Method and apparatus for fitting a visual prosthesis using electrically evoked responses
The invention is a method of automatically adjusting a visual prosthesis electrode array to the neural characteristics of an individual patient. By recording electrically evoked responses to a predetermined input stimulus, one can alter that input stimulus to the needs of an individual patient. A minimum input stimulus is applied to a patient, followed by recording an electrically evoked response to the input stimulus. By gradually increasing stimulus levels, one can determine the minimum input that creates a neural response, thereby identifying the threshold stimulation level. One can further determine a maximum level by increasing stimulus until a predetermined maximum neural response is obtained.
1. A method of fitting an implantable neural stimulator comprising in order the steps of:
a. providing an implantable neural stimulator including electrical components and an array of electrodes suitable to stimulate visual neurons;
b. providing a computer, the computer completing steps c through 1 under automatic control of the computer;
c. beginning recording electrically evoked response levels of measured neural activity;
d. stimulating the visual neurons with a series of electrical stimulation signals through a selected subset of the array of electrodes;
e. Stopping the recording electrically evoked response levels of measured neural activity;
f. filtering the response signals;
g. applying a wavelet transform to the response signals to filter out any artifacts caused by the electrical components in the implantable neural stimulator;
h. discarding the response signals with large artifacts;
i. averaging response signals measured during multiple epochs;
j. comparing each level of the response signals to each level of the stimulation signals;
k. determining a relationship between the each level of the stimulation signals and each level of the response signals;
l. repeating steps c-k for different selected subsets of the array of electrodes;
m. determining and storing desired levels of stimulation based upon the comparison of the levels of the response signals and the levels of the stimulation signals; and
m. stimulating the visual neurons with the implantable neural stimulator to induce the perception of vision based on the desired levels of stimulation signals.
2. The method according to claim 1 , wherein the step of stimulating the visual neurons includes increasing the electrical charge of the electrical stimulation signals until the neural activity is detected.
3. The method according to claim 1 , wherein said step of stimulating the visual neurons includes decreasing the electrical stimulation signals when a predetermined level of the neural activity is reached.
4. The method according to claim 1 , further comprising analysis of the delay period between the step of stimulating the visual neurons and the measurement of the neural activity, wherein the delay period varies according to a level of input stimulus, and this variation is used in the fitting process.
5. The method according to claim 1 , wherein storing a level of stimulation includes storing a threshold for each electrode in the array of electrodes.
6. The method according to claim 1 , further comprising ignoring the neural activity greater than a predetermined voltage.
7. The method according to claim 6 , wherein the predetermined voltage is between 40 μV and 60 μV.