Brain blankets for BMI

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Brain blankets for BMI

Early trials of a new method of brain-machine integration have produced promising results.

There are two established methods for brain-machine integration (BMI).

Insertion

The first involves placing electrodes deep into the brain itself.

The concept of an implantable electrode array

The implantation of modern neuroprosthetics is best described as dicey. A section of skull is opened up such that the brain is exposed. The neuroprosthetic is attached to the brain in the required area, and a hole drilled in the piece of skull that was removed. Wires are fed through the hole in the skull plate, and it is reattached and stitched into place. On the surface of the brain, the neuroprosthetic's electrodes individually seek out the strongest neuron signals they can, and bind with them.

The wires sticking out of the skull are then connected to a control microprocessor outside the body.

This implantation procedure has several immediate problems:

Risk of Infection

Implantation leaves the patient with a hole in their head, permanently. This has to be constantly tended to, to avoid any possibility of infection, which would otherwise seep down into the brain itself.

Damage to the brain

As most prosthetic devices are still not powerful enough to function on their own, without external power sources, and processing power too large to put on the electrode array itself, they have wires coming out of the scalp. If these wires are caught or pulled, there is the very real and immediate likelihood of direct brain trauma in the affected area.

Damage to the electrodes

The brain tolerates the invasion into its network by the electrodes, however the body does not. Over time, a layer of new tissue forms around the electrodes, blocking them off from interfacing with neurons. This is why the signal degrades over time.

Previously it had been thought that neurons dying off were the cause.

Externalisation

The second method involves attaching an electrode net onto the scalp. This is the same technique used for normal EEGs (electroencephalograms).

(EEG) is the measurement of electrical activity produced by the brain as recorded from electrodes placed on the scalp.

EEG based BMI works by reading the electrical field generated by the grain, detecting the tiny distortions in electrical field that leach through above the skull, and generalising their meaning from the rough area of the brain they came from.

General differences in brain activity produced by traditional EEG

There are several problems:

Dependability

At best, EEGs have a 70% chance of being right, where a general intent is being detected. For more specific queries where only a handful of neurons fire, the EEG grid is too low fidelity to pick up the changes.

Signal Dampening

Electrodes on the scalp can only detect electrical waves that have passed through the skull, producing a weak signal susceptible to interference from mains electricity and other sources.

A Third Way

The new method of BMI is a combination of the two.

Called electrocorticography or ECOG (sometimes known as intracranial EEG or icEEG, also referred to as subdural EEG or sdEEG), this new method involves a tiny, paper thin mesh being draped directly over the surface of the brain.

A hole is cut in the skull, rather a big hole, and a piece of the skull is removed. The net is then deployed over the brain. The net consists of a polymer sheet containing a grid of electrodes 2 millimetres in diameter and spaced 10 mm apart.

Very similar to a method used for detecting epilepsy, this net is left in place, whilst the skull section is surgically stapled back. As the electrodes are detecting electrical activity around them, rather than trying to interface with neurons directly, it does not matter if a sheaf of tissue develops round them - it is still greater fidelity than trying to penetrate scalp and skull.

Recent experiments

Recently, a three-way experiment by university teams in the US has demonstrated data to support this approach.

Schalk and colleagues at Albany Medical College, Washington University in St Louis, University of Washington, Seattle, and the University of Wisconsin at Madison, all US based, conducted studies using temporary grids, removed after the experimentation.

In them, live patients learned to control a computer cursor in two dimensions on a computer screen using their brain signals.

Most remarkably, all five did so in under 30 minutes, which is the performance usually seen from embedded implants, not EEG arrays.

Dedicated human trials will require electrode grids designed to be implanted permanently. Researchers at Washington University in St. Louis are testing new designs in live monkeys.

If successful, their goal is to seek approval from the US Food and Drug Administration for human trials.