Cortical implant

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.

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

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

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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

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.

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