Neurostimulation
Encyclopedia
Neurostimulation involves modulation of the nervous system
and electrically activate neurons in the body. The activation of neural elements in a part of the nervous system can be effectively facilitated by stimulation. Micro-electrodes are utilized to interface with excitable tissue
in order to either restore recording experiences to the implant recipient or control an effector organ. Neurostimulation technology improves the life quality of those who are severely paralyzed or suffering from profound losses to various sense organs. It serves as the key part of neural prosthetics for hearing aid, artificial vision, artificial limbs, and brain-machine interfaces. In the case of neural stimulation
, mostly an electrical stimulation is utilized and charge-balanced biphasic constant current waveforms or capacitively coupled charge injection approaches are adopted. Alternatively, the transcranial magnetic stimulation
has been proposed as a non-invasive method in which a magnetic field
causes neurostimulation.
used torpedo fish (electric ray) for relieving headaches. In the late 18th century, Luigi Galvani
discovered that the muscles of dead frog legs twitched when struck by direct current on the nervous system. The modulation of the brain
activity by electrical stimulation of the motor cortex in dogs was shown in 1870 that resulted in limb movement. From the late 18th century to today many milestones have been developed. Nowadays, sensory prosthetic devices, such as visual implants, cochlear implants, auditory midbrain implants, and spinal cord stimulators and also motor prosthetic devices, such as deep brain stimulators, Bion microstimulators, the brain control and sensing interface, and cardiac electro-stimulation devices are widely used.
. In this method, scheduled stimulation is applied to specific cortical or subcortical targets. There are available commercial devices that can deliver an electrical pulse at scheduled time intervals. Scheduled stimulation is hypothesized to alter the intrinsic neurophysiologic properties of epileptic networks. The most explored targets for scheduled stimulation are the anterior nucleus of the thalamus
and the hippocampus
. The anterior nucleus of the thalamus has been studied, which has shown a significant seizure reduction with the stimulator ‘‘on’’ versus ‘‘off’’ during several months after stimulator implantation. Moreover, the cluster headache
(CH) can be treated by using a temporary stimulating electrode at sphenopalatine ganglion (SPG). Pain relief is reported within several minutes of stimulation in this method.
(DBS) has shown benefits for movement disorders such as Parkinson’s disease, tremor
and dystonia
and affective disorders such as depression, obsessive-compulsive disorder
, Tourette syndrome
, chronic pain
and cluster headache. Since DBS can directly change the brain activity in a controlled manner, it is used to map fundamental mechanisms of brain functions along with neuroimaging methods. A simple DBS system consists of two different parts. First, tiny microelectrodes are implanted in the brain to deliver stimulation pulses to the tissue. Second, an electrical pulse generator
(PG) generates stimulation pulses, which is connected to the electrodes via microwires.
Physiological properties of the brain tissue, which may change with disease state, stimulation parameters, which include amplitude and temporal characteristics, and the geometric configuration of the electrode and the surrounding tissue are all parameters on which DBS of both the normal and the diseased brain depend on. In spite of a huge amount of studies on DBS, its mechanism of action is still not well understood.
Developing DBS microelectrodes is still challenging.
, failed back surgery syndrome, complex regional pain syndrome
, phantom limb
pain, ischemic limb pain, refractory unilateral limb pain syndrome, postherpetic neuralgia
and acute herpes zoster
pain. Another pain condition that is a potential candidate for SCS treatment is Charcot-Marie-Tooth (CMT) disease, which is associated with moderate to severe chronic extremity pain. SCS therapy consists of the electrical stimulation of the spinal cord to ‘mask’ pain. The gate theory proposed in 1965 by Melzack and Wall
provided a theoretical construct to attempt SCS as a clinical treatment for chronic pain. This theory postulates that activation of large diameter, myelinated primary afferent fibers suppresses the response of dorsal horn neurons to input from small, unmyelinated primary afferents.
A simple SCS system consists of three different parts. First, microelectrodes are implanted in the epidural space to deliver stimulation pulses to the tissue. Second, an electrical pulse generator implanted in the lower abdominal area or gluteal region while is connected to the electrodes via wires, and third a remote control to adjust the stimulus parameters such as pulse width and pulse rate in the PG. Improvements have been made in both the clinical aspects of SCS such as transition from subdural placement of contacts to epidural placement, which reduces the risk and morbidity of SCS implantation, and also technical aspects of SCS such as improving percutaneous leads, and fully implantable multi-channel stimulators. However, there are many parameters that need to be optimized including number of implanted contacts, contact size and spacing, and electrical sources for stimulation. The stimulus pulse width and pulse rate are important parameters that need to be adjusted in SCS, which are typically 400 us and 8–200 Hz respectively.
was developed by Baker in 1985. TMS uses a magnetic wire above the scalp
, which carries a sharp and high current pulse. A time variant magnetic filed is induced perpendicular to the coil due to the applied pulse which consequently generates an electric field based on Maxwell
law. The electric field provides the necessary current for a non-invasive and much less painful stimulation. There are two TMS devices called single pulse TMS and repetitive pulse TMS (rTMS) while the latter has greater effect but potential to cause seizure. TMS can be used for therapy particularly in psychiatry
, as a tool to measure central motor conduction and a research tool to study different aspects of human brain physiology such as motor function, vision, and language. The rTMS method has been used to treat epilepsy with rates of 8–25 Hz for 10 seconds. The other therapeutic uses of rTMS include parkinson diseases, dystonia and mood diseases. Also TMS can be used to determine the contribution of cortical networks to specific cognitive functions by disrupting activity in the focal brain region.
Cochlear implants have provided partial hearing to more than 120,000 persons worldwide as of 2008. The electrical stimulation is used in a cochlear implant to provide functional hearing in totally deafened persons. Cochlear implants include several subsystem components from the external speech processor and radio frequency
(RF) transmission link to the internal receiver, stimulator, and electrode arrays. Modern cochlear implant research started in the 1960s and 1970s. In 1961, a crude single electrode device was implanted in two deaf patients and useful hearing with electric stimulation was reported. The first FDA approved complete single channel device was released in 1984.
In cochlear implants, the sound is picked up by a microphone and transmitted to the behind-the-ear external processor to be converted to the digital data. The digitized data is then modulated on a radio frequency signal and transmitted to an antenna
inside a headpiece. The data and power carrier are transmitted through a pair of coupled coils to the hermetically sealed internal unit. By extracting the power and demodulating the data, electric current commands are sent to the cochlea
to stimulate the auditory nerve through microelectrodes. The key point is that the internal unit does not have a battery and it should be able to extract the required energy. Also to reduce the infection, data is transmitted wirelessly along with power. Inductively coupled coils are the best candidate for power and data telemetry. Parameters needed by the internal unit include the pulse amplitude, pulse duration, pulse gap, active electrode, and return electrode that are used to define a biphasic pulse and the stimulation mode. An example of the commercial devices include Nucleus 22 device that utilized a carrier frequency of 2.5 MHz and later in the newer revision called Nucleus 24 device, the carrier frequency was increased to 5 MHz. The internal unit in the cochlear implants is an ASIC
(Application-Specific Integrated Circuit
) chip that is responsible to ensure safe and reliable electric stimulation. Inside the ASIC chip, there is a forward pathway, a backward pathway, and control units. The forward pathway recovers digital information from the RF signal which includes stimulation parameters and some handshaking bits to reduce the communication error. The backward pathway usually includes a back telemetry voltage
sampler that reads the voltage over a period of time on the recording electrode. The stimulator block is responsible to deliver predetermined current by external unit to the microelectrodes. This block includes a reference current and a digital to analog converter to transform digital commands to an analog current.
Theoretical and experimental clinical evidences suggest that direct electrical stimulation of the retina might be able to provide some vision to subjects who have lost the photoreceptive elements of their retina
. Therefore, visual prostheses are developed to restore vision for the blind by using the stimulation. Depending upon which visual pathway location is targeted for neural stimulation, different approaches have been considered. Visual pathway consists mainly of the eye
, optic nerve
, lateral geniculate nucleus
(LGN), and visual cortex
. Therefore retinal, optic nerve and visual cortex stimulation are the three different methods used in visual prostheses.
Retinal degenerative diseases, such as retinitis pigmentosa
(RP) and age-related macular degeneration (AMD), are two likely candidate diseases in which retinal stimulation may be helpful. Three approaches called intraocular epiretinal, subretinal and extraocular transretinal stimulation are pursued in retinal devices that stimulate remaining retinal neural cells to bypass lost photoreceptors and allow the visual signal to reach the brain via the normal visual pathway. In epiretinal approach, electrodes are placed on the top side of the retina near ganglion cells, whereas the electrodes are placed under the retina in subretinal approaches. Finally, the posterior scleral surface of the eye is the place in which extraocular approach electrodes are positioned. Second Sight and the Humayun group at USC are the most active groups in the design of intraocular retinal prostheses. The ArgusTM 16 retinal implant is an intraocular retinal prosthesis utilizing video processing technologies.
Regarding to the visual cortex stimulation, Brindley, and Dobelle were the first ones who did the experiments and demonstrated that by stimulating the visual cortex most of the electrodes can produce visual percept.
LGN, which is located in the midbrain to relay signals from the retina to the visual cortex, is another potential area that can be used for stimulation. But this area has limited access due to surgical difficulty. The recent success of deep brain stimulation techniques targeting the midbrain has encouraged research to pursue the approach of LGN stimulation for a visual prosthesis.
(too fast a heart beat), heart failure, and even stroke
. Early implantable pacemakers worked only a short time and needed periodic recharging by an inductive link. These implantable pacemakers needed a pulse generator to stimulate heart muscles with a certain rate in addition to electrodes. Today, modern pulse generators are programmed non-invasively by sophisticated computerized machines using RF, obtaining information about the patient’s and device’s status by telemetry. Also they use a single hermetically sealed lithium iodide
(LiI) cell as the battery. The pacemaker circuitry includes sense amplifiers to detect the heart’s intrinsic electrical signals, which are used to track heart activity, rate adaptive circuitry, which determine the need for increased or reduced pacing rate, a microprocessor, memory to store the parameters, telemetry control for communication protocol and power supplies to provide regulated voltage.
Microelectrodes are one of the key components of the neurostimulation, which deliver the current to neurons. Typical microelectrodes have three main components: a substrate (the ‘‘carrier’’), a conductive metal layer, and an insulation material. In cochlear implants, microelectrodes are formed from platinum
-iridium
alloy. State-of-the-art electrodes include deeper insertion to better match the tonotopic place of stimulation to the frequency band assigned to each electrode channel, improving efficiency of stimulation, and reducing insertion related trauma. These cochlear implant electrodes are either straight or spiral such as Med El Combi 40+ and Advanced Bionics Helix microelectrodes respectively.
In visual implants, there are two types of electrode arrays called planar type or three dimensional needle or pillar type, where needle type array such as Utah array is mostly used for cortical and optic nerve stimulations and rarely used in retinal implants due to the possible damage of retina. However, a pillar-shaped gold electrode array on thin-film polyimide
has been used in an extraocular implant. On the other hand, planar electrode arrays are formed from flexible polymers, such as silicone
, polyimide, and parylene
as candidates for retinal implants.
Regarding to DBS microelectrodes an array, which can be controlled independently, distributed throughout the target nucleus would permit precise control of the spatial distribution of the stimulation, and thus, allow better personalized DBS. There are several requirements for DBS microelectrodes that include long lifetime without injury to the tissue or degradation of the electrodes, customized for different brain sites, long term biocompatibility
of the material, mechanically durable in order to reach the target without being damaged during handling by the implant surgeon, and finally uniformity of performance across the microelectrodes in a particular array. Tungsten
microwire, iridium microwires, and platinum–iridium microelectrodes are the examples of microelectrode used in DBS.
field, stimulation is used very frequently to test the strength of the connection between one bundle of cells to another by applying a small current in one cell which results in the release of neurotransmitters and measuring the postsynaptic potential
. Generally a short but high frequency current such as 100 Hz helps strengthening the connection called as long term potentiation. However, longer but low frequency current leads to weaken the connections called as long term depression. Finally, combining neurostimulation with neural recording devices can lead us to the brain-computer interface, which currently seems to be more than just a reality, but a daily necessity.
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
and electrically activate neurons in the body. The activation of neural elements in a part of the nervous system can be effectively facilitated by stimulation. Micro-electrodes are utilized to interface with excitable tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
in order to either restore recording experiences to the implant recipient or control an effector organ. Neurostimulation technology improves the life quality of those who are severely paralyzed or suffering from profound losses to various sense organs. It serves as the key part of neural prosthetics for hearing aid, artificial vision, artificial limbs, and brain-machine interfaces. In the case of neural stimulation
Stimulation
Stimulation is the action of various agents on nerves, muscles, or a sensory end organ, by which activity is evoked; especially, the nervous impulse produced by various agents on nerves, or a sensory end organ, by which the part connected with the nerve is thrown into a state of activity.The word...
, mostly an electrical stimulation is utilized and charge-balanced biphasic constant current waveforms or capacitively coupled charge injection approaches are adopted. Alternatively, the transcranial magnetic stimulation
Transcranial magnetic stimulation
Transcranial magnetic stimulation is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain...
has been proposed as a non-invasive method in which a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
causes neurostimulation.
History
The primary findings about stimulation originated from the idea to stimulate nerves for therapeutic purposes. The 1st recorded use of electrical stimulation for pain relief goes back to 46 AD, when Scribonius LargusScribonius Largus
Scribonius Largus was the court physician to the Roman emperor Claudius.About 47 AD, at the request of Gaius Julius Callistus, the emperor's freedman, he drew up a list of 271 prescriptions , most of them his own, although he acknowledged his indebtedness to his tutors, to friends and to the...
used torpedo fish (electric ray) for relieving headaches. In the late 18th century, Luigi Galvani
Luigi Galvani
Luigi Aloisio Galvani was an Italian physician and physicist who lived and died in Bologna. In 1791, he discovered that the muscles of dead frogs legs twitched when struck by a spark...
discovered that the muscles of dead frog legs twitched when struck by direct current on the nervous system. The modulation of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
activity by electrical stimulation of the motor cortex in dogs was shown in 1870 that resulted in limb movement. From the late 18th century to today many milestones have been developed. Nowadays, sensory prosthetic devices, such as visual implants, cochlear implants, auditory midbrain implants, and spinal cord stimulators and also motor prosthetic devices, such as deep brain stimulators, Bion microstimulators, the brain control and sensing interface, and cardiac electro-stimulation devices are widely used.
Brain Stimulation
Brain stimulation has potentials to treat some disorders such as epilepsyEpilepsy
Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...
. In this method, scheduled stimulation is applied to specific cortical or subcortical targets. There are available commercial devices that can deliver an electrical pulse at scheduled time intervals. Scheduled stimulation is hypothesized to alter the intrinsic neurophysiologic properties of epileptic networks. The most explored targets for scheduled stimulation are the anterior nucleus of the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
and the hippocampus
Hippocampus
The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...
. The anterior nucleus of the thalamus has been studied, which has shown a significant seizure reduction with the stimulator ‘‘on’’ versus ‘‘off’’ during several months after stimulator implantation. Moreover, the cluster headache
Cluster headache
Cluster headache, nicknamed "suicide headache", is a neurological disease that involves, as its most prominent feature, an immense degree of pain in the head. Cluster headaches occur periodically: spontaneous remissions interrupt active periods of pain. The cause of the disease is currently unknown...
(CH) can be treated by using a temporary stimulating electrode at sphenopalatine ganglion (SPG). Pain relief is reported within several minutes of stimulation in this method.
Deep Brain Stimulation (DBS)
Deep brain stimulationDeep brain stimulation
Deep brain stimulation is a surgical treatment involving the implantation of a medical device called a brain pacemaker, which sends electrical impulses to specific parts of the brain...
(DBS) has shown benefits for movement disorders such as Parkinson’s disease, tremor
Tremor
A tremor is an involuntary, somewhat rhythmic, muscle contraction and relaxation involving to-and-fro movements of one or more body parts. It is the most common of all involuntary movements and can affect the hands, arms, eyes, face, head, vocal folds, trunk, and legs. Most tremors occur in the...
and dystonia
Dystonia
Dystonia is a neurological movement disorder, in which sustained muscle contractions cause twisting and repetitive movements or abnormal postures. The disorder may be hereditary or caused by other factors such as birth-related or other physical trauma, infection, poisoning or reaction to...
and affective disorders such as depression, obsessive-compulsive disorder
Obsessive-compulsive disorder
Obsessive–compulsive disorder is an anxiety disorder characterized by intrusive thoughts that produce uneasiness, apprehension, fear, or worry, by repetitive behaviors aimed at reducing the associated anxiety, or by a combination of such obsessions and compulsions...
, Tourette syndrome
Tourette syndrome
Tourette syndrome is an inherited neuropsychiatric disorder with onset in childhood, characterized by multiple physical tics and at least one vocal tic; these tics characteristically wax and wane...
, chronic pain
Chronic pain
Chronic pain has several different meanings in medicine. Traditionally, the distinction between acute and chronic pain has relied upon an arbitrary interval of time from onset; the two most commonly used markers being 3 months and 6 months since the initiation of pain, though some theorists and...
and cluster headache. Since DBS can directly change the brain activity in a controlled manner, it is used to map fundamental mechanisms of brain functions along with neuroimaging methods. A simple DBS system consists of two different parts. First, tiny microelectrodes are implanted in the brain to deliver stimulation pulses to the tissue. Second, an electrical pulse generator
Pulse generator
A pulse generator is either an electronic circuit or a piece of electronic test equipment used to generate rectangular pulses. This article describes the test equipment.-Bench pulse generators:...
(PG) generates stimulation pulses, which is connected to the electrodes via microwires.
Physiological properties of the brain tissue, which may change with disease state, stimulation parameters, which include amplitude and temporal characteristics, and the geometric configuration of the electrode and the surrounding tissue are all parameters on which DBS of both the normal and the diseased brain depend on. In spite of a huge amount of studies on DBS, its mechanism of action is still not well understood.
Developing DBS microelectrodes is still challenging.
Spinal Cord Stimulation (SCS)
Spinal cord stimulation (SCS) is an effective therapy for the treatment of chronic and intractable pain including diabetic neuropathyDiabetic neuropathy
Diabetic neuropathies are neuropathic disorders that are associated with diabetes mellitus. These conditions are thought to result from diabetic microvascular injury involving small blood vessels that supply nerves in addition to macrovascular conditions that can culminate in diabetic neuropathy...
, failed back surgery syndrome, complex regional pain syndrome
Complex regional pain syndrome
Complex regional pain syndrome is a chronic progressive disease characterized by severe pain, swelling and changes in the skin. It often affects an arm or a leg and may spread to another part of the body.Though treatment is often unsatisfactory, early multimodal therapy can cause dramatic...
, phantom limb
Phantom limb
A phantom limb is the sensation that an amputated or missing limb is still attached to the body and is moving appropriately with other body parts. 2 out of 3 combat veterans report this feeling. Approximately 60 to 80% of individuals with an amputation experience phantom sensations in their...
pain, ischemic limb pain, refractory unilateral limb pain syndrome, postherpetic neuralgia
Postherpetic neuralgia
Postherpetic neuralgia is a neuralgia caused by the varicella zoster virus. Typically, the neuralgia is confined to a dermatomic area of the skin and follows an outbreak of herpes zoster in that same dermatomic area...
and acute herpes zoster
Herpes zoster
Herpes zoster , commonly known as shingles and also known as zona, is a viral disease characterized by a painful skin rash with blisters in a limited area on one side of the body, often in a stripe...
pain. Another pain condition that is a potential candidate for SCS treatment is Charcot-Marie-Tooth (CMT) disease, which is associated with moderate to severe chronic extremity pain. SCS therapy consists of the electrical stimulation of the spinal cord to ‘mask’ pain. The gate theory proposed in 1965 by Melzack and Wall
Wall
A wall is a usually solid structure that defines and sometimes protects an area. Most commonly, a wall delineates a building and supports its superstructure, separates space in buildings into rooms, or protects or delineates a space in the open air...
provided a theoretical construct to attempt SCS as a clinical treatment for chronic pain. This theory postulates that activation of large diameter, myelinated primary afferent fibers suppresses the response of dorsal horn neurons to input from small, unmyelinated primary afferents.
A simple SCS system consists of three different parts. First, microelectrodes are implanted in the epidural space to deliver stimulation pulses to the tissue. Second, an electrical pulse generator implanted in the lower abdominal area or gluteal region while is connected to the electrodes via wires, and third a remote control to adjust the stimulus parameters such as pulse width and pulse rate in the PG. Improvements have been made in both the clinical aspects of SCS such as transition from subdural placement of contacts to epidural placement, which reduces the risk and morbidity of SCS implantation, and also technical aspects of SCS such as improving percutaneous leads, and fully implantable multi-channel stimulators. However, there are many parameters that need to be optimized including number of implanted contacts, contact size and spacing, and electrical sources for stimulation. The stimulus pulse width and pulse rate are important parameters that need to be adjusted in SCS, which are typically 400 us and 8–200 Hz respectively.
Transcranial Magnetic Stimulation (TMS)
Compared to electrical stimulation that utilizes brief, high-voltage electric shock to activate neurons, which can potentially activate pain fibers, transcranial magnetic stimulation (TMS)Transcranial magnetic stimulation
Transcranial magnetic stimulation is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain...
was developed by Baker in 1985. TMS uses a magnetic wire above the scalp
Scalp
The scalp is the anatomical area bordered by the face anteriorly and the neck to the sides and posteriorly.-Layers:It is usually described as having five layers, which can conveniently be remembered as a mnemonic:...
, which carries a sharp and high current pulse. A time variant magnetic filed is induced perpendicular to the coil due to the applied pulse which consequently generates an electric field based on Maxwell
Maxwell
-People:* Maxwell , Grammy winning R&B/neo-soul singer* Maxwell Cabelino Andrade , a Brazilian footballer who plays for Spanish La Liga club FC Barcelona...
law. The electric field provides the necessary current for a non-invasive and much less painful stimulation. There are two TMS devices called single pulse TMS and repetitive pulse TMS (rTMS) while the latter has greater effect but potential to cause seizure. TMS can be used for therapy particularly in psychiatry
Psychiatry
Psychiatry is the medical specialty devoted to the study and treatment of mental disorders. These mental disorders include various affective, behavioural, cognitive and perceptual abnormalities...
, as a tool to measure central motor conduction and a research tool to study different aspects of human brain physiology such as motor function, vision, and language. The rTMS method has been used to treat epilepsy with rates of 8–25 Hz for 10 seconds. The other therapeutic uses of rTMS include parkinson diseases, dystonia and mood diseases. Also TMS can be used to determine the contribution of cortical networks to specific cognitive functions by disrupting activity in the focal brain region.
Cochlear Implants
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) transmission link to the internal receiver, stimulator, and electrode arrays. Modern cochlear implant research started in the 1960s and 1970s. In 1961, a crude single electrode device was implanted in two deaf patients and useful hearing with electric stimulation was reported. The first FDA approved complete single channel device was released in 1984.
In cochlear implants, the sound is picked up by a microphone and transmitted to the behind-the-ear external processor to be converted to the digital data. The digitized data is then modulated on a radio frequency signal and transmitted to an antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
inside a headpiece. The data and power carrier are transmitted through a pair of coupled coils to the hermetically sealed internal unit. By extracting the power and demodulating the data, electric current commands are sent to the cochlea
Cochlea
The cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth, making 2.5 turns around its axis, the modiolus....
to stimulate the auditory nerve through microelectrodes. The key point is that the internal unit does not have a battery and it should be able to extract the required energy. Also to reduce the infection, data is transmitted wirelessly along with power. Inductively coupled coils are the best candidate for power and data telemetry. Parameters needed by the internal unit include the pulse amplitude, pulse duration, pulse gap, active electrode, and return electrode that are used to define a biphasic pulse and the stimulation mode. An example of the commercial devices include Nucleus 22 device that utilized a carrier frequency of 2.5 MHz and later in the newer revision called Nucleus 24 device, the carrier frequency was increased to 5 MHz. The internal unit in the cochlear implants is an ASIC
ASIC
ASIC may refer to:* Application-specific integrated circuit, an integrated circuit developed for a particular use, as opposed to a customised general-purpose device.* ASIC programming language, a dialect of BASIC...
(Application-Specific Integrated Circuit
Application-specific integrated circuit
An application-specific integrated circuit is an integrated circuit customized for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC...
) chip that is responsible to ensure safe and reliable electric stimulation. Inside the ASIC chip, there is a forward pathway, a backward pathway, and control units. The forward pathway recovers digital information from the RF signal which includes stimulation parameters and some handshaking bits to reduce the communication error. The backward pathway usually includes a back telemetry voltage
sampler that reads the voltage over a period of time on the recording electrode. The stimulator block is responsible to deliver predetermined current by external unit to the microelectrodes. This block includes a reference current and a digital to analog converter to transform digital commands to an analog current.
Visual Prosthesis
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
. Therefore, visual prostheses are developed to restore vision for the blind by using the stimulation. Depending upon which visual pathway location is targeted for neural stimulation, different approaches have been considered. Visual pathway consists mainly of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
, optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
, lateral geniculate nucleus
Lateral geniculate nucleus
The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....
(LGN), and visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
. Therefore retinal, optic nerve and visual cortex stimulation are the three different methods used in visual prostheses.
Retinal degenerative diseases, such as retinitis pigmentosa
Retinitis pigmentosa
Retinitis pigmentosa is a group of genetic eye conditions that leads to incurable blindness. In the progression of symptoms for RP, night blindness generally precedes tunnel vision by years or even decades. Many people with RP do not become legally blind until their 40s or 50s and retain some...
(RP) and age-related macular degeneration (AMD), are two likely candidate diseases in which retinal stimulation may be helpful. Three approaches called intraocular epiretinal, subretinal and extraocular transretinal stimulation are pursued in retinal devices that stimulate remaining retinal neural cells to bypass lost photoreceptors and allow the visual signal to reach the brain via the normal visual pathway. In epiretinal approach, electrodes are placed on the top side of the retina near ganglion cells, whereas the electrodes are placed under the retina in subretinal approaches. Finally, the posterior scleral surface of the eye is the place in which extraocular approach electrodes are positioned. Second Sight and the Humayun group at USC are the most active groups in the design of intraocular retinal prostheses. The ArgusTM 16 retinal implant is an intraocular retinal prosthesis utilizing video processing technologies.
Regarding to the visual cortex stimulation, Brindley, and Dobelle were the first ones who did the experiments and demonstrated that by stimulating the visual cortex most of the electrodes can produce visual percept.
Cardiac Electrostimulation Devices
Implantable pacemakers were proposed for the first time in 1959 and became more sophisticated since then. The therapeutic application of pacemakers consists of numerous rhythm disturbances including some forms of tachycardiaTachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...
(too fast a heart beat), heart failure, and even stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
. Early implantable pacemakers worked only a short time and needed periodic recharging by an inductive link. These implantable pacemakers needed a pulse generator to stimulate heart muscles with a certain rate in addition to electrodes. Today, modern pulse generators are programmed non-invasively by sophisticated computerized machines using RF, obtaining information about the patient’s and device’s status by telemetry. Also they use a single hermetically sealed lithium iodide
Lithium iodide
Lithium iodide, or LiI, is a compound of lithium and iodine. When exposed to air, it becomes yellow in color, due to the oxidation of iodide to iodine.-Applications:...
(LiI) cell as the battery. The pacemaker circuitry includes sense amplifiers to detect the heart’s intrinsic electrical signals, which are used to track heart activity, rate adaptive circuitry, which determine the need for increased or reduced pacing rate, a microprocessor, memory to store the parameters, telemetry control for communication protocol and power supplies to provide regulated voltage.
Stimulation Microelectrode Technologies
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
-iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
alloy. State-of-the-art electrodes include deeper insertion to better match the tonotopic place of stimulation to the frequency band assigned to each electrode channel, improving efficiency of stimulation, and reducing insertion related trauma. These cochlear implant electrodes are either straight or spiral such as Med El Combi 40+ and Advanced Bionics Helix microelectrodes respectively.
In visual implants, there are two types of electrode arrays called planar type or three dimensional needle or pillar type, where needle type array such as Utah array is mostly used for cortical and optic nerve stimulations and rarely used in retinal implants due to the possible damage of retina. However, a pillar-shaped gold electrode array on thin-film polyimide
Polyimide
Polyimide is a polymer of imide monomers. The structure of imide is as shown. Polyimides have been in mass production since 1955...
has been used in an extraocular implant. On the other hand, planar electrode arrays are formed from flexible polymers, such as silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
, polyimide, and parylene
Parylene
Parylene is the tradename for a variety of chemical vapor deposited poly polymers used as moisture and dielectric barriers. Among them, Parylene C is the most popular due to its combination of barrier properties, cost, and other processing advantages.Parylene is green polymer chemistry...
as candidates for retinal implants.
Regarding to DBS microelectrodes an array, which can be controlled independently, distributed throughout the target nucleus would permit precise control of the spatial distribution of the stimulation, and thus, allow better personalized DBS. There are several requirements for DBS microelectrodes that include long lifetime without injury to the tissue or degradation of the electrodes, customized for different brain sites, long term biocompatibility
Biocompatibility
Biocompatibility is related to the behavior of biomaterials in various contexts. The term may refer to specific properties of a material without specifying where or how the material is used , or to more empirical clinical success of a whole device in...
of the material, mechanically durable in order to reach the target without being damaged during handling by the implant surgeon, and finally uniformity of performance across the microelectrodes in a particular array. Tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
microwire, iridium microwires, and platinum–iridium microelectrodes are the examples of microelectrode used in DBS.
Non-clinical (Laboratory) Stimulation Applications
In addition to the enormous usage of neurostimulation for clinical applications, it is also used widely in laboratories started dates back to 1920s by people link Delgado who used stimulation as an experimental manipulation to study basics of how the brain works. The primary works were on the reward center of the brain in which stimulation of those structures led to pleasure that requested more stimulation. Another most recent example is the electrical stimulation of the MT area of primary visual cortex to bias perception. In particular, the directionality of motion is represented in a regular way in the MT area. They presented monkeys with moving images on screen and monkey throughput was to determine what the direction is. They found that by systematically introducing some errors to the monkey’s responses, by stimulating the MT area which is responsible for perceiving the motion in another direction, the monkey responded to somewhere in between the actual motion and the stimulated one. This was an elegant use of stimulation to show that MT area is essential in the actual perception of motion. Within the memoryMemory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
field, stimulation is used very frequently to test the strength of the connection between one bundle of cells to another by applying a small current in one cell which results in the release of neurotransmitters and measuring the postsynaptic potential
Postsynaptic potential
Postsynaptic potentials are changes in the membrane potential of the postsynaptic terminal of a chemical synapse. Postsynaptic potentials are graded potentials, and should not be confused with action potentials although their function is to initiate or inhibit action potentials...
. Generally a short but high frequency current such as 100 Hz helps strengthening the connection called as long term potentiation. However, longer but low frequency current leads to weaken the connections called as long term depression. Finally, combining neurostimulation with neural recording devices can lead us to the brain-computer interface, which currently seems to be more than just a reality, but a daily necessity.
See also
- StimulusStimulusStimulus may refer to:*Stimulus , something external that influences an activity*Stimulus , a concept in behaviorism and perception*Input to a system in other fields*Economic stimulus:...
- Brain-computer interfaceBrain-computer interfaceA brain–computer interface , sometimes called a direct neural interface or a brain–machine interface , is a direct communication pathway between the brain and an external device...
- NeuroprostheticsNeuroprostheticsNeuroprosthetics is a discipline related to neuroscience and biomedical engineering concerned with developing neural prostheses....
- NeurostimulatorNeurostimulatorIn medical technology a neurostimulator, also called an implanted pulse generator is a battery powered device designed to deliver electrical stimulation to the brain.-Applications and use:...
- implanted pulse generator
- Transcutaneous electrical nerve stimulationTranscutaneous electrical nerve stimulationTranscutaneous electrical nerve stimulation is the use of electric current produced by a device to stimulate the nerves for therapeutic purposes...
- Sexual stimulationSexual stimulationSexual stimulation is any stimulus that leads to, enhances and maintains sexual arousal, and may lead to ejaculation and/or orgasm...