mammalian brain
A Real-Time Look at Value-Based Decision Making
Posted on by Dr. Francis Collins
All of us make many decisions every day. For most things, such as which jacket to wear or where to grab a cup of coffee, there’s usually no right answer, so we often decide using values rooted in our past experiences. Now, neuroscientists have identified the part of the mammalian brain that stores information essential to such value-based decision making.
Researchers zeroed in on this particular brain region, known as the retrosplenial cortex (RSC), by analyzing movies—including the clip shown about 32 seconds into this video—that captured in real time what goes on in the brains of mice as they make decisions. Each white circle is a neuron, and the flickers of light reflect their activity: the brighter the light, the more active the neuron at that point in time.
All told, the NIH-funded team, led by Ryoma Hattori and Takaki Komiyama, University of California at San Diego, La Jolla, made recordings of more than 45,000 neurons across six regions of the mouse brain [1]. Neural activity isn’t usually visible. But, in this case, researchers used mice that had been genetically engineered so that their neurons, when activated, expressed a protein that glowed.
Their system was also set up to encourage the mice to make value-based decisions, including choosing between two drinking tubes, each with a different probability of delivering water. During this decision-making process, the RSC proved to be the region of the brain where neurons persistently lit up, reflecting how the mouse evaluated one option over the other.
The new discovery, described in the journal Cell, comes as something of a surprise to neuroscientists because the RSC hadn’t previously been implicated in value-based decisions. To gather additional evidence, the researchers turned to optogenetics, a technique that enabled them to use light to inactivate neurons in the RSC’s of living animals. These studies confirmed that, with the RSC turned off, the mice couldn’t retrieve value information based on past experience.
The researchers note that the RSC is heavily interconnected with other key brain regions, including those involved in learning, memory, and controlling movement. This indicates that the RSC may be well situated to serve as a hub for storing value information, allowing it to be accessed and acted upon when it is needed.
The findings are yet another amazing example of how advances coming out of the NIH-led Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative are revolutionizing our understanding of the brain. In the future, the team hopes to learn more about how the RSC stores this information and sends it to other parts of the brain. They note that it will also be important to explore how activity in this brain area may be altered in schizophrenia, dementia, substance abuse, and other conditions that may affect decision-making abilities. It will also be interesting to see how this develops during childhood and adolescence.
Reference:
[1] Area-Specificity and Plasticity of History-Dependent Value Coding During Learning. Hattori R, Danskin B, Babic Z, Mlynaryk N, Komiyama T. Cell. 2019 Jun 13;177(7):1858-1872.e15.
Links:
Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)
Komiyama Lab (UCSD, La Jolla)
NIH Support: National Institute of Neurological Disorders and Stroke; National Eye Institute; National Institute on Deafness and Other Communication Disorders
Mammalian Brain Like You’ve Never Seen It Before
Posted on by Dr. Francis Collins
Credit: Gao et. al, Science
Researchers are making amazing progress in developing new imaging approaches. And they are now using one of their latest creations, called ExLLSM, to provide us with jaw-dropping views of a wide range of biological systems, including the incredibly complex neural networks within the mammalian brain.
In this video, ExLLSM takes us on a super-resolution, 3D voyage through a tiny sample (0.0030 inches thick) from the part of the mouse brain that processes sensation, the primary somatosensory cortex. The video zooms in and out of densely packed pyramidal neurons (large yellow cell bodies), each of which has about 7,000 synapses, or connections. You can also see presynapses (cyan), the part of the neuron that sends chemical signals; and postsynapes (magenta), the part of the neuron that receives chemical signals.
At 1:45, the video zooms in on dendritic spines, which are mushroom-like nubs on the neuronal branches (yellow). These structures, located on the tips of dendrites, receive incoming signals that are turned into electrical impulses. While dendritic spines have been imaged in black and white with electron microscopy, they’ve never been presented before on such a vast, colorful scale.
The video comes from a paper, published recently in the journal Science [1], from the labs of Ed Boyden, Massachusetts Institute of Technology, Cambridge, and the Nobel Prize-winning Eric Betzig, Janelia Research Campus of the Howard Hughes Medical Institute, Ashburn, VA. Like many collaborations, this one comes with a little story.
Four years ago, the Boyden lab developed expansion microscopy (ExM). The technique involves infusing cells with a hydrogel, made from a chemical used in disposable diapers. The hydrogel expands molecules within the cell away from each other, usually by about 4.5 times, but still locks them into place for remarkable imaging clarity. It makes structures visible by light microscopy that are normally below the resolution limit.
Though the expansion technique has worked well with a small number of cells under a standard light microscope, it hasn’t been as successful—until now—at imaging thicker tissue samples. That’s because thicker tissue is harder to illuminate, and flooding the specimen with light often bleaches out the fluorescent markers that scientists use to label proteins. The signal just fades away.
For Boyden, that was a problem that needed to be solved. Because his lab’s goal is to trace the inner workings of the brain in unprecedented detail, Boyden wants to image entire neural circuits in relatively thick swaths of tissue, not just look at individual cells in isolation.
After some discussion, Boyden’s team concluded that the best solution might be to swap out the light source for the standard microscope with a relatively new imaging tool developed in the Betzig lab. It’s called lattice light-sheet microscopy (LLSM), and the tool generates extremely thin sheets of light that illuminate tissue only in a very tightly defined plane, dramatically reducing light-related bleaching of fluorescent markers in the tissue sample. This allows LLSM to extend its range of image acquisition and quickly deliver stunningly vivid pictures.
Telephone calls were made, and the Betzig lab soon welcomed Ruixuan Gao, Shoh Asano, and colleagues from the Boyden lab to try their hand at combining the two techniques. As the video above shows, ExLLSM has proved to be a perfect technological match. In addition to the movie above, the team has used ExLLSM to provide unprecedented views of a range of samples—from human kidney to neuron bundles in the brain of the fruit fly.
Not only is ExLLSM super-resolution, it’s also super-fast. In fact, the team imaged the entire fruit fly brain in 2 1/2 days—an effort that would take years using an electron microscope.
ExLLSM will likely never supplant the power of electron microscopy or standard fluorescent light microscopy. Still, this new combo imaging approach shows much promise as a complementary tool for biological exploration. The more innovative imaging approaches that researchers have in their toolbox, the better for our ongoing efforts to unlock the mysteries of the brain and other complex biological systems. And yes, those systems are all complex. This is life we’re talking about!
Reference:
[1] Cortical column and whole-brain imaging with molecular contrast and nanoscale resolution. Gao R, Asano SM, Upadhyayula S, Pisarev I, Milkie DE, Liu TL, Singh V, Graves A, Huynh GH, Zhao Y, Bogovic J, Colonell J, Ott CM, Zugates C, Tappan S, Rodriguez A, Mosaliganti KR, Sheu SH, Pasolli HA, Pang S, Xu CS, Megason SG, Hess H, Lippincott-Schwartz J, Hantman A, Rubin GM, Kirchhausen T, Saalfeld S, Aso Y, Boyden ES, Betzig E. Science. 2019 Jan 18;363(6424).
Links:
Video: Expansion Microscopy Explained (YouTube)
Video: Lattice Light-Sheet Microscopy (YouTube)
How to Rapidly Image Entire Brains at Nanoscale Resolution, Howard Hughes Medical Institute, January 17, 2019.
Synthetic Neurobiology Group (Massachusetts Institute of Technology, Cambridge)
Eric Betzig (Janelia Reseach Campus, Ashburn, VA)
NIH Support: National Institute of Neurological Disorders and Stroke; National Human Genome Research Institute; National Institute on Drug Abuse; National Institute of Mental Health; National Institute of Biomedical Imaging and Bioengineering
