Motor cortex and the systems BCIs target

Every intracortical BCIBrain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. Related: DecoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. , Closed loopClosed loop A system in which the output feeds back to influence the input, such as a user adjusting their brain activity while watching the cursor it controls. . Glossary entry paper mentions where the array went: “the hand knob of precentral gyrus,” “area 6v,” “dorsal premotor cortex.” Those are not incidental. The location decides what the neurons encode, how reliably, and for how long, and the choices the field has made are the result of thirty years of learning what works. Here is the map, from the strip that drives the muscles to the areas that plan, and the reasoning behind each implant site.

Primary motor cortex

A strip along the precentral gyrus, just in front of the central sulcus, laid out as a distorted body map: legs at the top near the midline, hand and arm on the lateral surface, face and tongue lower down. Its large layer 5 pyramidal neurons send axons all the way to the spinal cord, and their firing is the most direct cortical correlate of movement. The hand region is a knob-shaped fold you can see on an MRI, and it is where BrainGate and most other human arrays have gone, because hand movement is what people with paralysis want back, because the area is superficial and accessible, and because its neurons are tuned to the direction and speed of intended movement in a way that has decoded well since the population vectorPopulation vector A way to read out intended movement direction by adding up the preferred directions of many motor cortex neurons, weighted by how fast each is firing. Related: DecoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. , Rate codeRate code The idea that a neuron carries information in how many spikes per second it fires. . Glossary entry.

Tuning here is broad and mixed: a neuron cares about direction, speed, force, posture, and the target, in proportions that shift with context. That mixture is why linear decodersDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. Related: ClassifierClassifier An algorithm that assigns each trial a label, such as left hand or right hand, after being trained on labelled examples. , Kalman filterKalman filter An estimator that blends a prediction from a motion model with a noisy measurement, weighting each by how much it trusts it; the decoder behind most intracortical cursor control. , Brain-computer interface (BCI)Brain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. . Glossary entry work moderately and why the manifold view has been productive.

Premotor and supplementary motor areas

In front of primary motor cortex. Dorsal premotor cortex plans reaches, especially their direction and target, before movement begins; its activity during a delay period predicts the coming movement, which is useful for a decoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. Related: ClassifierClassifier An algorithm that assigns each trial a label, such as left hand or right hand, after being trained on labelled examples. , Kalman filterKalman filter An estimator that blends a prediction from a motion model with a noisy measurement, weighting each by how much it trusts it; the decoder behind most intracortical cursor control. , Brain-computer interface (BCI)Brain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. . Glossary entry that wants to act early. Ventral premotor cortex, lower down, is involved in grasping and in the mouth and face. The supplementary motor area, on the midline, is about sequences and self-initiated movement.

Posterior parietal cortex

Behind the central sulcus, where vision meets movement. It encodes goals and intended endpoints rather than trajectories, in the coordinates of eye and hand. The Caltech group has implanted here in humans and decoded intended reach targets and even imagined speech, showing that “intention” in a fairly abstract sense is readable. Parietal signals are less dependent on the exact movement, which could make them more stable, and more abstract, which makes them harder to turn into continuous control.

The speech motor area

Speech is movement: lips, tongue, jaw, larynx. The part of ventral precentral gyrus and adjacent ventral premotor cortex that drives those articulators (area 6v in the current shorthand) is where the 2023 speech BCIsBrain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. Related: DecoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. , Closed loopClosed loop A system in which the output feeds back to influence the input, such as a user adjusting their brain activity while watching the cursor it controls. . Glossary entry put their arrays. The neurons there encode which articulator is moving and how, and a decoderDecoder The algorithm that maps neural signals to intended actions, such as a classifier or a Kalman filter. Related: ClassifierClassifier An algorithm that assigns each trial a label, such as left hand or right hand, after being trained on labelled examples. , Kalman filterKalman filter An estimator that blends a prediction from a motion model with a noisy measurement, weighting each by how much it trusts it; the decoder behind most intracortical cursor control. , Brain-computer interface (BCI)Brain-computer interface (BCI) A system that turns measured brain activity into control of something outside the body, from a cursor to a wheelchair to a speech synthesizer. . Glossary entry trained on a few hours of attempted speech maps that to phonemes and then, with a language model, to words at 60 to 80 words per minute. No thoughts are read; attempted speech movements are. That distinction is the whole ethical and technical story.

What the location decides

Signal type. Primary motor cortex: continuous kinematics, good for cursors and arms. Premotor: plans and targets, good for discrete selection. Parietal: goals, more abstract. Speech motor: articulators.

Stability. Superficial gyral surface is easier to implant and gives cleaner recordings; sulcal walls, where much of the area actually lies, are harder to reach with rigid arrays, which is one argument for flexible electrodesElectrode The conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Related: Silver/silver-chloride (Ag/AgCl)Silver/silver-chloride (Ag/AgCl) The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. , Electrode impedanceElectrode impedance How hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. , Half-cell potentialHalf-cell potential The DC voltage that appears wherever a metal meets an electrolyte; two electrodes with different half-cell potentials produce an offset that can swamp the signal. . Glossary entry.

Redundancy. Hand knob is small; arrays of 100 channels sample perhaps a hundred neurons of a population of millions, and that has been enough, because the manifold is low-dimensional and redundantly represented.

Risk. Every implant site trades benefit against the surgery and the tissue response. The field has stayed with sites whose function is well understood and whose loss, should the implant damage them, would not add to what the person has already lost.