The family room of Bernd Kuhn's childhood home has all the hallmarks of a natural history museum.
Its walls are lined with glass cases and drawers full of fossils he and his father dug up in the hills of southern Germany. They have thousands of specimens, ranging from small plants, fish, and corals to parts of massive mammals, including wooly mammoth and prehistoric rhino.
Among the collection is a 13 million-year-old pomegranate that Kuhn discovered and was named punica kuhnii after his father, Reinhard Kuhn. Over the years, he amassed a collection of thousands of mammalian teeth from which he donated 2000 samples to the natural history museum in Stuttgart.
"All my thinking life, I have been fossil hunting," said Kuhn, a AAAS member and professor at the Okinawa Institute of Science and Technology Graduate University (OIST) in Japan.
Such work requires a huge amount of patience; a skill Kuhn has practiced over the past 20 years while hunting for something completely different—a way to see voltage changes in a single neuron in live animals.
It's a project that he started as an undergraduate in Germany, worked on for his Ph.D., and which he continues today as the head of the Optical Neuroimaging Unit at OIST.
"If I had known then, I might not have done it," he half-jokes. "Some things just take more time."
Given his passion for fossils, it's surprising he didn't become an paleontologist or archeologist.
"I knew too many of those and it is not always a good life," he said. "There are very few positions and you need a lot of luck to get a reasonably good position."
So he went in the complete opposite direction—physics. He attended University of Ulm, a small university with a biophysics professor, Peter Fromherz, who was famous for detecting neuronal signals with field effect transistors. Kuhn wanted to work with him, so took on a project that no one else in the lab wanted to take because it sounded less fancy than the brain-computer interface and also difficult —working with a team of chemists to develop a novel dye that changes fluorescence color when there is a voltage change in the neuron membrane.
The electrical change is the most basic and minute signal of information processing in the brain. Being able to "see" this would enable researchers to better understand how information flows through the brain networks.
Most other researchers use calcium changes to see brain activity. Reading the actual voltage change is about 10,000 times harder, because the signal is much smaller and much faster. That means a lot of signals go unseen because they don't always trigger calcium changes.
The appeal of the florescent dye, called ANNINE-6, is that its mechanism is very simple—it only responds to voltage changes. . Researchers inject a droplet of dye into a cell and the dye seeps into the membrane. Then they use a two-photon microscope to image the minute fluorescence change as the voltage changes across the membrane.
But while simple at first, the dye was very toxic to the cells.
"It was a sophisticated way to kill neurons," quipped Kuhn, who followed Fromherz to the Max Planck Institute of Biochemistry to finish his undergraduate diploma and his Ph.D.
Eventually, he and collaborators at the Max Planck Institute for Medical Research were able to figure out how to use the dye so it no longer killed cells in culture or brain slices, and also produced large fluorescence changes they could record. Then they succeeded in doing this in living animals, seeing signals throughout brain regions but not yet in a single neuron.
Kuhn then spent five years at Princeton University learning more about neuroscience, in vivo imaging techniques and virology. He now combines all his skills and interests in physics, chemistry, biology, neuroscience and virology at OIST, where he conducts sophisticated single-cell imaging.
He and his team at OIST are trying to accomplish the final piece of the voltage-imaging puzzle by reading voltage changes in dendrites of single neurons not only in anesthetized mice, which do not move, but also in awake mice that can even run during experiments.
"In the past 30 years we have learned a lot about single neurons from brain slices," Kuhn said. "But with brain slices, all the inputs are cut away. We want to learn how neurons live and behave in a completely physiological environment."
For the next phase, they want to simultaneously measure the voltage and calcium changes in different parts of a neuron, including the dendrite and the main cell body, in a waking animal. This would enable them to see not just the incoming signal in the dendrite but also the information as it is passed along from the cell body to the next neuron. Then, they could correlate single cell input-output relations to actual behavior.
While many people are skeptical it can work, Kuhn and his colleagues are optimistic they will have results to share next year, which will mark the 20th anniversary of the dye's development. Kuhn's patience might finally pay off.
"It's very exciting," Kuhn said. "The goal is very close."