US 20050021106 A1
The present disclosure suggests methods of selecting a stimulation site for stimulating a patient's brain or methods of effectuating a neural-function of a patient associated with an impaired body function. In one exemplary implementation, such a neural function may be effectuated by selecting a stimulation site, positioning at least a first electrode at the stimulation site, and applying an electrical potential to pass a current through the first electrode. If one aspect, this stimulation site may be selected by a) identifying a second body function that is a corollary to the impaired body function, and b) determining a corollary location of the patient's brain that is associated with the second body function and is ipsilateral to the impaired body function.
33. A method for effectuating a neural function related to a body part affected by a stroke, comprising:
identifying an ipsilateral hemisphere in the brain relative to the affected body part;
implanting an electrode at a cortical stimulation site selected to promote the neural function related to the affected body part, the cortical stimulation site being at least proximate to the cortex in the ipsilateral hemisphere of the brain; and
applying an ipsilateral electrical stimulation to the cortical stimulation site.
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45. A method for effectuating a neural function related to a body part affected by a stroke, comprising:
positioning an electrode at a cortical stimulation site selected to promote the neural function related to the affected body part, the cortical stimulation site being at least proximate to the cortex in an ipsilateral hemisphere of the brain relative to the affected body part; and
applying an ipsilateral electrical stimulation to the cortical stimulation site.
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This application is a continuation-in-part of U.S. application Ser. No. 10/260,720, filed on Sep. 27, 2002, which claims the benefit of U.S. application Ser. No. 60/325,872 filed on Sep. 28, 2001 and which is a continuation-in-part of U.S. application Ser. No. 09/802,808, filed on Mar. 8, 2001, which, in turn, claims the benefit of U.S. Provisional Application No. 60/217,981, filed Jul. 31, 2000. Each of these applications is incorporated by reference herein in its entirety.
Several embodiments of methods and apparatus in accordance with the invention are related to electrically stimulating a region in the cortex or other area of the brain to bring about a lasting change in a physiological function and/or a mental process of a patient.
A wide variety of mental and physical processes are controlled or influenced by neural activity in particular regions of the brain. The neural-functions in some areas of the brain (i.e., the sensory or motor cortices) are organized according to physical or cognitive functions. Several other areas of the brain also appear to have distinct functions in most individuals. In the majority of people, for example, the occipital lobes relate to vision, the left interior frontal lobes relate to language, and the cerebral cortex appears to be involved with conscious awareness, memory, and intellect.
Many problems or abnormalities can be caused by damage, disease and/or disorders in the brain. Effectively treating such abnormalities may be very difficult. For example, a stroke is a common condition that damages the brain. Strokes are generally caused by emboli (e.g., obstruction of a vessel), hemorrhages (e.g., rupture of a vessel), or thrombi (e.g., clotting) in the vascular system of a specific region of the brain. Such events generally result in a loss or impairment of a neural function (e.g., neural functions related to facial muscles, limbs, speech, etc.). Stroke patients are typically treated using various forms of physical therapy to rehabilitate the loss of function of a limb or another affected body part. Stroke patients may also be treated using physical therapy plus an adjunctive therapy such as amphetamine treatment. For most patients, however, such treatments are minimally effective and little can be done to improve the function of an affected body part beyond the recovery that occurs naturally without intervention.
The problems or abnormalities in the brain are often related to electrical and/or chemical activity in the brain. Neural activity is governed by electrical impulses or “action potentials” generated in neurons and propagated along synaptically connected neurons. When a neuron is in a quiescent state, it is polarized negatively and exhibits a resting membrane potential typically between −70 and −60 mV. Through chemical connections known as synapses, any given neuron receives excitatory and inhibitory input signals or stimuli from other neurons. A neuron integrates the excitatory and inhibitory input signals it receives, and generates or fires a series of action potentials when the integration exceeds a threshold potential. A neural firing threshold, for example, may be approximately −55 mV.
It follows that neural activity in the brain can be influenced by electrical energy supplied from an external source such as a waveform generator. Various neural functions can be promoted or disrupted by applying an electrical current to the cortex or other region of the brain. As a result, researchers have attempted to treat physical damage, disease and disorders in the brain using electrical or magnetic stimulation signals to control or affect brain functions.
Transcranial electrical stimulation is one such approach that involves placing an electrode on the exterior of the scalp and delivering an electrical current to the brain through the scalp and skull. Another treatment approach, transcranial magnetic stimulation, involves producing a high-powered magnetic field adjacent to the exterior of the scalp over an area of the cortex. Yet another treatment approach involves direct electrical stimulation of neural tissue using implanted electrodes.
The neural stimulation signals used by these approaches may comprise a series of electrical or magnetic pulses that can affect neurons within a target neural population. Stimulation signals may be defined or described in accordance with stimulation signal parameters including pulse amplitude, pulse frequency, duty cycle, stimulation signal duration, and/or other parameters. Electrical or magnetic stimulation signals applied to a population of neurons can depolarize neurons within the population toward their threshold potentials. Depending upon stimulation signal parameters, this depolarization can cause neurons to generate or fire action potentials. Neural stimulation that elicits or induces action potentials in a functionally significant proportion of the neural population to which the stimulation is applied is referred to as supra-threshold stimulation; neural stimulation that fails to elicit action potentials in a functionally significant proportion of the neural population is defined as sub-threshold stimulation. In general, supra-threshold stimulation of a neural population triggers or activates one or more functions associated with the neural population, but sub-threshold stimulation by itself does not trigger or activate such functions. Supra-threshold neural stimulation can induce various types of measurable or monitorable responses in a patient. For example, supra-threshold stimulation applied to a patient's motor cortex can induce muscle fiber contractions in an associated part of the body.
Although electrical or magnetic stimulation of neural tissue may be directed toward producing an intended type of therapeutic, rehabilitative, or restorative neural activity, such stimulation may result in collateral neural activity. In particular, neural stimulation delivered beyond a certain intensity, period of time, level, or amplitude can give rise to seizure activity and/or other types of collateral activity. It will be appreciated that collateral neural activity may be undesirable and/or inconvenient in a neural stimulation situation.
The human brain has two hemispheres that are connected via the corpus callosum. Each hemisphere of the brain generally exerts majority control over motor functions and/or sensory functions on the opposite or “contralateral” side of the patient's body. Hence, for example, the left hemisphere of the brain has majority control over movement of the right arm and right leg. Through transcallosal connections, though, each hemisphere of the brain exerts some degree of control over the functions on the same or “ipsilaterial” side of the patient's body. Hence, the right hemisphere of the brain may have some involvement in controlling movement of the right arm and right leg.
Some studies have concluded that damage to or disorders of the cerebral cortex on one hemisphere can induce long-term changes in the structure and function of a homotopic location of the contralateral hemisphere, namely a location on the undamaged cortex that is at about the same position as the position of the damaged tissue in the opposite cortex. Damage to the cortex in one hemisphere may impact the contralateral homotopic cortex in a variety of fashions, including causing increased cortical thickness, dendritic growth and/or elimination, neuronal hyperexcitability, and synaptogenesis. See, e.g., Nudo, “Recovery After Damage to Motor Cortical Areas,” Current Opinion in Neurobiology, 1999, 9:740-747, the entirety of which is incorporated herein by reference. See also Keyvani et al., “Suppression of Proteasome C2 Contralateral to Ischemic Lesions in Rat Brain,” Brain Research 858, (2000) 386-392.
The following disclosure describes several methods and apparatus for intracranial electrical stimulation to treat or otherwise effectuate a change in neural-functions of a patient. Several embodiments of methods in accordance with the invention are directed toward enhancing or otherwise inducing neuroplasticity to effectuate a particular neural-function. Neuroplasticity refers to the ability of the brain to change or adapt over time. It was once thought adult brains became relatively “hard wired” such that functionally significant neural networks could not change significantly over time or in response to injury. It has become increasingly more apparent that these neural networks can change and adapt over time so that meaningful function can be regained in response to brain injury. An aspect of several embodiments of methods in accordance with the invention is to provide the appropriate triggers for adaptive neuroplasticity. These appropriate triggers appear to cause or enable increased synchrony of functionally significant populations of neurons in a network.
Electrically enhanced or induced neural stimulation in accordance with several embodiments of the invention excites a portion of a neural network involved in a functionally significant task such that a selected population of neurons can become more strongly associated with that network. Because such a network will subserve a functionally meaningful task, such as motor relearning, the changes are more likely to be lasting because they are continually being reinforced by natural use mechanisms. The nature of stimulation in accordance with several embodiments of the invention ensures that the stimulated population of neurons links to other neurons in the functional network. It is expected that this occurs because action potentials are not actually caused by the stimulation, but rather are caused by interactions with other neurons in the network. Several aspects of the electrical stimulation in accordance with selected embodiments of the invention simply allows this to happen with an increased probability when the network is activated by favorable activities, such as rehabilitation or limb use.
The methods in accordance with the invention can be used to treat brain damage (e.g., stroke, trauma, etc.), brain disease (e.g., Alzheimer's, Pick's, Parkinson's, etc.), and/or brain disorders (e.g., epilepsy, depression, etc.). The methods in accordance with the invention can also be used to enhance functions of normal, healthy brains (e.g., learning, memory, etc.), or to control sensory functions (e.g., pain).
Certain embodiments of methods in accordance with the invention electrically stimulate the brain at a stimulation site where neuroplasticity is occurring. The stimulation site may be different than the region in the brain where neural activity is typically present to perform the particular function according to the functional organization of the brain. In one embodiment in which neuroplasticity related to the neural-function occurs in the brain, the method can include identifying the location where such neuroplasticity is present. This particular procedure may accordingly enhance a change in the neural activity to assist the brain in performing the particular neural function. In an alternative embodiment in which neuroplasticity is not occurring in the brain, an aspect is to induce neuroplasticity at a stimulation site where it is expected to occur. This particular procedure may thus induce a change in the neural activity to instigate performance of the neural function. Several embodiments of these methods are expected to produce a lasting effect on the intended neural activity at the stimulation site.
The specific details of certain embodiments of the invention are set forth in the following description and in
1. Embodiments of Electrically Enhancing Neural Activity
The method 100 includes a diagnostic procedure 102 involving identifying a stimulation site. In one approach, the stimulation site may be a location of the brain where an intended neural activity related to the neural-function is or is expected to be present. In another approach, the stimulation site may be a location of the brain that supports or is expected to support the intended neural-function.
The diagnostic procedure 102 may include identifying one or more exterior anatomical landmarks on the patient that correspond to such neurological regions and/or structures within the brain. The external anatomical landmarks serve as reference points for locating a structure of the brain where an intended neural activity may occur. Thus, one aspect of the diagnostic procedure 102 may include referencing the stimulation site on the brain relative to external anatomical landmarks.
More specifically, identifying an anatomical landmark may include visually determining the location of one or more reference structures (e.g., visible cranial landmarks), and locating underlying brain regions or structures (e.g., the motor strip and/or the Sylvian fissure) relative to the external location of the reference structures in a manner understood by those skilled in the art. Such reference structures may include, for example, the bregma, the midsagittal suture, and/or other well-known cranial landmarks. The methods for locating the underlying brain structure typically involve measuring distances and angles relative to the cerebral topography as known in the art of neurosurgery.
In another embodiment, the diagnostic procedure 102 includes generating the intended neural activity in the brain from a “peripheral” location that is remote from the normal location, and then determining where the intended neural activity is actually present in the brain. In an alternative embodiment, the diagnostic procedure 102 can be performed by identifying a stimulation site where neural activity has changed in response to a change in the neural-function. For example, the patient's brain may be scanned for neural activity associated with the impaired neural-function as the patient regains use of an affected limb or learns a task over a period of time. In yet another embodiment, the diagnostic procedure 102 may involve identifying a stimulation site where a corollary neural activity is present. This corollary neural activity may correspond to an unaffected or unimpaired physiological function or mental process that is a counterpart or corollary to an impaired or affected physiological function or mental process, e.g., movement of an unimpaired limb that is contralateral to an impaired homotypic limb.
The method 100 continues with an implanting procedure 104 involving positioning first and second electrodes relative to the identified stimulation site, and a stimulating procedure 106 involving applying an electrical current between the first and second electrodes. Many embodiments of the implanting procedure 104 position two or more electrodes at the stimulation site, but other embodiments of the implanting procedure involve positioning only one electrode at the stimulation site and another electrode remotely from the stimulation site. As such, the implanting procedure 104 of the method 100 can include implanting at least one electrode at the stimulation site. The procedures 102-106 are described in greater detail below.
The brain 200 of
The neural activity in the first region 210, however, can be impaired. In one embodiment, the diagnostic procedure 102 begins by taking an image of the brain 200 that is capable of detecting neural activity to determine whether the intended neural activity associated with the particular neural function of interest is occurring at the region of the brain 200 where it normally occurs according to the functional organization of the brain.
The two hemispheres 202 and 204 of the brain 200 are connected via the corpus callosum, which facilitates information transfer between the hemispheres 202 and 204. Although each hemisphere 202 or 204 generally exerts majority control over motor and/or sensory functions on the opposite or contralateral side of the patient's body, each hemisphere typically also exerts some level of control and/or influence over motor and/or sensory functions on the same or ipsilateral side of the patient's body. Moreover, through transcallosal connections, neural activity in one hemisphere may influence neural activity, e.g., neuroplasticity, in the opposite hemisphere. The location in the brain 200 that exerts influence on an ipsilateral body function frequently is proximate to or subsumed within the location of the brain associated with a corollary body function. Hence, as suggested in
The diagnostic procedure 102 may utilize the neuroplasticity that occurs in the brain to identify the location of a stimulation site that is expected to be more responsive to the results of an electrical, magnetic, sonic, genetic, biologic, and/or pharmaceutical procedure to effectuate the desired neural-function. One embodiment of the diagnostic procedure 102 involves generating the intended neural activity remotely from the first region 210 of the brain, and then detecting or sensing the location in the brain where the intended neural activity has been generated. The intended neural activity can be generated by applying an input that causes a signal to be sent to the brain. For example, in the case of a patient having an impaired limb, the affected limb is moved and/or stimulated while the brain is scanned using a known imaging technique that can detect neural activity (e.g., functional MRI, positron emission tomography, etc.). In one specific embodiment, the affected limb can be moved by a practitioner or the patient, stimulated by sensory tests (e.g., pricking), or subject to peripheral electrical stimulation. The movement/stimulation of the affected limb produces a peripheral neural signal from the limb that is expected to generate a response neural activity in the brain. The location in the brain where this response neural activity is present can be identified using the imaging technique.
Several particular embodiments for peripherally generating the intended neural activity are expected to be useful for therapies that involve patients who have lost volitional control of a body part. Volitional movement of a body part involves several parts of the cortex. For example, the prefrontal cortex is where the decision to move the body part occurs, the pre-motor cortex then generates the particular instructions for performing the movement, and the motor cortex then uses these instructions to send the appropriate electrical pulses to the body part via the spinal cord. The loss of volitional movement of a body part is usually caused by damage to the motor cortex. Some of the following embodiments for generating the intended neural activity may locate the site on the cortex where the brain is recruiting neurons related to the functionality of the impaired body part even though the patient is incapable of moving it.
The passive movement of the impaired body part is expected to provide a good indication of the location in the cortex where the brain is performing neural activity that controls the impaired body part. Passively moving the impaired body part produces neural signals that travel through the spinal cord to the cortex. The neural signals then produce neural activity at a site in the brain that is associated with the function of the impaired body part. In one embodiment, this neural activity defines the “intended neural activity.” The site of the intended neural activity generated by the passive movement of the impaired body part correlates well with active movement of the impaired body part. Thus, by passively moving an impaired body part and monitoring the intended neural activity that occurs in response to the passive motion, the location of the intended neural activity allows one to select, in selection phase 114, the stimulation site for applying an electrical therapy or another type of therapy.
As suggested in
In the particular example of
In circumstances where neural activity occurs at multiple locations in response to moving the impaired body part, the selection phase 114 may further include selecting one or more of the areas of the brain exhibiting such neural activity as a stimulation site. In one particular embodiment, the selection phase 114 includes selecting a stimulation site from a plurality of potential stimulation sites exhibiting such neural activity that is contralateral to a damaged location of the brain and ipsilateral to an impaired body function. Hence, in the specific example of
Movement of the contralateral homotypic body part can be accomplished in a variety of ways. In some of the embodiments discussed above, the patient may have little or no volitional control over movement of an impaired body part, necessitating intervention by an operator or a device to move the impaired body part. Because the contralateral homotypic body part often will have largely unimpaired function, movement of the homotypic body part in the generating phase 140 may be volitional movement of the body part by the patient. For example, the patient may be requested to flex or otherwise move the fingers of the left hand. In another embodiment, the contralateral homotypic body part may be moved passively as discussed above in connection with
The diagnostic procedure 102 of
In another embodiment, a generating phase (not shown) may instead comprise volitional exercise of the actual impaired body function. For example, to enhance the ability to learn a particular task (e.g., playing a musical instrument or memorizing information), the neural activity may be monitored (in a manner analogous to the monitoring phase 142 of
The embodiment of the diagnostic procedure 102 shown in
In certain applications of the embodiment shown in
The diagnostic procedure 102 in
The embodiment of the diagnostic procedure 102 shown in
Another benefit of several embodiments of the diagnostic procedures described above with reference to
The monitoring phase 152 of this embodiment of the diagnostic procedure 102 involves measuring the response to the electrical stimulation that was applied to the cortex in the generating phase 150. The response can be detected using electrical sensors at the impaired body part or by detecting movement of the impaired body part. If no response is detected, then the particular area of the cortex to which the stimulation was applied is not likely the motor control area of the cortex associated with performing neural activity for the impaired body part. The diagnostic procedure 102 can accordingly further include a decision phase 153 in which the practitioner or a computer decides to apply stimulation to another area of the cortex by moving a TMS device to another location, or selecting another electrode configuration on the electrode array. If a response is detected at the impaired body part, then the area of the cortex to which the stimulation was applied is likely involved in performing the neural function for the impaired body part. The diagnostic procedure 102 in this embodiment can also decide to test alternate cortical stimulation regions or locations at the decision phase 153 even when a response to the stimulation is detected to further refine the area of the cortex that is performing the neural activity of the impaired body part. After testing one or, more typically, several different cortical stimulation locations, the diagnostic procedure 102 can proceed to the selection phase 154 in which the area of the cortex that provided a desired response in the impaired body part is selected as the site to apply therapeutic electrical stimulation. If the generating phase 150 generates neural activity at multiple locations of the brain, the selection phase 154 may comprise selecting a stimulation site from these multiple potential stimulation sites. In one particular embodiment, the selection phase 154 comprises selecting a stimulation site that is ipsilateral to an impaired body part and contralateral to a damaged location of the brain 200 that was formerly associated with the impaired body part.
An alternative embodiment of the diagnostic procedure 102 involves identifying a stimulation site at a second location of the brain where the neural activity has changed in response to a change in the neural-function of the patient. This embodiment of the method does not necessarily require that the intended neural activity be generated by peripherally actuating or stimulating a body part. For example, the brain can be scanned for neural activity associated with the impaired neural-function as a patient regains use of an affected limb or learns a task over a period of time. This embodiment, however, can also include peripherally generating the intended neural activity remotely from the brain explained above.
In still another embodiment, the diagnostic procedure 102 involves identifying a stimulation site at a location of the brain where the intended neural activity is developing to perform the neural-function. This embodiment is similar to the other embodiments of the diagnostic procedure 102, but it can be used to identify a stimulation site at (a) the normal region of the brain where the intended neural activity is expected to occur according to the functional organization of the brain and/or (b) a different region where the neural activity occurs because the brain is recruiting additional matter to perform the neural-function. This particular embodiment of the method involves monitoring neural activity at one or more locations where the neural activity occurs in response to the particular neural-function of interest. For example, to enhance the ability to learn a particular task (e.g., playing a musical instrument, memorizing, etc.), the neural activity can be monitored while a person performs the task or thinks about performing the task. The stimulation sites can be defined by the areas of the brain where the neural activity has the highest intensity, the greatest increases, and/or other parameters that indicate areas of the brain that are being used to perform the particular task.
Several embodiments of methods for enhancing neural activity in accordance with the invention are expected to provide lasting results that promote the desired neural-function. Before the present invention, electrical and magnetic stimulation techniques typically stimulated the normal locations of the brain where neural activity related to the neural-functions occurred according to the functional organization of the brain. Such conventional techniques, however, may not be effective because the neurons in the “normal locations” of the brain may not be capable of carrying out the neural activity because of brain damage, disease, disorder, and/or because of variations of the location specific to individual patients. Several embodiments of methods for enhancing neural activity in accordance with the invention overcome this drawback by identifying a stimulation site based on neuroplastic activity that appears to be related to the neural-function. By first identifying a location in the brain that is being recruited to perform the neural activity, it is expected that therapies (e.g., electrical, magnetic, genetic, biologic, and/or pharmaceutical) applied to this location will be more effective than conventional techniques. This is because the location that the brain is recruiting for the neural activity may not be the “normal location” where the neuro activity would normally occur according to the functional organization of the brain. Therefore, several embodiments of methods for enhancing neural activity in accordance with the invention are expected to provide lasting results because the therapies are applied to the portion of the brain where neural activity for carrying out the neural-function actually occurs in the particular patient.
2. Electrically Inducing Desired Neural Activity
The method 100 for effectuating a neural-function can also be used to induce neural activity in a region of the brain where such neural activity is not present. As opposed to the embodiments of the method 100 described above for enhancing existing neural activity, the embodiments of the method 100 for inducing neural activity initiate the neural activity at a stimulation site where it is estimated that neuroplasticity will occur. In this particular situation, an image of the brain seeking to locate where neuroplasticity is occurring may be similar to
A stimulation site may be identified by estimating where the brain will likely recruit neurons for performing the neural-function. In one embodiment, the location of the stimulation site is estimated by defining a region of the brain that is proximate to the normal location where neural activity related to the neural-function is generally present according to the functional organization of the brain. An alternative embodiment for locating the stimulation site includes determining where neuroplasticity has typically occurred in patients with similar symptoms. For example, if the brain typically recruits a second region of the cortex to compensate for a loss of neural activity in the normal region of the cortex, then the second region of the cortex can be selected as the stimulation site either with or without imaging the neural activity in the brain.
Several embodiments of methods for inducing neural activity in accordance with the invention are also expected to provide lasting results that initiate and promote a desired neural-function. By first estimating the location of a stimulation site where desired neuroplasticity is expected to occur, therapies applied to this location may be more effective than conventional therapies for reasons that are similar to those explained above regarding enhancing neural activity. Additionally, methods for inducing neural activity may be easier and less expensive to implement because they do not require generating neural activity and/or imaging the brain to determine where the intended neural activity is occurring before applying the therapy.
3. Applications of Methods for Electrically Stimulating Regions of the Brain
The foregoing methods for enhancing existing neural activity or inducing new neural activity are expected to be useful for many applications. As explained above, several embodiments of the method 100 involve determining an efficacious location of the brain to enhance or induce an intended neural activity that causes the desired neural-functions to occur. Additional therapies can also be implemented in combination with the electrical stimulation methods described above. Several specific applications using embodiments of electrical stimulation methods in accordance with the invention either alone or with adjunctive therapies will now be described, but it will be appreciated that the methods in accordance with the invention can be used in many additional applications.
a. General Applications
The embodiments of the electrical stimulation methods described above are expected to be particularly useful for rehabilitating a loss of mental functions, motor functions and/or sensory functions caused by damage to the brain. In a typical application, the brain has been damaged by a stroke or trauma (e.g., automobile accident). The extent of the particular brain damage can be assessed using functional MRI or another appropriate imaging technique as explained above with respect to
Several specific applications are expected to have a stimulation site in the cortex because neural activity in this part of the brain effectuates motor functions and/or sensory functions that are typically affected by a stroke or trauma. In these applications, the electrical stimulation can be applied directly to the pial surface of the brain or at least proximate to the pial surface (e.g., the dura mater, the fluid surrounding the cortex, or neurons within the cortex). Suitable devices for applying the electrical stimulation to the cortex are described in detail with reference to
The electrical stimulation methods can also be used with adjunctive therapies to rehabilitate damaged portions of the brain. In one embodiment, the electrical stimulation methods can be combined with behavioral therapy and/or drug therapies to rehabilitate an affected neural function. For example, if a stroke patient has lost the use of a limb, the patient can be treated by applying the electrical therapy to a stimulation site where the intended neural activity is present while the affected limb is also subject to physical therapy. An alternative embodiment can involve applying the electrical therapy to the stimulation site and chemically treating the patient using amphetamines or other suitable drugs.
The embodiments of the electrical stimulation methods described above are also expected to be useful for treating brain diseases, such as Alzheimer's, Parkinson's, and other brain diseases. In this application, the stimulation site can be identified by monitoring the neural activity using functional MRI or other suitable imaging techniques over a period of time to determine where the brain is recruiting material to perform the neural activity that is being affected by the disease. It may also be possible to identify the stimulation site by having the patient try to perform an act that the particular disease has affected, and monitoring the brain to determine whether any response neural activity is present in the brain. After identifying where the brain is recruiting additional matter, the electrical stimulation can be applied to this portion of the brain. It is expected that electrically stimulating the regions of the brain that have been recruited to perform the neural activity which was affected by the disease will assist the brain in offsetting the damage caused by the disease.
The embodiments of the electrical stimulation methods described above are also expected to be useful for treating neurological disorders, such as depression, passive-aggressive behavior, weight control, and other disorders. In these applications, the electrical stimulation can be applied to a stimulation site in the cortex or another suitable part of the brain where neural activity related to the particular disorder is present. The embodiments of electrical stimulation methods for carrying out the particular therapy can be adapted to either increase or decrease the particular neural activity in a manner that produces the desired results. For example, an amputee may feel phantom sensations associated with the amputated limb. This phenomenon can be treated by applying an electrical pulse that reduces the phantom sensations. The electrical therapy can be applied so that it will modulate the ability of the neurons in that portion of the brain to execute sensory functions.
b. Pulse Forms and Potentials
The electrical stimulation methods in accordance with the invention can use several different pulse forms to facilitate or effectuate the desired neuroplasticity.
The pulses can be a bi-phasic or monophasic stimulus that is applied to achieve a desired potential in a sufficient percentage of a population of neurons at the stimulation site. In one embodiment, the pulse form has a frequency of approximately 2-1000 Hz, but the frequency may be particularly useful in the range of approximately 40-200 Hz. For example, initial clinical trials are expected to use a frequency of approximately 50-100 Hz. The pulses can also have pulse widths of approximately 10 μs-100 ms, or more specifically the pulse width can be approximately 20-200 μs. For example, a pulse width of 50-100 μs may produce beneficial results.
It is expected that one particularly useful application of the invention involves enhancing or inducing neuroplasticity by raising the membrane potential of neurons to bring the neurons closer to the threshold level for firing an action potential. Because the stimulation raises the membrane potential of the neurons, it is expected that these neurons are more likely to “fire” an action potential in response to excitatory input at a lower level.
The actual electrical potential applied to electrodes implanted in the brain to achieve a subthreshold potential stimulation will vary according to the individual patient, the type of therapy, the type of electrodes, and other factors. In general, the pulse form of the electrical stimulation (e.g., the frequency, pulse width, wave form, and voltage potential) is selected to raise the potential in a sufficient number neurons at the stimulation site to a level that is less than a threshold potential for a statistical portion of the neurons in the population. The pulse form, for example, can be selected so that the applied voltage of the stimulus achieves a change in the potential of approximately 10%-95%, and more specifically of 60%-80%, of the difference between the unstimulated resting potential and the threshold potential.
In one specific example of a subthreshold application for treating a patient's hand, electrical stimulation is not initially applied to the stimulation site. Although physical therapy related to the patient's hand may cause some activation of a particular population of neurons that is known to be involved in “hand function,” only a low level of activation might occur because physical therapy only produces a low level of action potential generation in that population of neurons. However, when the subthreshold electrical stimulation is applied, the membrane potentials of the neurons in the stimulated population are elevated. These neurons now are much closer to the threshold for action potential formation such that when the same type of physical therapy is given, this population of cells will have a higher level of activation because these cells are more likely to fire action potentials.
Subthreshold stimulation may produce better results than simply stimulating the neurons with sufficient energy levels to exceed the threshold for action potential formation. One aspect of subthreshold stimulation is to increase the probability that action potentials will occur in response to the ordinary causes of activation—such as behavioral therapy. This will allow the neurons in this functional network to become entrained together, or “learn” to become associated with these types of activities. If neurons are given so much electricity that they continually fire action potentials without additional excitatory inputs (suprathreshold stimulation), this will create “noise” and disorganization that will not likely cause improvement in function. In fact, neurons that are “overdriven” soon deplete their neurotransmitters and effectively become silent.
The application of a subthreshold stimulation is very different than suprathreshold stimulation. Subthreshold stimulation in accordance with several embodiments of the invention, for example, does not intend to directly make neurons fire action potentials with the electrical stimulation in a significant population of neurons at the stimulation site. Instead, subthreshold stimulation attempts to decrease the “activation energy” required to activate a large portion of the neurons at the stimulation site. As such, subthreshold stimulation in accordance with certain embodiments of the invention is expected to increase the probability that the neurons will fire in response to the usual intrinsic triggers, such as trying to move a limb, physical therapy, or simply thinking about movement of a limb, etc. Moreover, coincident stimulation associated with physical therapy is expected to increase the probability that the action potentials that are occurring with an increased probability due to the subthreshold stimulation will be related to meaningful triggers, and not just “noise.”
The stimulus parameters set forth above, such as a frequency selection of approximately 50-100 Hz and an amplitude sufficient to achieve an increase of 60% to 80% of the difference between the potential and the threshold potential are specifically selected so that they will increase the resting membrane potential of the neurons, thereby increasing the likelihood that they will fire action potentials, without directly causing action potentials in most of the neuron population. In addition, and as explained in more detail below with respect to
1. Implantable Stimulation Apparatus with Integrated Pulse Systems
The embodiment of the stimulation apparatus 600 shown in
Several embodiments of the stimulation apparatus 600 are expected to be more effective than existing transcranial electrical stimulation devices and transcranial magnetic stimulation devices. It will be appreciated that much of the power required for transcranial therapies is dissipated in the scalp and skull before it reaches the brain. In contrast to conventional transcranial stimulation devices, the stimulation apparatus 600 is implanted so that the electrodes are at least proximate to the pial surface of the brain 708. Several embodiments of methods in accordance with the invention can use the stimulation apparatus 600 to apply an electrical therapy directly to the pia mater 708, the dura mater 706, and/or another portion of the cortex 709 at significantly lower power levels than existing transcranial therapies. For example, a potential of approximately 1 mV to 10 V can be applied to the electrodes 660; in many instances a potential of 100 mV to 5 V can be applied to the electrodes 660 for selected applications. It will also be appreciated that other potentials can be applied to the electrodes 660 of the stimulation apparatus 600 in accordance with other embodiments of the invention.
Selected embodiments of the stimulation apparatus 600 are also capable of applying stimulation to a precise stimulation site. Again, because the stimulation apparatus 600 positions the electrodes 660 at least proximate to the pial surface 708, precise levels of stimulation with good pulse shape fidelity will be accurately transmitted to the stimulation site in the brain. It will be appreciated that transcranial therapies may not be able to apply stimulation to a precise stimulation site because the magnetic and electrical properties of the scalp and skull may vary from one patient to another such that an identical stimulation by the transcranial device may produce a different level of stimulation at the neurons in each patient. Moreover, the ability to focus the stimulation to a precise area is hindered by delivering the stimulation transcranially because the scalp, skull and dura all diffuse the energy from a transcranial device. Several embodiments of the stimulation apparatus 600 overcome this drawback because the electrodes 660 are positioned under the skull 700 such that the pulses generated by the stimulation apparatus 600 are not diffused by the scalp 702 and skull 700.
2. Integrated Pulse Systems for Implantable Stimulation Apparatus
The pulse system 630 shown in
2. Implantable Stimulation Apparatus with External Pulse Systems
The stimulation apparatus 3100, however, does not have an internal pulse system carried by the portion of the device that is implanted in the skull 700 of the patient 500. The stimulation apparatus 3100 receives electrical pulses from an external pulse system 3130. The external pulse system 3130 can have an electrical connector 3132 with a plurality of contacts 3134 configured to engage the contacts within the receptacle 3120. The external pulse system 3130 can also have a power supply, controller, pulse generator, and pulse transmitter to generate the electrical pulses. In operation, the external pulse system 3130 sends electrical pulses to the stimulation apparatus 3100 via the connector 3132, the receptacle 3120, and the lead line 3124.
3. Alternate Embodiments of Implantable Stimulation Apparatus
From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims. All of the U.S. Patents, U.S. Patent applications, and other references noted above are incorporated herein by reference.