Sub-threshold stimulation to precondition neurons for supra-threshold stimulation
In order to generate the smallest phosphenes possible, it is advantageous to selectively stimulate smaller cells. By hyperpolarizing the somas of the large cells selectively with sub-threshold anodic ‘pre-pulse’ stimuli (making them more difficult to stimulate) and then selectively depolarize the smaller cells one can selectively stimulate smaller cells. Alternatively, one can hyperpolarize the dendrites of the cells with larger dendritic fields by applying sub-threshold anodic currents on surrounding electrodes and then depolarizing the smaller cells in the center. Further, one can manipulate the phases of an individual biphasic wave to affect selective stimulation resulting in more focal responses. It is possible to increase resolution with the ‘pre-pulse’ described above. One can also effect resolution by modifying the pulse order of the cathodic and anodic phases. Further, one can isolate the effect of the phases by separating them in time (long inter-phase interval) or by making one of the phases long and low amplitude—always keeping equal total charge for the two phases. As an example, one can preferentially stimulate smaller ganglion cells by providing a longer sub-threshold anodic pulse balanced with a shorter supra-threshold cathodic pulse. Preferentially stimulating the smaller ganglion cells will allow stimulation of different brightness levels while maintaining high spatial resolution.
1. A neural stimulator comprising:
means for providing a first supra-threshold pulse on a stimulating electrode suitable to contact neural tissue;
means for providing a first sub-threshold pulse on an electrode near said stimulating electrode, said electrode near said stimulating electrode being suitable to contact neural tissue;
means for providing a second supra-threshold pulse on said stimulating electrode; and
means for providing a second sub-threshold pulse on said electrode near said stimulating electrode,
wherein the first sub-threshold pulse and the first supra-threshold pulse are charge balanced pulses.
2. The neural stimulator according to claim 1 , further comprising
means for determining a desired stimulation level;
means for controlling the duration of said first and second sub-threshold pulses according to said desired stimulation level; and
means for controlling the amplitude of said first and second supra-threshold pulses according to said desired stimulation level.
3. The neural stimulator according to claim 1 , wherein said first sub-threshold pulse is anodic and said first supra-threshold pulse is cathodic.
4. The neural stimulator according to claim 1 , wherein said neural stimulator is suitable to stimulate visual neurons to create the perception of a visual image.
5. The neural stimulator according to claim 4 , wherein said desired stimulation level is a desired brightness.
6. The neural stimulator according to claim 4 , wherein said visual neurons are retinal neurons.
7. The neural stimulator according to claim 1 , wherein said first sub-threshold pulse is cathodic and said first supra-threshold pulse is anodic.
8. The neural stimulator of claim 1 , wherein the first sub-threshold pulse starts simultaneously with the first supra-threshold pulse and the second sub-threshold pulse starts simultaneously with the second supra-threshold pulse.
9. The neural simulator of claim 1 , wherein the second sub-threshold pulse and the second supra-threshold pulse are charge balanced pulses.
10. The neural simulator of claim 1 , wherein the second sub-threshold pulse is opposite and equal to the first sub-threshold pulse and the second supra-threshold pulse is opposite and equal to the first supra-threshold pulse.
11. The neural stimulator of claim 1 , wherein:
the first supra-threshold pulse is provided during a first supra-threshold pulse time period;
the first sub-threshold pulse is provided during a first sub-threshold pulse time period overlapping with the first supra-threshold pulse time period;
the second supra-threshold pulse is provided during a second supra-threshold pulse time period;
the second sub-threshold pulse is provided during a second sub-threshold pulse time period overlapping with the second supra-threshold pulse time period;
the first supra-threshold pulse time period is separated in time from the second supra-threshold pulse time period; and
the first sub-threshold pulse time period is separated in time from the second sub-threshold pulse time period.