Waveforms for selective stimulation of central nervous system neurons

[0002] This application claims priority from and hereby expressly incorporates by reference U.S. provisional application No. 60/235,417 filed Sep. 26, 2000.

[0004] The present invention relates generally to electrical stimulation of the human central nervous system for treatment of neurological disorders in humans and other mammals. It is necessary to achieve the desired effect(s) that stimuli applied in the central nervous system activate the targeted elements (those neural elements, either local cells or axons of passage, that produce the desired effect(s)) without activation of the non-targeted neural elements (those neural elements, either local cells or axons of passage, that produce undesired effect(s) or do not produce the desired effect(s)). Thus, the ability to activate (stimulate) selectively either local neuron cells or axons of passage is required for device efficacy.

[0005] Monophasic cathodic stimuli have enabled some selective stimulation of axons of passage over local cells in CNS tissue. This is illustrated graphically in FIG. 1 wherein the line AF represents percent activation of axons of passage (fibers) and the line AC represents percentage activation of local neuronal cells for a given stimulus amplitude (pulse duration (pd=0.20 milliseconds (ms). As shown, activation of 70% of fibers is possible with activation of only 10% of local cells. Conversely, monophasic anodic stimuli have enabled some selective stimulation of local cells over fibers. This phenomenon is illustrated graphically in FIG. 2, again using the lines AF and AC to represent percent activation of fibers and local cells in a given sample of CNS tissue depending upon stimulus amplitude (pd=0.20 ms). Here, activation of 70% of the cells is possible with activation of only 25% of the fibers. Unfortunately, as is well known in the art, monophasic stimuli are not usable in practice owing to the fact that the stimuli are not charge-balanced, a fact that leads to tissue damage and electrode corrosion.

[0006] Use of symmetric, charge-balanced stimuli greatly decreases selectively. The effects of using an anodic first pulse (pd=0.20 ms) immediately followed by a cathodic second pulse (pd=0.20 ms) are shown in FIG. 3, wherein the lines AF and AC represent percent activation of fibers and local cells, respectively. It can be seen that a stimulus amplitude that activated 70% of the fibers also activated 15% of local cells. The effects of using a cathodic first pulse (pd=0.20 ms) immediately followed by an anodic second pulse (pd=0.2. ms) are shown in FIG. 4 by the lines AF and AC that represent percent activation of fibers and local cells, respectively. A stimulus amplitude sufficient to activate 70% of the cells also activated 100% of fibers of passage.

[0007] In light of the foregoing deficiencies and others associated with known techniques for obtaining either selective activation of axons of passage (fibers) over local cells, or for obtaining selective activation of local cells over fibers, a need has been identified for a novel and unobvious method and apparatus for selective stimulation (activation) of CNS neurons, i.e., stimulation of local neuron cells with minimal stimulation of axons of passage or stimulation of axons of passage with minimal stimulation of local neuron cells.

[0008] Asymmetric charge-balanced stimulation waveforms are defined and used for CNS stimulation with selective stimulation of either axons neuronal cell bodies or axon fibers of passage in favor of the other. A pre-pulse is followed by an opposite polarity stimulation pulse, with the pre-pulse being relatively low-amplitude and long-duration as compared to the stimulation pulse. The pre-pulse and stimulation pulse are charge-balanced to prevent tissue damage and electrode corrosion. The polarity of the pre-pulse and stimulation pulse control the selectivity of stimulation with respect to neuronal cell bodies versus axon fibers of passage. The waveform used in the stimulation method optionally includes a zero-amplitude phase having a duration of 0 to 500 microseconds.

[0009] One advantage of the present invention resides in the provision of waveforms for selective stimulation of central nervous system neurons.

[0010] Another advantage of the present invention is found in the provision of a method and apparatus for applying stimuli in the central nervous system to activate the targeted elements (those neural elements, either local cells or axons of passage, that produce the desired effect(s)) without activation of the non-targeted neural elements (those neural elements, either local cells or axons of passage, that produce undesired effect(s) or do not produce the desired effect(s)).

[0011] Still other benefits and advantages of the invention will become readily apparent to one of ordinary skill in the art to which the invention pertains upon reading this specification.

[0012] The invention comprises a plurality of components and arrangements of components, and a variety of steps and arrangements of steps, preferred embodiments of which are disclosed herein and illustrated in the accompanying drawings, wherein:

[0013] FIG. 1 graphically illustrates percent activation of local cells and axons of passage (fibers) in CNS tissue using a monophasic cathodic stimulus pulse of varying amplitude (pulse duration=0.20 milliseconds);

[0014] FIG. 2 graphically illustrates percent activation of local cells and axons of passage (fibers) in CNS tissue using a monophasic anodic stimulus pulse of varying amplitude (pulse duration=0.20 milliseconds);

[0015] FIG. 3 graphically illustrates percent activation of local cells and axons of passage (fibers) in CNS tissue using a symmetric, charge-balanced, biphasic stimulus pulse (anodic first pulse) of varying amplitude (pulse duration=0.20 milliseconds);

[0016] FIG. 4 graphically illustrates percent activation of local cells and axons of passage (fibers) in CNS tissue using a symmetric, charge-balanced, biphasic stimulus pulse (cathodic first pulse) of varying amplitude (pulse duration=0.20 milliseconds);

[0017] FIG. 5 diagrammatically illustrates a stimulation waveform generator adapted for selective stimulation of central nervous system neurons in accordance with the present invention;

[0018] FIG. 6 is a flow chart that illustrates a method for selective stimulation of local cells in CNS tissue according to the present invention;

[0019] FIG. 7 graphically illustrates a waveform generated in accordance with the method disclosed in FIG. 6;

[0020] FIG. 8 graphically illustrates percent activation of local cells and axons of passage (fibers) in CNS tissue using an example stimulus pulse generated according to the method disclosed in FIG. 6 depending upon the amplitude of the anodic stimulation pulse;

[0021] FIG. 9 is a flow chart that illustrates a method for selective stimulation of fibers in CNS tissue according to the present invention;

[0022] FIG. 10 graphically illustrates a waveform generated in accordance with the method disclosed in FIG. 9;

[0023] FIG. 11 graphically illustrates percent activation of local cells and axons of passage (fibers) in CNS tissue using an example stimulus pulse generated according to the method disclosed in FIG. 9 depending upon the amplitude of the cathodic stimulation pulse; and,

[0024] FIG. 12 graphically illustrates percent activation of fibers as a function of percent activation of local cells for the waveforms.

[0025] Referring to FIG. 5, a stimulation waveform generator G is diagrammatically illustrated. The hardware of the generator G is conventional. Likewise, a conventional electrode E, such as a metal microelectrode or a multiple-contact electrode, is inserted into the desired location in the brain or spinal cord (shown as “CNS” in FIG. 5). The generator G, itself, can be external to the human or other mammal. When implanted, a metal case of the generator G can act as a reference electrode. Alternatively, the electrode E is bi-polar and includes a reference electrode as a part thereof. If the electrode E is mono-polar, a separate reference electrode REF may be implanted into the central nervous system CNS or elsewhere in the body of the human or other mammal to be stimulated if the case of the stimulator is not used as a reference electrode.

[0026] Unlike known stimulation generators, the stimulation waveform generator G is configured or programmed to output novel and unobvious stimulation waveforms in accordance with the present invention. A method for generating a first waveform in accordance with the present invention using the generator G is disclosed in FIG. 6. An example of a first waveform generated in accordance with the method described in FIG. 6 is illustrated at WVC in FIG. 7 and is particularly adapted for selective stimulation of local cells over axons of passage (fibers) in CNS tissue.

[0027] Referring to both FIGS. 6 and 7, the method for selective stimulation of local cells in CNS tissue comprises a step S100 of generating a relatively long-duration, low-amplitude cathodic pre-pulse CPP. This cathodic pre-pulse has the effect of increasing the excitability of the local neuronal cells and decreasing the excitability of the axonal fibers in CNS tissue, without acting as a stimulation pulse that is intended to evoke the desired CNS response. Subsequent to the step S100, the method comprises a step S110 of pausing with no stimulation amplitude as shown at VP1 in FIG. 7. It is most preferred that this pause VP1 last from 0 and 500 microseconds. After the step S110, the method comprises a step S120 of transmitting a relatively short-duration, high-amplitude anodic stimulation pulse ASP after which the amplitude of the waveform WVC returns to 0. The anodic stimulation pulse ASP is intended to evoke the desired CNS response.

[0028] The cathodic pre-pulse CPP is relatively long compared to the anodic stimulation pulse ASP. In one embodiment, the duration of the cathodic pre-pulse is 8 to 12 times the duration of the anodic stimulation pulse. For example, a cathodic pre-pulse CPP having a pulse duration pd=1.00 millisecond (ms) and an anodic stimulation pulse ASP having a pulse duration pd=0.10 ms can be used in accordance with the present invention. To preserve charge-balancing, the charge delivered by the cathodic pre-pulse CPP is equal in magnitude and opposite in polarity relative to the charge delivered by the anodic stimulation pulse ASP.

[0029] FIG. 8 graphically illustrates percent activation of fibers AF and percent activation of cells AC using a stimulation waveform WVC generated in accordance with the method of FIG. 6. It can be seen that an anodic stimulation pulse ASP having an amplitude (approximately 35 μA) sufficient to activate 70% of local cells activated only 20% of fibers.

[0030] A method for generating a second waveform in accordance with the present invention using the generator G is disclosed in FIG. 9. An example of a second waveform generated in accordance with the method described in FIG. 9 is illustrated at WVF in FIG. 10 and is particularly adapted for selective stimulation of axon fibers of passage (fibers) over neuronal cell bodies in CNS tissue.

[0031] Referring to both FIGS. 9 and 10, the method for selective stimulation of fibers in CNS tissue comprises a step S200 of generating a relatively long-duration, low-amplitude anodic pre-pulse APP. This anodic pre-pulse has the effect of increasing the excitability of the axon fibers of passage and decreasing the excitability of the neuronal cell bodies in CNS tissue, without acting as a stimulation pulse that is intended to evoke the desired CNS response. Subsequent to the step S200, the method comprises a step S210 of pausing with no stimulation amplitude as shown at VP2 is FIG. 10. In a preferred embodiment, the pause VP2 has a duration of 0 to 500 microseconds. After the step S210, the method comprises a step S220 of transmitting a relatively short-duration, high-amplitude cathodic stimulation pulse CSP after which the amplitude of the waveform WVF returns to 0. The cathodic stimulation pulse CSP is intended to evoke the desired CNS response.

[0032] The anodic pre-pulse APP is relatively long compared to the cathodic stimulation pulse CSP. In one embodiment, the duration of the anodic pre-pulse is 8 to 12 times the duration of the cathodic stimulation pulse CSP. For example, an anodic pre-pulse APP having a pulse duration pd=0.20 ms and an cathodic stimulation pulse CSP having a pulse duration pd=0.02 ms can be used in accordance with the present invention. To preserve charge-balancing, the charge delivered by the anodic pre-pulse APP is equal in magnitude and opposite in polarity relative to the charge delivered by the cathodic stimulation pulse CSP.

[0033] FIG. 11 graphically illustrates percent activation of fibers AF and percent activation of cells AC using a stimulation waveform WVF generated in accordance with the method of FIG. 9. It can be seen that a cathodic stimulation pulse CSP having an amplitude (approximately 30 μA) sufficient to activate 70% of axon fibers of passage activated only 5% of neuronal cells.

[0034] FIG. 12 illustrates percent activation of axon fibers of passage as a function of percent activation of neuronal cells using the waveforms WVC, WVF in accordance with the present invention. The line A1 represents possible activation of axon fibers of passage in favor of neuronal cells using the waveform WVF. Likewise, the line A2 represents possible activation of neuronal cells in favor of axon fibers of passage using the waveform WVC.

[0035] The first and second waveforms WVC,WVF have application in neural prosthetic devices that use electrodes places in the brain (e.g., visual or auditory prostheses or deep brain stimulation devices) or spinal cord (e.g., control of micturition or motor function).

[0036] Modifications and alterations will occur to those of ordinary skill in the art to which the invention pertains upon reading and understanding the foregoing disclosure. It is intended that the invention be construed as encompassing all such modifications and alterations insofar as they are encompassed by the following claims considered literally or according to the doctrine of equivalents.