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 method of stimulating the perception of light in visual neural tissue comprising:
providing an anodic pulse which is sub-threshold for small ganglion cells and hyper polarizing for large ganglion cells and
providing a cathodic pulse which is supra-threshold for small ganglion cells.
2. The method according to claim 1 , wherein said anodic pulse precedes said cathodic pulse.
3. The method according to claim 1 , wherein said anodic pulse is longer in duration than said cathodic pulse to provide a charge balanced biphasic pulse.
4. The method according to claim 1 , wherein said cathodic pulse is provided on a stimulating electrode and anodic pulse is provided on an electrode near said stimulating electrode.
5. The method according to claim 4 , wherein pulses on said stimulating electrode and said electrode near said stimulating electrode are balanced biphasic pulses.
6. The method according to claim 1 , wherein said cathodic pulse varies in amplitude based on a preferred brightness and said anodic pulse varies in duration to balance said cathodic pulse.
7. A method of electrically stimulating artificial vision comprising:
providing a plurality of electrodes suitable to contact visual neural tissue;
collecting a visual field and providing a stimulation signal according said visual field;
determining a desired brightness according to said stimulation signal;
providing an anodic pulse which is sub threshold for small ganglion cells and hyper polarizing for large ganglion cells wherein the duration of said anodic pulse is determined by said desired brightness; and
providing a supra-threshold cathodic pulse wherein the amplitude of said supra-threshold cathodic pulse is determined by said desired brightness.
8. The method according to claim 7 , wherein said cathodic pulse is provided on a stimulating electrode and said anodic pulse is provided on an electrode near said stimulating electrode.
9. The method according to claim 8 , wherein pulses on said stimulating electrode and said electrode near said stimulating electrode are balanced biphasic pulses.