Cortical implant

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]

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 an auditory brainstem implant and a cochlear implant that have been successful in restoring hearing to deaf patients. There have also been some studies that have used microelectrode arrays to take readings from the auditory cortex in 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.[5] With studies like this, advancements can be made that could lead to new auditory prostheses.

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.[6] 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

By mimicking the natural coding of the brain with electrical stimulation, researchers look to replace compromised hippocampal regions and restore function.[7] 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.[8]

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 [9] 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.