The ability to manipulate specific neurons and their circuits in awake-behaving animals would have been considered science fiction just a decade ago. With the advent of optogenetics, a technique that allows expression of light-activated proteins in defined populations of neurons, scientists can directly activate or silence neural activity with millisecond timing and cell-type precision. When combined with mice performing behavioral tasks, optogenetics can provide a causal link between neural activity and animal behavior.

Despite the successful use of optogenetics in many studies, technical constraints have limited its application. To deliver light into the brain and control optogenetically 'tagged' neurons during behavior, animals are often implanted with bulky optical devices and long cables connected to light sources, such as lasers and light-emitting diodes (LEDs). These can impair an animal's natural movements and limit the environmental conditions under which behavior can be studied.

To address this limitation, Ada Poon and colleagues at Stanford University (CA) developed a miniature and wirelessly powered optogenetic implant (Nat. Methods 12, 969–974; 2015). Consisting of a small power coil and micro-LED, the device is only 4 mm in diameter and 50 mg in weight, a fraction of the size and weight of previous optogenetic implants. The device is fully biocompatible and can be implanted subcutaneously under the scalp or skin of mice without any protruding components. Because the implants are powered wirelessly and have a built-in LED, optical cables and power connectors are eliminated, leaving mice untethered and free to move naturally. Poon and colleagues tested the ability of their new implants to manipulate neural activity in mice under a variety of conditions that would be too difficult with bulkier and tethered implants. Using immunostaining and behavioral measurements, they confirmed successful optogenetic activation of neurons as mice maneuvered through enriched environments with tunnels and other enclosures, and under social conditions with multiple mice interacting in the same testing chamber.

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The study provides proof of concept for a novel optogenetic implant and gives a glimpse into the future of optogenetics. Without the constraints of bulkier implants, optogenetics could be unleashed to study how specific neurons and brain circuits drive complex behaviors, such as social decisions and communication. In a press release, Poon and colleagues say they also hope the implant will open the door to new treatments for movement disorders and other mental health conditions.