A tiny wearable scanner has been used to track chemical activity in the brains of unrestrained animals for the first time. By revealing neurological circuitry as the subjects perform normal tasks, researchers say, the technology could greatly broaden the understanding of learning, addiction, depression, and other conditions.
The device was designed to be used with rats—the main animal model used by behavioral neuroscientists. But the researchers who developed the device, at Brookhaven National Laboratory, say it would be straightforward to engineer a similar device for people.
Positron emission tomography, or PET, is already broadly used in neuroscience research and in clinical treatment. It allows researchers to track the location of radioactively labeled neurotransmitters (the chemicals that carry signals between neurons) or drugs within the brain. Images of the way neurotransmitters and drugs move through the brain can reveal the processes that underpin normal behavior such as learning as well as pathologies including addiction. PET has been used to map drug-binding sites in the brains of addicts and healthy people, and to study how those sites change over time and with therapy.
A conventional PET scanner is so large that these studies have to be performed with the subject lying inside a large tube. Large photomultiplier tubes amplify signals from gamma rays emitted by labeled chemicals in the brain. The signals then pass through a desk-sized rack of electronics that process them and map them to a particular region of the brain. To get good readings during animal studies, the subjects are typically anaesthetized or restrained. What’s being measured is not normal waking behavior.
“We have very limited data about what brains do in the real world,” says Paul Glimcher, professor of neuroscience, economics, and psychology at New York University. Glimcher was not involved with the work.
The new portable scanner is designed to provide the same information about brain chemistry while an animal behaves naturally. It is small and lightweight enough that a rat can carry it around on its head. “[The rat] can move freely, interact with other animals, and at the same time we can make a 3-D map of, for example, dopamine receptors throughout the brain,” says David Schlyer, a senior scientist at Brookhaven who led the work.
Schlyer’s group worked for years to engineer a miniature PET scanner that could be worn by a moving subject. The device consists of a metal ring hanging from a support structure that helps support its weight and allows the rat to move around. The rat’s head goes inside the ring, which contains both detectors and electronics.
The key to miniaturizing the device, Schlyer says, was integrating all the electronics for each detector in the ring on a single, specialized chip. An avalanche photodiode also replaces the large photomultiplier tubes of conventional PET, amplifying the signals emitted by the labeled chemicals in the brain. “The rats take about an hour to acclimate, then begin behaving normally,” says Schlyer. The Brookhaven device is described this week in the journal Nature Methods.