Traditionally practiced by pale nerds tuning lasers and mirrors and lenses and crystals in pitch-black underground labs, optics has for decades been the domain of the physicist. But beginning roughly ten years ago, brain researchers began venturing into this dark world. It turns out that optics can supplement – and in some cases replace – the traditional use of electrodes as primary measuring tools for neuroscience.
A traditional neuroscience study begins with the surgical implantation of metal electrodes into the brain of a test animal, often a rabbit or a mouse. These electrodes are targeted to particular types of brain cells, found in particular regions of the brain. The animal is then subjected to a range of external stimuli. Electrical activity of the neurons in contact with the electrode tip is recorded on a computer.
Careful analysis and processing of these raw electrical signals then ascribes them to individual cells firing in particular patterns. These patterns are then correlated to the processes of learning, memory, sensory processing and other brain functions. In this way, electrode-based studies rely on passive observations.
However, optics combined with genetics – a field now known as optogenetics – allows researchers to directly control brain function with extreme precision instead of merely observing it. This has been a major development in neuroscience, wowing many in the field.
Optogenetic studies first introduce a carrier virus to the brain of the animal. A gene that encodes a light-sensitive ion channel protein is loaded into a virus and targeted to particular neurons of interest. (Brains contain many different types of neurons.) A flash of light, provided by a surgically implanted fiber-optic tip, opens the ion channels, causing the neuron to fire.
Thus, brain cells can be quite literally turned on and off. The possibilities of this powerful technique are nearly limitless. Examples include: Restoring eyesight after retinal damage by firing the remaining cells in the vision circuits of the brain; switching on and off a pathway that generates hunger impulses; and training the brain to suppress obsessive-compulsive behavior. There are no current plans to perform optogenetic studies on humans, however.
For this neuro-revolution, biologists should thank physicists who study optics. Lasers have undergone a technological advancement comparable to that of computers over the past 50 years: They are simpler, more reliable and more affordable than ever before. Optical fibers that bend, curve, and snake into tiny spaces such as the inside of a skull have similarly matured. Non-specialists can now assemble and operate commercially available optical systems.
While this is bad news for unemployed physicists, it is exactly the type of practical development that physics should deliver.