Categories of Neurotechnology
A multitude of neurotechnologies, with different maturity levels, belong to one or more types of medical neurotechnology. For example, there may be technologies for physical modification that also involve stimulation. In that case, the ethical, legal, social, and cultural issues for both classifications should be considered.
Some neurotechnologies have been used for decades and are part of the common medical practice to diagnose or treat neural pathologies, while others are more recent and currently available only in clinical or preclinical trials. In this section, we provide some examples for each of the several types of medical neurotechnologies, while a more exhaustive (but not comprehensive) list of existing and anticipated technologies can be found in the tables at the end of each type of medical neurotechnology.
Recording/Sensing
These are neurotechnologies capable of capturing information about the functional activity of the nervous system. For example, action potentials, ionic currents, activity-dependent blood flow and/or oxygenation, or other signals that reveal information about the activity of the central and/or peripheral nervous system.
Stimulating/Actuating
These are neurotechnologies capable of stimulating, inhibiting, or modulating the activity of the nervous system.
Closed-loop Operation
These are neurotechnologies that combine recording/sensing with stimulation/actuation where the recorded signals (or other relevant biomarkers sensed by implanted or extracorporeal devices) are used to regulate the stimulation/modulation in a feedback control loop. In other types of non-stimulating closed-loop technologies, the user’s biomarkers are used to control external devices and sensory feedback (such as visual feedback) is used by the user to modulate its own biomarkers to better regulate the therapeutic effect.
Direct Physical and Biological Modification
These are neurotechnologies that physically alter the nervous system by modifying physiology, and/or specific systems or sub-systems. Examples include genetic manipulation combined with associated stimulation and/or recording devices (e.g., optogenetics combined with optical stimulators), devices or technologies whose primary function is to alter neural cells and/or their connections (e.g., gamma knife).
Recording/Sensing Neurotechnology for Medical Applications
To better diagnose and treat medical conditions it is often helpful to access and measure the state of the brain and the nervous system. For the case of neural recording, there are different types of signals that can be measured and that have been identified as viable proxies of brain and nervous system activity: namely, electric and chemical signals. These measurements can be used for diagnosis and to inform treatment efficacy. The surface electroencephalogram (EEG) can be used to measure the electrical activity of the brain by placing electrodes on the surface of the scalp. The changes in the electric potential measured by the electrodes can provide information about which areas of the brain are activated by an external stimulus. For instance, EEG is used to test the auditory system of newborns. While the baby is asleep, simple sounds are played and clinicians look at the EEG recordings to see if the brain receives the signals from the auditory sensory system, responding with characteristic electrical waves, called evoked potentials. In other applications, the mental state of a participant being recorded can be inferred from analyzing the frequencies of the electrical waves generated by their brain, which changes according to the participant’s level of attention, drowsiness, relaxation, etc.
Imaging techniques can be used to measure biochemical cerebral activity, for instance, active brain areas show an increased level of local blood oxygenation that can be measured with magnetic resonance imaging (MRI). MRI can be used, for example, to check the brain damage after a stroke. The impaired brain areas appear as dark or light spots that do not look like the normal brain tissue.
EEG and MRI are two examples of non-invasive techniques that are commonly used to record neural activity, but many others are available and are mainly used for diagnostic purposes. Each technology has its application according to the temporal and spatial resolution needed.

Table 1.1 Examples of Existing and Anticipated Recording/Sensing Medical Neurotechnologies
Stimulating/Actuating Neurotechnology for Medical Applications
Neurotechnologies designed to provide treatment often rely on direct interaction with the nervous system to supply an external input or stimulation. Since we are not considering drugs in the scope of this framework, electrical inputs are the most common type of stimulation that can be provided with neurotechnologies. With non-invasive technologies, it is possible to stimulate brain activity with surface electrodes ( transcranial direct current stimulation — [tDCS]) or to generate strong magnetic fields that in turn generate electrical fields within the brain (transcranial magnetic stimulation — [TMS]). In these cases, the stimulation is typically less precise and localized when compared to invasive techniques.
Invasive methods like deep brain stimulation (DBS) foresee a more precise injection of current pulses through implanted electrodes targeting deeper regions of the brain. These stimulation technologies allow interfering with biological electric activity, up- or down-regulating the targeted neuronal populations, according to the desired effect. DBS has been successfully used to treat motor symptoms in drug-resistant Parkinson’s disease patients.

Table 1.2 Examples of Existing and Anticipated Stimulating/Modulating Medical Neurotechnologies
Closed-Loop Neurotechnology for Medical Applications
Closed-loop or feedback-controlled neurotechnologies combine measurement and stimulation with the goal of more accurately controlling the state of a specific physiological signal for therapeutic purposes. Thus, neurotechnologies belonging to this category should be able to both read from and stimulate the nervous system. For example, closed-loop systems to prevent seizures use intracranial electrodes to monitor brain signals. When those signals indicate an impending seizure, electrical stimulation is automatically delivered through intracranial electrodes to prevent the seizure onset.
To date, one system to suppress severe epileptic seizures, the NeuroPace RNS, has been granted FDA approval in the US (FDA, 2022). Other types of closed loop systems operate like a thermostat to regulate a physiological variable. For instance, using similar electrical recording and stimulation capabilities, researchers are working on techniques to adjust the amplitude or frequency of an electrical stimulus to maintain a variable—like movement—within a specified region. Closed-loop systems can provide more patient-specific therapies, and rapidly adjust in real-time to maintain stable operation throughout the day. In many cases these closed-loop systems can also be more energy-efficient. Note that certain technologies, such as intracortical BCIs used for the real-time control of a computer interface or physical end effector, may be classified as neurophysiologically closed-loop (AKA “human-in-the-loop”) systems even when they do not incorporate any direct neural stimulation feedback. The reason is that these systems use recorded neural activity to control an external apparatus, which provides direct visual feedback to the controller (the user) about how to modulate its own activity to achieve better control. Hence, from a neurophysiological standpoint, the sensory feedback effectively closes the loop. These “human-in-the-loop” systems using open-loop BCI controllers should be distinguished from BCI systems that use an internal (“closed”) control loop to automatically modulate the output of the BCI decoder based on error signals measured by the system.

Table 1.3 Examples of Existing and Anticipated Closed-loop Medical Neurotechnologies
Direct Physical and Biological Modification Neurotechnology for Medical Applications
While technologies in the previous categories often use electrical signals, more recent technologies have been using methods to directly interact with physical and biological modifications. While many experiments have been tried on animal models, applications on humans are still in an early development stage. An example of this category is optogenetics. This is a technique that foresees the genetic modification of neurons that become light-sensitive. Then, it is possible to modulate neuronal activity providing not an electrical, but an optical stimulation. As a recent example, this technique has been tested in a blind patient to partially restore his visual function. The injection with an adeno-associated viral vector made the ganglion neurons in the patient’s retina sensitive to light stimulation. Special goggles worn by the patient measure the local changes in light intensity and send corresponding light pulses onto the retina in real time to activate the genetically modified neurons (Sahel, Boulanger-Scemama, Pagot, et al, 2021).

Table 1.4 Examples of Existing and Anticipated Direct Physical and Biological Modulation Medical Neurotechnologies