Rockefeller University researchers have taken a step closer to achieving the current "holy grail" of brain science – the ability to look into a living brain and see all the neurons firing in real time, with the subject free to move around and perform tasks.

Neurons within a three-dimensional section of mouse brain were genetically altered to flouresce as they signal to one another(Credit: Rockefeller University)

The multidisciplinary team announced last month that it has created a breakthrough tool that uses a special light-sculpting technique to record the activity of thousands of neurons in a three dimensional section of a mouse's brain, while the animal walked on a treadmill.

Although every neuroscience story seems to feature pictures of neurons "lighting up", in reality there's no visual cue to show that electrical activity between brain cells is actually happening.

This is major barrier to neuroscience, which aims to understand how sensory input leads to behavior and learning. Scientists need a tool that can capture short-lived events within large amounts of brain tissue so they can map connections between neurons, examine how the cells interact in real time and observe how the dynamics within different regions of the brain influence behavior.

Understanding the molecular mechanisms underpinning the brain's processes such as decision-making, movement, and sleep could have huge implications. What would it mean if we understood how the brain of a champion athlete, a stroke victim or a sleep apnea patient worked?

Dr Alipasha Vaziri, lead researcher in the Rockefeller study, has been dedicated for the last six years to the task of developing and using imaging techniques that can be used across species to capture neuronal activity over large areas of the brain at high speeds and single cell resolution.

In recent years, scientists have had success in genetically engineering neurons to produce a protein that fluoresces when its exposed to the electrical signal within the neuron, making activated neurons easier to observe. Vaziri was involved in successfully developed an imaging technique that enabled his team to record the electrical activity inside the brain of a 302-neuron roundworm brain, before moving on to the 100,000 neuron brain of a larval zebrafish.

Scaling up the technique to study a living rodent was more challenging: a mouse's brain is more complex, at around 70 million neurons, and the brain is opaque, unlike the more transparent worm and larval fish brains.

The required microscopic imaging system would have to include a way of efficiently exciting fluorescence from genetically engineered mouse brain cells while moving quickly enough to scan and capture the activity of thousands of the cells in a three dimensional section of brain.

They accomplished the task by using a technique based on photographic light sculpting. Short pulses of laser light are dispersed into their spectral components and brought back together to generate a sculpted excitation sphere. The sphere is then scanned to illuminate the neurons within a plane, then refocused on another layer of neurons above or below, allowing neuron signals to be recorded in three dimensions.

In this way, they recorded the activity in most of the mouse's cortical column, a section responsible for planning movement. The researchers believe this will be able to help them understand brain computation as a whole and are now working to capture the activity of the entire cortical column.

"Progress in neuroscience, and many other areas of biology, is limited by the available tools," Vaziri says. "By developing increasingly faster, higher-resolution imaging techniques, we hope to be able to push the study of the brain into new frontiers."

You can read the paper about the study at Nature Methods.

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