver a total amount of current to the patient regardless of what portion of the total amount of current each respective electrode in the functional set delivers;

providing one or more constant current sources, each having a supply terminal and a return terminal, wherein the current supplied at the supply terminal is equal to current returned to the return terminal;

coupling the one or more constant current sources to the electrodes such that each supply terminal and each return terminal is coupled to no more than the electrodes of a single one of the functional sets, if any, or to a single one of the electrodes not included in one of the functional set;

monitoring a voltage being produced across the constant current source's supply and return terminals to provide at least an indirect measure of resistance; and

in response to one of the monitored voltages exceeding a threshold value:

reducing an amount of current passing through a corresponding one of the electrodes; and

increasing an amount of current being delivered at a different one or more of the electrodes by an amount approximately equal to the amount of current that is reduced.

2. The method of claim 1 , wherein a rate of change associated with reducing or increasing the amounts of current flowing does not exceed approximately 1 milliamp per second.

3. The method of claim 1 , further comprising:

coupling more than one of the supply terminals or more than one of the return terminals to the electrodes of a single one of the functional sets, if any, or to a single one of the electrodes, if any, not included in the functional sets.

4. The method of claim 1 , further comprising:

delivering a substantially constant amount of current to each respective functional set, if any; and

delivering a substantially constant amount of current to each respective electrode, if any, not included in one of the functional sets.

5. The method of claim 4 further comprising:

limiting a rate of change associated with the current being delivered to each electrode to no more than approximately 1 milliamp per second.

6. The method of claim 1 , wherein the plurality of electrodes includes at least one functional set.

7. The method of claim 1 , wherein the plurality of electrodes includes at least one electrode not included in one of the functional sets.

8. The method of claim 1 , wherein identifying the one or more functional sets comprises considering one or more of the following factors:

physical proximity of same polarity electrodes to one another;

electrical resistance between adjacent same polarity electrodes; and

physical arrangement of electrodes having a first polarity relative to electrodes of a second polarity.

9. The method of claim 8 wherein a functional set can include one or more of the following:

a pair of same polarity electrodes that are within about 4 centimeters, more preferably less than about 2 centimeter of one another and more preferably touching one another;

a pair of same polarity electrodes that have a resistance between them of less than about 1 kOhm, less than about 200 ohms, less than about 100 Ohms, less than about 10 Ohms, less than about 1 Ohm or close to zero ohms; and

a same polarity electrode or plurality of same polarity electrodes surrounded by three or more electrodes having an opposite polarity.

10. The method of claim 1 , wherein two or more electrodes that are physically in contact with one another form a functional set.

11. The method of claim 1 , wherein identifying whether two or more of the electrodes form a functional set comprises:

measuring or estimating a resistance from one electrode to another electrode of the same polarity.

12. The method of claim 1 , wherein identifying whether two or more of the electrodes form a functional set comprises identifying one or more electrodes of a first polarity substantially surrounded by four electrodes of a second polarity different than the first.

13. The method of claim 1 , wherein each electrode can be included in no more than one of the functional sets.

14. The method of claim 1 , wherein the one or more electrodes that experience an increase in the amount of current flowing are in a functional set with the electrode that experiences a reduction in the amount of current flowing.

15. The method of claim 1 , wherein the desired therapeutic effect can be achieved as long as the electric field strength produced by the current flow at any location within the target tissue is not changed by more than approximately 50%, or wherein a peak electric field in the brain is not changed by more than 15%, or wherein an area of cortical surface that is greater than 50%, 75%, or 90% or the peak cortical field does not change more than 10 cm2, or wherein an average electric field in the target tissue does not change by more than approximately 50% OR any combination of the foregoing.

16. The method of claim 1 , wherein the two or more electrodes forming the functional set are substantially surrounded by a group of outer peripheral electrodes of the opposite polarity.

17. The method of claim 1 , further comprising:

substantially changing the distribution of the total current across the electrodes of the functional set.

18. A method comprising:

coupling a plurality of electrodes to a patient's head;

identifying whether two or more of the electrodes form a functional set, such that a desired therapeutic effect can be achieved when the two or more electrodes deliver a total amount of current to the patient regardless of what portion of the total amount of current each respective electrode in the functional set delivers;

providing one or more constant current sources, each having a supply terminal and a return terminal, wherein the current supplied at the supply terminal is equal to current returned to the return terminal; and

coupling the one or more constant current sources to the electrodes such that each supply terminal and each return terminal is coupled to no more than the electrodes of a single one of the functional sets, if any, or to a single one of the electrodes not included in one of the functional set,

wherein the one or more constant current sources are alternating current sources and wherein at any particular time during operation, each terminal of the one or more constant current sources is acting as either a supply terminal or a return terminal.

19. The method of claim 18 , further comprising:

monitoring an electrical resistance between the supply terminal and return terminal for one or more of the constant current sources while current is being delivered by the one or more constant current sources to the patient.

20. The method of claim 18 , further comprising:

monitoring a voltage being produced across the constant current source's supply and return terminals to provide at least an indirect measure of resistance.

21. A method comprising:

coupling a plurality of electrodes to a patient's head;

identifying whether two or more of the electrodes form a functional set, such that a desired therapeutic effect can be achieved when the two or more electrodes deliver a total amount of current to the patient regardless of what portion of the total amount of current each respective electrode in the functional set delivers, wherein identifying whether two or more of the electrodes form a functional set comprises identifying one or more electrodes of a first polarity substantially surrounded by four electrodes of a second polarity different than the first;

providing one or more constant current sources, each having a supply terminal and a return terminal, wherein the current supplied at the supply terminal is equal to current returned to the return terminal;

coupling the one or more constant current sources to the electrodes such that each supply terminal and each return terminal is coupled to no more than the electrodes of a single one of the functional sets, if any, or to a single one of the electrodes not included in one of the functional set;

delivering current to the patient's body from the supply terminals of four constant current sources through the one or more electrodes of the first polarity; and

returning current from the patient's body to the return terminals of the four constant current sources through the four electrodes of the second polarity.

22. A method comprising:

coupling a plurality of electrodes to a patient's head;

identifying whether two or more of the electrodes form a functional set, such that a desired therapeutic effect can be achieved when the two or more electrodes deliver a total amount of current to the patient regardless of what portion of the total amount of current each respective electrode in the functional set delivers, wherein identifying whether two or more of the electrodes form a functional set comprises identifying one or more electrodes of a first polarity substantially surrounded by four electrodes of a second polarity different than the first;

providing one or more constant current sources, each having a supply terminal and a return terminal, wherein the current supplied at the supply terminal is equal to current returned to the return terminal;

coupling the one or more constant current sources to the electrodes such that each supply terminal and each return terminal is coupled to no more than the electrodes of a single one of the functional sets, if any, or to a single one of the electrodes not included in one of the functional set;

delivering current to the patient's body from the supply terminals of four constant current sources through the four electrodes of the second polarity, respectively; and

returning current from the patient's body to the return terminals of the four constant current sources through the one or more electrodes of the first polarity.

23. A method comprising:

coupling a plurality of electrodes to a patient's head;

identifying whether two or more of the electrodes form a functional set, such that a desired therapeutic effect can be achieved when the two or more electrodes deliver a total amount of current to the patient regardless of what portion of the total amount of current each respective electrode in the functional set delivers, wherein identifying whether two or more of the electrodes form a functional set comprises identifying one or more electrodes of a first polarity substantially surrounded by four electrodes of a second polarity different than the first;

providing one or more constant current sources, each having a supply terminal and a return terminal, wherein the current supplied at the supply terminal is equal to current returned to the return terminal;

coupling the one or more constant current sources to the electrodes such that each supply terminal and each return terminal is coupled to no more than the electrodes of a single one of the functional sets, if any, or to a single one of the electrodes not included in one of the functional set;

delivering current to the patient's body from the supply electrode of a single one of the constant current sources through the one or more electrodes of the first polarity; and

returning current from the patient's body to the return terminal of the single one of the constant current sources through the four electrodes of the second polarity.

24. A method comprising:

coupling a plurality of electrodes to a patient's head;

identifying whether two or more of the electrodes form a functional set, such that a desired therapeutic effect can be achieved when the two or more electrodes deliver a total amount of current to the patient regardless of what portion of the total amount of current each respective electrode in the functional set delivers, wherein identifying whether two or more of the electrodes form a functional set comprises identifying one or more electrodes of a first polarity substantially surrounded by four electrodes of a second polarity different than the first;

providing one or more constant current sources, each having a supply terminal and a return terminal, wherein the current supplied at the supply terminal is equal to current returned to the return terminal;

coupling the one or more constant current sources to the electrodes such that each supply terminal and each return terminal is coupled to no more than the electrodes of a single one of the functional sets, if any, or to a single one of the electrodes not included in one of the functional set;

delivering current to the patient's body from the supply terminal of a single one of the constant current sources through the four electrodes of the second polarity; and

returning current from the patient's body to the return terminal of the single one of the constant current sources through the one or more electrodes of the first polarity.