Say “prosthetic” and most of us think of artificial limbs, important devices that help people regain essential functions lost to injury or disease. Modern prosthetics, however, are a long way from that image. Today, they’re often an integration of electronics and living tissue and can support much more than physical locomotion. New bionic technologies aid hearing and sight. Soon, they may help thinking itself. Some researchers are even looking beyond replacement of lost capabilities to providing capabilities we weren’t born with. If they succeed, the effect may revise our concepts of what defines a “normal” human.
Arms, hands, and legs
Artificial limbs are already pretty sophisticated and function very effectively (some bionic legs are actually considered an unfair advantage in running competitions!). Although they can be made to look very natural, though, what’s been missing has been natural control, i.e. operating limbs by simply willing them to move. That situation may not last for much longer. Neural control systems use a transducer implanted in the motor cortex of the brain to deliver control commands to the prosthetic in response to the user’s intent. Such devices are already going through human testing and results have been encouraging, as patients were able to reach, grasp, and even drink coffee with direct brain control.
Replacing the senses
Two sensory prosthetics – cochlear implants and artificial retinas – are designed to supplant hearing and vision loss, and are already part of current medical practice.
Although cochlear implants have been around for some time (the first patent was issued in 1974), they certainly fall within the class of brain-based prosthetics. These implants convert sound from a microphone, worn outside the head, to electrical signals that are then delivered directly to the auditory nerve. Almost 220,000 people worldwide use these devices.
Artificial retinas are chips containing photosensitive arrays that are implanted in or on the retina and convert light energy to electrical signals fed to the optic nerve. This technology is pretty recent (US approval was only granted in 2012), and the vision obtainable from these devices is still crude. To an otherwise-blind person, however, artificial retinas are a major boost to life quality.
Memories from microchips
Another technology even more in the research stage has big ambitions: to eventually provide support to people with memory impairments. Memory prosthetics won’t just transduce neural signals, like limb control or sensory devices. They’ll emulate replicate neural functions within the brain.
A research team involving Wake Forest University and the University of Southern California implanted a microchip into the brains of mice. Chip leads were inserted on each side of the hippocampus, a brain center involved in memory formation. The researchers then trained the mice to press one of two bars to obtain a food reward and measured the neural patterns that entered and left the hippocampus during successful trials. By reproducing that neural code and delivering it to the brain via the microchip, the team found that the mice made fewer mistakes on later trials. Furthermore, by blocking the input neurons, the researchers found that mice still remembered the task with only the microchip stimulation.
This work has now been extended to monkeys and human testing may not be far off. The hope is that a memory prosthesis like this might help people with Alzheimer’s disease who have difficulty forming long term memories. The precedent for this kind of medical intervention has been around since 1993: Deep brain stimulation reduces pain, tremor, and other conditions by electrically interrupting neural events with cortical implants. Although some people might be wary of “thought control” from technologies like this, it’s pretty unlikely. While science can model the memory signals for a food reward in mice, most human memories and cognitive functions are distributed throughout the brain and are hard to measure, and harder to manipulate.
Better than we are?
While the motivation for most prosthetic work has been to restore normal levels of human functioning, other efforts are exploring ways to surpass those levels. Recently, for example, infrared cameras were attached to implanted electrodes in the brains of rats. When the cameras detected infrared light, they stimulated the whisker neurons of the rats, effectively “steering” the rats toward the special light sources in their cages. In other words, the device allowed the rats to navigate using information beyond their own sensory capabilities. Blending human neural systems with electronics like this might someday offer opportunities to see, smell, and touch the world in new ways.
The use of technologies that work directly with the brain is already generating discussions about ethics in research and medical practice. What conditions and what people will benefit from such devices? Could there be side effects and, more importantly, how could we detect them? Do the benefits wear off? Are these effects reversible?
Prosthetics have succeeded in helping people restore lost functions. Now, the integration of prosthetics with the brain has dramatically expanded what can be achieved and might even expand what “normal” human functioning can be.