Neural Prostheses


A neural prosthesis is a device that aims to restore or replace the functions of the nervous system that are lost to disease or injury. Examples include devices to improve hearing, vision, motor and cognitive functions. Neural prostheses artificially stimulate the nervous system to convey sensory information, activate paralysed muscles or modulate the excitability of neural circuits to improve conditions such as chronic pain, epilepsy or tremor. Some neuroprostheses also record activity from the nervous system, which can be useful for patients who have difficulty moving or communicating. These devices can decipher the intention of the user or detect ongoing brain events such as seizures by recording neural signals directly from the brain. Emerging neuroprostheses aim to ‘close the loop’ using recorded neural activity to control stimulation delivered elsewhere in the nervous system with the goal of improving function.

Key Concepts

  • Neural prostheses aim to replace lost function or control activity within the nervous system following injury or disease.
  • Sensory neural prostheses transduce external events (e.g. sound, light) into artificial stimulation delivered to the nervous system.
  • Motor neural prostheses aim to restore functional movement to weak or paralysed muscles.
  • Neuromodulatory devices utilise electrical stimulation to modulate the excitability of neural circuits to improve symptoms of neurological diseases such as pain, tremor and the occurrence of seizures.
  • Recording neuroprostheses aim to detect the user's intention or brain state in order to enable communication or control of external devices.
  • Closed‐loop neuroprostheses utilise neural recording to control functional or symptom‐relieving stimulation.
  • Methods are emerging for optical and magnetic stimulation of the nervous system to supplement the electrical stimulation techniques currently in use.

Keywords: brain–computer interface; deep brain stimulation; functional electrical stimulation; intraspinal microstimulation; cochlear implant; retinal prosthesis; neuromodulation; ultrasound

Figure 1. . An external microphone and speech processor relay sound information to an external transmitter, which transmits it to the internal receiver. Here, it is converted to a stimulation pattern that is delivered to the cochlea via the implanted electrode array. Inset images detail the electrode array with multiple contacts ascending the spiral anatomy of the cochlea to activate the auditory nerves sensitive to different wavelengths of sound. Figure Copyright MRC Cognition and Brain Sciences Unit, used by kind permission.
Figure 2. Visual prostheses. An example visual prosthesis system and its implantation. The Argus II (Second Sight Medical Products) consists of (a) a camera mounted on glasses and a video processing unit (VPU). The implanted components (b) include the electrode array and a coil for wireless communication and stimulation. In this design, the retinal stimulation is delivered through a 6 × 10 electrode array (c), visualised with optical coherence tomography in (d). Reproduced with permission from Humayun et al. (2012) © Elsevier.
Figure 3. . High‐frequency brain signals are recorded from micro‐electrodes that penetrate the cortex of the brain to resolve the timing of individual neuron action potentials. Moderate frequency signals representing the activity of many thousands of neurons are recorded with electrocortigraphic (ECoG) electrodes on the brain surface. Low‐frequency activity representing the average activity of hundreds of thousands of neurons is recorded from electroencephalography (EEG) electrodes on the surface of the scalp. These signals are then processed to extract information about the intent of the user and control a brain–computer interface, such as the movement of a robotic arm, control of a computer cursor or for written communication.
Figure 4. . Electrodes implanted on the surface of the brain (electrocorticography – ECoG) or penetrating within the cortex of the brain record neural activity that can be translated into stimulation of muscles to enable functional movements after paralysis. Reproduced from Scott SH (2008) © Nature Publishing Group.
Figure 5. . Intraspinal microstimulation (ISMS) utilises electrodes placed within the spinal cord to activate specific neural circuits within the gray matter or area of spinal cord cell bodies. Epidural stimulation utilises electrodes placed on the dorsal surface of the spinal cord, above the dura or protective covering of the spinal cord. In most cases, stimulation is delivered below a site of injury to restore functional activity to the areas disconnected from descending brain input after injury. Epidural stimulation preferentially targets sensory fibres near the dorsal surface of the cord, whereas penetrating stimulation can reach the motor neuron cell bodies in the ventral horn. Reproduced with permission from Mondello SE, Kasten MR, Horner PJ and Moritz CT (2014) Creative Commons Attribution License.


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Further Reading

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Kasten, Michael R, Ievins, Aiva M, and Moritz, Chet T(Jan 2015) Neural Prostheses. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0024011]