Cortical implant

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A cortical implant is a subset of neuroprosthetics that is in direct connection with the cerebral cortex of the brain. By directly interfacing with different regions of the cortex, the cortical implant can provide stimulation to an immediate area and provide different benefits, depending on its design and placement. A typical cortical implant is an implantable microelectrode array, which is a small device through which a neural signal can be received or transmitted.

Contents

The goal of a cortical implant and neuroprosthetic in general is "to replace neural circuitry in the brain that no longer functions appropriately." [1]

Overview

Cortical implants have a wide variety of potential uses, ranging from restoring vision to blind patients or helping patients with dementia. With the complexity of the brain, the possibilities for these brain implants to expand their usefulness are nearly endless. Some early work in cortical implants involved stimulation of the visual cortex, using implants made from silicone rubber. [2] Since then, implants have developed into more complex devices using new polymers, such as polyimide. There are two ways that cortical implants can interface with the brain, either intracortically (direct) or epicortically (indirect). [3] Intracortical implants have electrodes that penetrate into the brain, while epicortical implants have electrodes that stimulate along the surface. Epicortical implants mainly record field potentials around them and are generally more flexible compared to their intracortical counterparts. Since the intracortical implants go deeper into the brain, they require a stiffer electrode. [2] However, due to micromotion in the brain, some flexibility is necessary in order to prevent injury to the brain tissue.

Visual implants

Certain types of cortical implants can partially restore vision by directly stimulating the visual cortex. [4] Early work to restore vision through cortical stimulation began in 1970 with the work of Brindley and Dobelle. With their initial experimentation, some patients were able to recognize small images at fairly close distances. Their initial implant was based on the surface of the visual cortex and it did not provide as clear of images that it could, with an added downside of damage to surrounding tissues. More recent models, such as the "Utah" Electrode Array use deeper cortical stimulation that would hypothetically provide higher resolution images with less power needed, thus causing less damage. One of the major benefits to this method of artificial vision over any other visual prosthetic is that it bypasses many neurons of the visual pathway that could be damaged, potentially restoring vision to a greater number of blind patients. [4]

However, there are some issues that come with direct stimulation of the visual cortex. As with all implants, the impact of their presence over extended periods of time must be monitored. If an implant needs to be removed or re-positioned after a few years, complications can occur. The visual cortex is much more complex and difficult to deal with than the other areas where artificial vision are possible, such as the retina or optic nerve. The visual field is much easier to process in different locations other than the visual cortex. In addition, each areas of the cortex is specialized to deal with different aspects of vision, so simple direct stimulation will not provide complete images to patients. Lastly, surgical operations dealing with brain implants are extremely high-risk for patients, so the research needs to be further improved. However, cortical visual prostheses are important to people who have a completely damaged retina, optic nerve or lateral geniculate body, as they are one of the only ways they would be able to have their vision restored, so further developments will need to be sought out. [4]

Advancements in visual implants focus on stimulating specific areas of the visual cortex. The middle temporal (MT) region, crucial for perceiving motion, is a key target for electrical stimulation to create smooth motion artificially. Precise electrode implantation in MT poses a challenge due to its location, which is surrounded by sulci. Ongoing research explores multi-area stimulation between MT and primary visual cortex (V1), aiming to understand its impact on generating phosphenes (visual illusion) and motion perception. This multi-area approach, targeting different regions in the visual system, holds promise for improving the clarity and performance of visual implants, offering a potential avenue for more effective vision restoration. [5]

Auditory implants

While there has been little development in developing an effective auditory prosthesis that directly interfaces with the auditory cortex, there are some devices, such as a cochlear implant, and an auditory brainstem implant, introduced by Dr. William House and his team, that have been successful in restoring hearing to deaf patients. [6] The cochlear implant targets the cochlear or auditory nerve, and individuals who have issues with this nerve can never benefit from it. As an alternative, the auditory brainstem prosthesis can be used. [7]

There have also been some studies that have used microelectrode arrays to take readings from the auditory cortex of animals. One study has been performed on rats to develop an implant that enabled simultaneous readings from both the auditory cortex and the thalamus. The readings from this new microelectrode array were similar in clarity to other readily available devices that did not provide the same simultaneous readings. [8] With studies like this, advancements can be made that could lead to new auditory prostheses.

To address the challenges faced by conventional auditory prostheses, many unconventional auditory prostheses, such as bone conduction implants and middle ear implants are still under ongoing research. The bone conduction prosthesis stimulates the cochlea by triggering skull vibrations. The middle ear prosthesis, either partially or completely implanted, triggers direct vibration of the ossicular chain (ossicles or ear bones). Despite the complications these prostheses may cause, their purpose is to enhance the transmission of sound vibrations into the inner ear and, consequently, improve hearing abilities. [9]

Cognitive implants

Some cortical implants have been designed to improve cognitive function. These implants are placed in the prefrontal cortex or the hippocampus. Implants in the prefrontal cortex help restore attention, decision-making and movement selection by duplicating the minicolumnar organization of neural firings. [10] A hippocampal prosthetic aims to help with restoration of a patient's full long-term memory capabilities. Researchers are trying to determine the neural basis for memory by finding out how the brain encodes different memories in the hippocampus.

A patient thinks about moving a mouse pointer. The brain-computer interface takes that thought and translates it on the screen. InterfaceNeuronaleDirecte-tag.svg
A patient thinks about moving a mouse pointer. The brain-computer interface takes that thought and translates it on the screen.

By mimicking the natural coding of the brain with electrical stimulation, researchers look to replace compromised hippocampal regions and restore function. [11] Treatment for several conditions that impact cognition such as stroke, Alzheimer's disease and head trauma can benefit from the development of a hippocampal prosthetic. Epilepsy has also been linked to dysfunction in the CA3 region of the hippocampus. [12]

Brain-computer interfaces

A Brain-computer interface (BCI) is a type of implant that allows for a direct connection between a patient's brain and some form of external hardware. Since the mid-1990s, the amount of research done on BCI's in both animal and human models has grown exponentially. Most brain-computer interfaces are used for some form of neural signal extraction, while some attempt to return sensation through an implanted signal. [3] As an example of signal extraction, a BCI may take a signal from a paraplegic patient's brain and use it to move a robotic prosthetic. Paralyzed patients get a great amount of utility from these devices because they allow for a return of control to the patient. Current research for brain-computer interfaces is focused on determining which regions of the brain can be manipulated by an individual. A majority of research focuses on the sensorimotor region of the brain, using imagined motor actions to drive the devices, while some studies have sought to determine if the cognitive control network would be a suitable location for implantations. This region is a "neuronal network that coordinates mental processes in the service of explicit intentions or tasks," driving the device by intent, rather than imagined motion [13] An example of returning sensation through an implanted signal would be developing a tactile response for a prosthetic limb. Amputees have no touch response in artificial limbs, but through an implant in their somatosensory cortex could potentially give them an artificial sense of touch.

A current example of a brain-computer interface would be the BrainGate, a device developed by Cyberkinetics. This BCI is currently undergoing a second round of clinical trials as of May 2009. An earlier trial featured a patient with a severe spinal cord injury, with no control over any of his limbs. He succeeded in operating a computer mouse with only thoughts. Further developments have been made that allow for more complex interfacing, such as controlling a robotic arm.

The applications of BCIs have been emerging over the years, particularly in addressing the challenges posed by neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Parkinson’s disease (PD), Alzheimer’s disease (AD), and spinal muscular atrophy (SMA). [14]

In AD, a progressive fatal neurodegenerative disorder, BCIs face challenges due to cognitive decline. Some innovative studies used a technique called “classical conditioning with functional magnetic resonance imaging (fMRI) and BCIs.”. The main idea was to form a connection between certain intentional mental activities or thoughts and emotional responses or stimuli. Despite limitations, this novel approach seems to hold potential for the neurorehabilitation of AD. [14]

BCIs also play a role in enhancing motor function by translating neuronal firing into motor commands in PD, which is characterized by motor impairments. Research using local field potentials from deep brain stimulation (DBS) electrodes has shown improvements in motor functions. Neurofeedback through BCIs, based on electroencephalography (EEG) or fMRI, has been explored to regulate brain activity. BCIs with EEG feedback primarily aim to specifically detect intentional movements, with the goal of reducing neurological tremors when combined with technologies like functional electrical stimulation (FES). [14]

Moreover, BCIs offer potential improvements in muscle control in SMA patients, those who suffer from neurodegeneration in the anterior horn of the spinal cord, resulting in progressive muscle weakness. Some studies with SMA patients have explored integrating BCIs into control systems to enable remote devices such as TVs and telephones. Other studies have focused on enabling SMA individuals to manipulate a robotic arm using surface electromyography (sEMG). [14]

Advantages

Perhaps one of the biggest advantages that cortical implants have over other neuroprostheses is being directly interfaced with the cortex. Bypassing damaged tissues in the visual pathway allows for a wider range of treatable patients. These implants can also act as a replacement for damage tissues in the cortex. The idea of biomimicry allows for the implant to act as an alternate pathway for signals.

Disadvantages

Having any sort of implant that is directly connected to the cortex presents some issues. A major issue with cortical implants is biocompatibility, or how the body will respond to a foreign object. If the body rejects the implant, then the implant will be more of a detriment to the patient instead of a benefit. In addition to biocompatibility, once the implant is in place, the body may have an adverse reaction to it over an extended period of time, rendering the implant useless. [15] Implanting a microelectrode array can cause damage to the surrounding tissue. Development of scar tissue around the electrodes can prevent some signals from reaching the neurons the implant is meant to. Most microelectrode arrays require neuronal cell bodies to be within 50 μm of the electrodes to provide the best function, and studies have shown that chronically implanted animals have significantly reduced cell density within this range. [15] Implants have been shown to cause neurodegeneration at the site of implantation as well.

Neural coding represents a difficulty faced by cortical implants, and in particular, implants dealing with cognition. Researchers have found difficulty in determining how the brain codes distinct memories. For example, the way the brain codes the memory of a chair is vastly different from the way it codes for a lamp. With a full understanding of the neural code, more progress can be made in developing a hippocampal prosthetic that can more effectively enhance memory.

Due to the uniqueness of every patient's cortex, it is difficult to standardize procedures involving direct implantation. [4] There are many common physical features between brains, but an individual gyrus or sulcus (neuroanatomy) can be different when compared. This leads to difficulties because it causes each procedure to be unique, thus taking longer to perform. In addition, the nature of a microelectrode array intended effect is limited due to the stated variance's presented in association with individual cortex uniqueness i.e. differences. Present day microelectrode arrays are also constrained due their physical size, and achievable data processing/capability rates; which continue to be governed in relation to the characteristics dictated in accordance with Moore's Law.

Future developments

As more research is performed on, further developments will be made that will increase the viability and usability of cortical implants. Decreasing the size of the implants would help with keeping procedures less complicated and reducing the bulk. The longevity of these devices is also being considered as developments are made. The goal with the development of new implants is "to avoid the hydrolytic, oxidative and enzymatic degradation due to the harsh environment of the human body or at least to slow it down to a minimum which enables the interface to work over a long time period, before it finally has to be exchanged." [2] With extended operational lifetimes, fewer operations would need to be performed for maintenance, allowing for The amount of polymers that are now able to be used for neural implants has increased, allowing for a greater diversity of devices. As technology improves, researchers are able to more densely place electrodes into arrays, permitting high selectivity. [2] Other areas of investigation are the battery packs that power these devices. Effort has been made to try and reduce the overall size and bulkiness of these packs to make them less obtrusive for the patient. Reducing the amount of power each implant requires is also of interest, as this will reduce the amount of heat the implant makes, therefore reducing the risk of damage to the surrounding tissues.

Related Research Articles

<span class="mw-page-title-main">Brain–computer interface</span> Direct communication pathway between an enhanced or wired brain and an external device

A brain–computer interface (BCI), sometimes called a brain–machine interface (BMI) or smartbrain, is a direct communication pathway between the brain's electrical activity and an external device, most commonly a computer or robotic limb. BCIs are often directed at researching, mapping, assisting, augmenting, or repairing human cognitive or sensory-motor functions. They are often conceptualized as a human–machine interface that skips the intermediary component of the physical movement of body parts, although they also raise the possibility of the erasure of the discreteness of brain and machine. Implementations of BCIs range from non-invasive and partially invasive to invasive, based on how close electrodes get to brain tissue.

<span class="mw-page-title-main">Phosphene</span> Visual illusion

A phosphene is the phenomenon of seeing light without light entering the eye. The word phosphene comes from the Greek words phos (light) and phainein. Phosphenes that are induced by movement or sound may be associated with optic neuritis.

BrainGate is a brain implant system, currently under development and in clinical trials, designed to help those who have lost control of their limbs, or other bodily functions, such as patients with amyotrophic lateral sclerosis (ALS) or spinal cord injury. The Braingate technology and related Cyberkinetic’s assets are now owned by privately held Braingate, Co. The sensor, which is implanted into the brain, monitors brain activity in the patient and converts the intention of the user into computer commands.

Brain implants, often referred to as neural implants, are technological devices that connect directly to a biological subject's brain – usually placed on the surface of the brain, or attached to the brain's cortex. A common purpose of modern brain implants and the focus of much current research is establishing a biomedical prosthesis circumventing areas in the brain that have become dysfunctional after a stroke or other head injuries. This includes sensory substitution, e.g., in vision. Other brain implants are used in animal experiments simply to record brain activity for scientific reasons. Some brain implants involve creating interfaces between neural systems and computer chips. This work is part of a wider research field called brain–computer interfaces.

Sensory substitution is a change of the characteristics of one sensory modality into stimuli of another sensory modality.

Neuroprosthetics is a discipline related to neuroscience and biomedical engineering concerned with developing neural prostheses. They are sometimes contrasted with a brain–computer interface, which connects the brain to a computer rather than a device meant to replace missing biological functionality.

Neural engineering is a discipline within biomedical engineering that uses engineering techniques to understand, repair, replace, or enhance neural systems. Neural engineers are uniquely qualified to solve design problems at the interface of living neural tissue and non-living constructs.

<span class="mw-page-title-main">Retinal implant</span>

A retinal implant is a visual prosthesis for restoration of sight to patients blinded by retinal degeneration. The system is meant to partially restore useful vision to those who have lost their photoreceptors due to retinal diseases such as retinitis pigmentosa (RP) or age-related macular degeneration (AMD). Retinal implants are being developed by a number of private companies and research institutions, and three types are in clinical trials: epiretinal, subretinal, and suprachoroidal. The implants introduce visual information into the retina by electrically stimulating the surviving retinal neurons. So far, elicited percepts had rather low resolution, and may be suitable for light perception and recognition of simple objects.

In neuroscience, single-unit recordings provide a method of measuring the electro-physiological responses of a single neuron using a microelectrode system. When a neuron generates an action potential, the signal propagates down the neuron as a current which flows in and out of the cell through excitable membrane regions in the soma and axon. A microelectrode is inserted into the brain, where it can record the rate of change in voltage with respect to time. These microelectrodes must be fine-tipped, impedance matching; they are primarily glass micro-pipettes, metal microelectrodes made of platinum, tungsten, iridium or even iridium oxide. Microelectrodes can be carefully placed close to the cell membrane, allowing the ability to record extracellularly.

<span class="mw-page-title-main">Electrocorticography</span>

Electrocorticography (ECoG), a type of intracranial electroencephalography (iEEG), is a type of electrophysiological monitoring that uses electrodes placed directly on the exposed surface of the brain to record electrical activity from the cerebral cortex. In contrast, conventional electroencephalography (EEG) electrodes monitor this activity from outside the skull. ECoG may be performed either in the operating room during surgery or outside of surgery. Because a craniotomy is required to implant the electrode grid, ECoG is an invasive procedure.

A visual prosthesis, often referred to as a bionic eye, is an experimental visual device intended to restore functional vision in those with partial or total blindness. Many devices have been developed, usually modeled on the cochlear implant or bionic ear devices, a type of neural prosthesis in use since the mid-1980s. The idea of using electrical current to provide sight dates back to the 18th century, discussed by Benjamin Franklin, Tiberius Cavallo, and Charles LeRoy.

Microelectrode arrays (MEAs) are devices that contain multiple microelectrodes through which neural signals are obtained or delivered, essentially serving as neural interfaces that connect neurons to electronic circuitry. There are two general classes of MEAs: implantable MEAs, used in vivo, and non-implantable MEAs, used in vitro.

Neurostimulation is the purposeful modulation of the nervous system's activity using invasive or non-invasive means. Neurostimulation usually refers to the electromagnetic approaches to neuromodulation.

<span class="mw-page-title-main">Neurotrophic electrode</span> Intracortical device designed to read the electrical signals of the brain

The neurotrophic electrode is an intracortical device designed to read the electrical signals that the brain uses to process information. It consists of a small, hollow glass cone attached to several electrically conductive gold wires. The term neurotrophic means "relating to the nutrition and maintenance of nerve tissue" and the device gets its name from the fact that it is coated with Matrigel and nerve growth factor to encourage the expansion of neurites through its tip. It was invented by neurologist Dr. Philip Kennedy and was successfully implanted for the first time in a human patient in 1996 by neurosurgeon Roy Bakay.

A hippocampus prosthesis is a type of cognitive prosthesis. Prosthetic devices replace normal function of a damaged body part; this can be simply a structural replacement or a rudimentary, functional replacement.

Cortical stimulation mapping (CSM) is a type of electrocorticography that involves a physically invasive procedure and aims to localize the function of specific brain regions through direct electrical stimulation of the cerebral cortex. It remains one of the earliest methods of analyzing the brain and has allowed researchers to study the relationship between cortical structure and systemic function. Cortical stimulation mapping is used for a number of clinical and therapeutic applications, and remains the preferred method for the pre-surgical mapping of the motor cortex and language areas to prevent unnecessary functional damage. There are also some clinical applications for cortical stimulation mapping, such as the treatment of epilepsy.

<span class="mw-page-title-main">Photovoltaic retinal prosthesis</span>

Photovoltaic retinal prosthesis is a technology for restoring sight to patients blinded by degenerative retinal diseases, such as retinitis pigmentosa and age-related macular degeneration (AMD), when patients lose the 'image capturing' photoreceptors, but neurons in the 'image-processing' inner retinal layers are relatively well-preserved. This subretinal prosthesis is designed to restore a patients' sight by electrically stimulating the surviving inner retinal neurons, primarily the bipolar cells. Photovoltaic retinal implants are completely wireless and powered by near-infrared illumination (880nm) projected from the augmented-reality glasses. Therefore, they do not require such complex surgical methods as needed for other retinal implants, which are powered via extraocular electronics connected to the retinal array by a trans-scleral cable. Optical activation of the photovoltaic pixels allows scaling the implants to thousands of electrodes.

Stentrode is a small stent-mounted electrode array permanently implanted into a blood vessel in the brain, without the need for open brain surgery. It is in clinical trials as a brain–computer interface (BCI) for people with paralyzed or missing limbs, who will use their neural signals or thoughts to control external devices, which currently include computer operating systems. The device may ultimately be used to control powered exoskeletons, robotic prosthesis, computers or other devices.

A chronic electrode implant is an electronic device implanted chronically into the brain or other electrically excitable tissue. It may record electrical impulses in the brain or may stimulate neurons with electrical impulses from an external source.

Neural dust is a hypothetical class of nanometer-sized devices operated as wirelessly powered nerve sensors; it is a type of brain–computer interface. The sensors may be used to study, monitor, or control the nerves and muscles and to remotely monitor neural activity. In practice, a medical treatment could introduce thousands of neural dust devices into human brains. The term is derived from "smart dust", as the sensors used as neural dust may also be defined by this concept.

References

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