Objective: Neuroprosthetics are artificial devices or systems designed to generate, restore or modulate a range of neurally mediated functions. These include sensorimotor, visual, auditory, cognitive affective and volitional functions that have been impaired or lost from congenital anomalies, traumatic brain injury, infection, amputation or neurodevelopmental and neurodegenerative disorders. Cochlear implants, visual prosthetics, deep brain stimulation, brain-computer interfaces, brain-to-brain interfaces and hippocampal prosthetics can bypass, replace or compensate for dysfunctional neural circuits, brain injury and limb loss. They can enable people with these conditions to gain or regain varying degrees of control of thought and behavior. These direct and indirect interventions in the brain raise general ethical questions about weighing the potential benefit of altering neural circuits against the potential harm from neurophysiological and psychological sequelae. Other ethical questions are more specific to the therapeutic goals of particular neuroprosthetics and the conditions for which they are indicated. These include informed consent, agency, autonomy (free will) and identity.

Approach: This review is an analysis and discussion of these questions. It also includes consideration of social justice issues such as how to establish and implement fair selection criteria in providing access to neuroprosthetic research and balancing technological innovation with patients' best interests.

Main results: Neuroprosthetics can restore or improve motor and mental functions in bypassing areas of injury or modulating dysregulation in neural circuits. As enabling devices that integrate with these circuits, neuroprosthetics can restore varying degrees of autonomous agency for people affected by neurological and psychiatric disorders. They can also re-establish the connectedness and continuity of the psychological properties they had before injury or disease onset and thereby re-establish their identity. Neuroprosthetics can maximize benefit and minimize harm for people affected by damaged or dysfunctional brains and improve the quality of their lives.

Significance: Provided that adequate protections are in place for research subjects and patients, the probable benefit of research into and therapeutic applications of neuroprosthetics outweighs the risk and therefore can be ethically justified. Depending on their neurogenerative potential, there may be an ethical obligation to conduct this research. Advances in neuroscience will generate new ethical and philosophical questions about people and their brains. These questions should shape the evolution and application of novel techniques to better understand and treat brain disorders.