Noninvasive imaging of neuronal activity has been a goal for scientists for many years, yet until recently, imaging technology has not been sufficiently advanced to allow images to be produced as deep as the hippocampus without damaging the brain.
Back in 2013, Gabriel Popescu of the University of Illinois reported that his team had developed a form of phase-contrast microscopy – called spatial light interference microscopy or SLIM – that could be used to show neurons forming a neural network in real-time.
Now, Chris Xu, the Mong Family Foundation Director of Cornell Neurotech – Engineering, says his team of researchers have developed a new noninvasive imaging technique that has allowed them to observing large-scale neuronal activity deep in the brain with incredible clarity.
The team’s noninvasive imaging technique – three-photon microscopy – was used to show protein-labelled (GCaMP6) neurons in the subcortex of an intact mouse brain.
The team received funding for their research under former president Obama’s BRAIN (Brain Research through Advancing Innovative Neurotechnologies) initiative, the aim of which is to provide funding to accelerate the development of new imaging technologies that can produce dynamic, noninvasive images of the inner workings of the brain.
Around four years ago, the team produced a proof-of-concept which showed how three-photon microscopy could produce deeper images of the brain than was possible using standard two-photon microscopy, first developed in in 1990.
Chris Xu was aware that three-photon microscopy could allow even clearer and deeper images of the brain to be obtained as early as 1995, although laser technology at the time was not sufficiently advanced.
However, using a new laser developed according to Wu’s specifications, the team has successfully used 3-photon microscopy to make large-scale observations of neuronal activity. The team was able to observe neurons firing using green florescent protein (GLP) labelled neurons. The neurons flashed green each time they fired, with the images captured with incredible clarity. The team used their new microscopy technique to record spontaneous neuronal activity of around 150 neurons simultaneously. The images produced were far superior to those produced using 2-photon microscopy.
By observing mouse behavior and mapping it to neuron activity, it would be possible to connect physical actions to the activity of specific neurons. Xu says, “We can see function; we can see how the mouse thinks.” Now that the team has turned its proof-of-concept into a working microscope, the new technique can be used for neuroscience research.
Previously, neuroscientists could only obtain images so deep into the brain by removing the top part of the cortex or inserting a glass plug deep into the brain. The new imaging technique can be used without any such drastic actions.
The potential of the noninvasive imaging technique is considerable. Since the imaging technique can be used to show neuronal activity deep into the brain – as deep as the hippocampus – it could greatly enhance studies into Alzheimer’s disease.
The paper, In vivo three-photon imaging of activity of GCaMP6-labeled neurons deep in intact mouse brain, was recently published in the journal, Nature Methods.