Vision may be the most valued human sense. However, millions of people experience vision loss, which severely affects their daily lives. Through the iVision project, a team of neuroscientists, engineers and physicians is striving to restore the vision of people who have lost it or never had it. Signals sent by stimulating brain tissue through microcoils can make the function of the eye superfluous and create images of reality in the brain’s visual cortex. A few major technical challenges remain before this goal can be achieved.
Holding a magnetic coil over a person’s head can induce vertigo. This simple observation was made a long time ago and since then numerous experiments have been carried out to investigate how magnetic stimulation affects the brain and how to apply this for treatment. Since 2002, people with depression have been treated with great success through transcranial magnetic stimulation: short magnetic pulses to stimulate nerve cells in specific areas of the brain. Now the same principle can restore vision for people who have lost it.
“We took a pair of glasses and attached a camera, and this produces a signal that is received in the brain through an implant in the visual cortex, bypassing the eyes and artificially recreating a copy in the brain. Electric pulses have previously been used, but magnetic stimulation can create a more controlled and focused signal. The challenge is to produce microcoils that create a strong enough signal without having to send excessive electric current through the brain,” explains Anpan Han, Senior Researcher, Department of Civil and Mechanical Engineering, Technical University of Denmark, Kongens Lyngby.
Implantable microcoils
Although researchers still do not fully understand how transcranial magnetic stimulation works, it has proved to be a huge success in treatment and an invaluable method of stimulating the brain for people with cardiovascular, sensory and nervous system diseases. The technique has also inspired a new generation of implantable microcoil magnetic stimulation devices.
“This is a novel way of stimulating neurons. Stimulating with an electrode produces a monopole field, with the electrical field radiating in all directions. A magnetic coil creates a dipole field in the neurons – like a small unidirectional magnet. This makes it useful for artificial vision, because the effect is easier to control and focus and can be made shorter and more precise,” says Anpan Han.
However, magnetic stimulation has major limitations, especially for treating people with chronic diseases and for influencing regions deep inside the brain. The magnetic coils are too big. This problem was solved when researchers from Harvard Medical School and the Cleveland Clinic created the first implantable microcoils that were 0.5 by 1 mm. A main driving force behind that study was Shelley I. Fried, Associate Professor, Department of Neurosurgery, Massachusetts General Hospital, Harvard Medical School, Boston, MA, USA, who also has a key role in the iVision project.
“Conventional electrodes cannot create specific patterns of neural activity around each electrode, whereas the microcoils can selectively activate some neurons, such as the vertically oriented ones, while avoiding others, such as horizontally oriented ones. This ensures direction and strength in the signals rather than diffusing them and losing strength,” he explains.
The vertically oriented pyramidal neurons appear to be responsible for processing input from both the auditory cortex (hearing) and the somatosensory sensory cortex and thus the visual cortex. Microcoil magnetic stimulation is therefore extremely promising.
“To assess effectiveness, we tested both the classic electrodes and microcoils in anesthetised mice. The electrical stimulation often extended more than 1 mm from the stimulation site, whereas activation by magnetic stimulation was limited to about one third of the area around the stimulation site. This suggests that the effectiveness of magnetic stimulation leads to a marked improvement,” says Shelley I. Fried.
The human brain cannot tolerate 100,000 amperes
The researchers behind iVision hope that the new microcoils will limit the spread of the signal from the stimulating electrode beyond the local area and that they can create an implant that can more closely duplicate the normal patterns of neural activity among people with excellent vision.
“Implants already exist but are incredibly expensive and do not provide very high overall resolution. Another potential advantage of microcoils is that magnetic fields pass more easily through biological material and are less susceptible to changes in the tissue surrounding the implant. We therefore think that the microcoils will be safer and more stable over time than electrical coils,” explains Shelley I. Fried.
The researchers still need to solve perhaps the greatest challenge: manufacturing the microcoils.
“We use chip technology that is designed for making things on a flat surface. A coil, by definition, is not flat. So we have only been able to produce half a coil rather than a whole one. This means that we have to apply excessively high electrical current. Creating 1 megapixel right now would require 100,000 amperes. The human brain cannot tolerate this,” says Anpan Han.
A stronger magnetic field therefore needs to be created while reducing the current. Physicists usually strengthen a magnetic field by adding more windings and putting a more magnetic material in the middle.
“To minimise tissue damage, the implant must also be as thin and small as possible. Making these coils small enough, with enough turns and with a core in the middle, is incredibly difficult. But using advanced lithography and thin-film diamond, we can carve these microcoils out of silicon that is sufficiently stiff to make very small and less tissue-damaging devices. This has never been done before, so we have no manual, but we are well on our way and absolutely convinced that one day we can help people restore their vision or see for the first time ever,” concludes Anpan Han.
