Method and apparatus to provide safety checks for neural stimulation
In electrically stimulating neural tissue it is important to prevent over stimulation and unbalanced stimulation which would cause damage to the neural tissue, the electrode, or both. It is critical that neural tissue in not subjected to any direct current or alternating current above a safe threshold. Further, it is important to identify defective electrodes as continued use may result in neural and further electrode damage. The present invention presents system and stimulator control mechanisms to prevent damage to neural tissue.
1. A method of determining good electrodes of a perception inducing array of electrodes, comprising:
mounting the array of electrodes to neural tissue;
providing a controller configured to perform calculations;
providing a memory for storing multiple impedances over time for each electrode and when the electrode is determined to be a good electrode for each electrode;
wherein the controller is programmed to
a) obtain multiple individual impedances over time, including a most recent impedance, of an electrode tissue interface, and storing them in said memory, for a plurality of electrodes of the perception inducing array of electrodes while the electrode array is mounted to said neural tissue;
b) calculate an average of said impedances while the electrode array is mounted to the neural tissue;
c) calculate a difference between said average and said most recent impedance for each electrode;
d) determine good electrodes suitable to stimulate neural tissue without damage, wherein a good electrode is determined based on having said most recent impedance being between a predetermined minimum and a predetermined maximum acceptable impedance and previously being recorded in said memory as a good electrode;
e) store information about said good electrodes in the memory;
f) configure the array of electrodes to provide stimulation to the neural tissue to create neural perceptions using only the good electrodes.
2. The method according to claim 1 , wherein said average is a median.
3. The method according to claim 1 , wherein said average is a mean.
4. The method according to claim 1 , further comprising calculating a standard deviation of said impedances, and wherein said predetermined value is a multiple of said standard deviation.
5. The method according to claim 4 , wherein said predetermined value is four times said standard deviation.
6. The method according to claim 1 , further comprising repeating said steps of calculating based on a subset of said electrodes after eliminating from further calculation electrodes previously determined to be not good electrodes.
7. The method according to claim 4 , wherein said standard deviation is recalculated based on a subset of said electrodes after eliminating from further calculation electrodes previously determined to be not good electrodes.
8. The method according to claim 4 , wherein said standard deviation is compared with a predetermined minimum value and replaced with the predetermined minimum value if the predetermined minimum value is higher than the standard deviation.
9. The method according to claim 6 , further comprising maintaining a record of said impedances over time and determining not good electrodes by impedance charges over time.
10. The method according to claim 1 , further comprising stimulating visual neurons to form artificial vision.
11. The method of claim 6 , further comprising calculating a standard deviation of said impedances, and wherein said predetermined value is a multiple of said standard deviation.
12. The method according to claim 1 , further comprising disabling electrodes other than said good electrodes.