A new tack in neuroprosthetic interfaces is being born out of pulses of light rather than electricity. Termed the roll-off-the-tongue 'optogenetic neuromodulation', the technique is being pioneered by Medtronic, a UK-based medical R&D firm, in the hope that it will enable a far greater level of accuracy and single-cell stimulation than is possible with electrode interfaces.

Currently, all deep and surface brain implants function in the same manner: an electrode array is either wrapped around the brain as in ECoG arrays, or a much smaller electrode array is thrust into the brain. The two approaches can of course be combined in a single implant, but both suffer from being electric-based. The brain's neurons are electrochemical. This means they use a combination of electric micro-pulses, and chemical signalling to deliver message packets between each other.

The electrical data, our implants can duplicate, but the chemical control signal, they cannot. This means our implants operate at a crude level compared to neurons. Rather than sending out signals to state which adjacent cells should respond to electrical pulses, our neurostimulators just send out a pulse which is picked up by every neuron in the area - they have no way to signal individual cells to ignore the pulses.

This is where optogenetic neuromodulation comes in. It works off of a simple premise: specific brain cells are genetically engineered to respond to specific patterns of light upon the surface of the cell, as if it was an electrochemical signal. This allows the pairing of optical computer components and brain cells directly. Glass electrodes placed amongst the modified cells pick up the electrical discharges of those cells, acting as neural readers, without impeding light signals from above. The pulsed light comprises specific frequencies, usually in the infra-red, aimed at individual cells. These then fire into neighbouring neurons when the light affects each directly. Laser light is typically used for a level of precision of individual cells.

The whole mass of modified neural tissue with its embedded electrode arrays can then be surgically grafted onto (or into) an existing neural structure such as a brain. The light pulses convey data to the modified neurons, and they reach out to, and send data into the original neurons. Since the actual connection to the brain is neuron to neuron, many of the long-term problems with electrode placement can be side-stepped.

Medtronic aims to use their device for research groundwork at first - developing a greater understanding of how neurostimulation's electrical therapies actually affect the brain. Beyond that, they have expressed desire to develop the light-based system into stand-alone interface methods.

While academic scientists are developing new tools to deliver light to the brain, Medtronic is developing an optogenetically based implant for commercial use. The module, which is approximately the size and shape of a small USB flash drive, has wireless data links, a power management unit, a microcontroller, and an optical stimulator. It uses a fibre-optic wire to direct light from a blue or green LED at target neurons in the brain. The company plans to market the device to neuroscience researchers and use it for in-house research on the effects of DBS.

Use of Optogenetics on lab mice

Future Usage

It is hard to say with any certainty at this stage of development what the future holds for optical brain interfaces. Certainly the technique works, and can penetrate brain matter non-surgically, and without interference to the natural processes of neurons above and below the target area.

It may prove to be a viable method of shallow brain stimulation and activity reading, and it is certainly readily apparent this will enhance our understanding of how the brain functions. However, using implants based off of this technology, does require grafting organic material which differs in genetic signature to the client, into the brain. It is questionable at best, if this will prove to be a long-term asset in implanted devices, or a feature that will hold usage back.

Another point worthy of note is that optic pathways are being explored since we cannot use neurostimulators and read brain activity in response at the same time - the stimulator's pulse blocks everything else out. It may well be that through using this approach as a research tool, we find ways to improve neurostimulator patters such that they connect more effectively with the cells around them.

For the time being at least, this approach then, should be considered a research tool, and not a vector towards a new type of neuroprosthetic, at least until use in humans is well underway.

References

Beginnings In Optogenetics

Medtronic

A Brain Implant that Uses Light

Optogenetic Neuromodulation (PDF)