In a quiet laboratory, tucked away in the Rangos Life Sciences building in East Baltimore, an assortment of electronic gadgets, vertically stacked on ceiling-high metal racks, measure the brain activity of a laboratory mouse as it decides which of two spouts will deliver a satisfying drop of water. As the mouse begins to contemplate its move, the animal’s nerve cells fire spikes of electric current, sounding like crackling radio noise on a nearby audio monitor.
These trains of electrical pulses, although discordant to the ears, contain an organized code of how the brain processes information coming in from the world. By measuring how nerve cells in the decision-making part of the brain fire electric pulses when mice perform a “think before you act” task, Johns Hopkins scientist Jeremiah Cohen and his team probe a fundamental question in neuroscience: What happens in the brain when we think, plan and make decisions?
Cognition, or the way we think, is the driving force behind all purposeful behaviors. In the early 20th century, many psychologists, called behaviorists, abandoned an introspection approach to understanding how the mind controls behavior. Instead, they studied behavior in terms of reward-based learning, the idea that organisms repeat behaviors if their actions lead to positive consequences, just like dogs can be made to perform tricks repeatedly with treats, or mice can be made to press levers for food or lick spouts for water.
Early behaviorists also viewed the brain as an impenetrable black box because mental processes that underlie cognition were difficult to measure with the limited tools available during their time.
Today, science and technology have advanced in ways that behavior psychologists of the last century would have never imagined — the entire genetic makeup of organisms is known and the processing power of computers has increased a trillion-fold since the 1950s. While scientists still use reward-based learning to study decision-making and behavior, they can now use the power of genetics to make nerve cells glow, trigger them to fire on command, and track how they are connected together to form intricate circuits in the brain.
“The repertoire of experimental tools accessible for neuroscientists today is unprecedented,” says Cohen. “We can now study decision-making in the brain at a level of detail that was not possible even two decades ago."
Cohen first started studying cognition as a graduate student at Vanderbilt University. At that time, his research focused on understanding how nerve cells in the front of the brain, called the prefrontal cortex, help monkeys decide where to look. Although non-human primates were a good animal model to study decision-making, he felt limited in the number of genetic tools he could use for his research.
“Inserting fluorescent proteins into particular nerve cells to label them is much harder in monkeys,” says Cohen. “I knew very little about the type of cells I was recording from or what other kinds of cells they were connected to in the brain. I was studying little islands of cells, but without much context.”
When Cohen started his own laboratory five years ago, he continued to study cognition but chose a different animal model — the mouse.
Unlike with primates, the main benefit of mice is the availability of genetically altered strains, which allow a variety of manipulations, like labeling cells with fluorescent proteins or making nerve cells express light-sensitive genes so that nerve cell “firing” can be experimentally controlled.
Cohen wanted to take advantage of the power of mouse genetics to focus on the role of the brain’s chemical messengers, called neuromodulators, in cognition. Most neuromodulators, like serotonin, dopamine and norepinephrine, play an essential role in regulating mood, allotting attention, controlling movement and making decisions. These biological molecules are produced by very small pockets of cells, but they broadcast their signals all over the brain, particularly to the prefrontal cortex — the seat of cognition.
Almost all of the nerve cells in the prefrontal cortex are influenced by the action of neuromodulators. But Cohen was interested in studying only those cells in the prefrontal cortex that showed the hallmarks of decision-making, that is, the cells that could both retain information for longer periods of time and also rapidly forget what has been learned to acquire new information.
“So, if you think of a mouse foraging in the forest, there has to be some cells in its prefrontal cortex that have a way of remembering the value of a fruit versus a blade of grass, and then these cells must be able to change those values if, say, the fruit is rotten,” explains Cohen.
Very quickly, he found himself facing the same impediment as he did in graduate school — finding specific cells in the prefrontal cortex that exclusively control decision-making.
Cohen and Bari
Cohen and his graduate student Bilal Bari found a solution to this technical challenge by using tools developed by neuroscientists in the Janelia Research Campus of the Howard Hughes Medical Institute. They used light-sensitive proteins in prefrontal cortex cells that connected to other brain areas that were also involved in decision-making.
“By shining blue light on the prefrontal cortex, I could turn on only those nerve cells that had light-sensitive proteins,” says Bari. “This was the magic bullet I needed to weed out other cells in the prefrontal cortex and focus on those playing a role in cognition.”
Cohen and Bari had just overcome one hurdle when another more critical challenge arose — designing mouse experiments that appropriately model how monkeys and humans make decisions.
Like primates, mice constantly make choices relating to their primary needs, such as deciding where to go to quench their thirst or finding a food source that is abundant. So, Cohen and Bari settled on exploring decision-making involved in a simple behavior common among mice, monkeys and humans: foraging for water.
Designing a setup that could accommodate both the gadgetry needed to record brain activity and the apparatus needed for the water-foraging task became Bari’s next challenge.
To measure activity from nerve cells, Bari connected a wire to each mouse’s brain to record electrical currents and an optic fiber to turn on cells with light-activated proteins. Also, within the tiny space under a surgical microscope, Bari positioned a waterspout a few millimeters away from the mouse’s nose. While the mouse was immobilized, he used a nearby aerosolizer and puffed into the air specific odors, like strawberry, lemon and spearmint, while at the same time providing a drop of water from the spout. Likewise, he puffed another odor without providing water in the spout. At the end of two weeks of training, the mouse only licked if it was cued by a particular scent.
All the ingredients needed for the experiments were finally in place. Bari could now record electrical activity from specific groups of cells in the prefrontal cortex while his mice were performing a decision-making task.
Despite the success Bari was having in getting his experiments up and going, he quickly recognized that the mice relied more on their memory rather than complex decision-making to make a choice about licking the waterspout.
“The mice passively observed what was happening in their environment without much control about whether they would get the water reward,” says Bari. “We needed the mice to make a choice to get a water reward based on their previous experience in finding rewards.”
Bari tweaked the original experiment to now have two spouts that randomly switched between letting out water and withholding it. He once again trained mice to lick the spout if they smelled certain odors. But this time, to get a water drop from the correct spout, mice had to remember which one delivered water in the previous round capturing, in essence, how mice would have to remember sources of water in their natural environment. This new experiment, says Bari, had just enough nuance to see mouse decision-making in action.
Just before Bari let loose the odor cue for licking, when the animal was deciding between which of two spout it wanted to lick, he found that only the cells labeled with light-activated proteins fired continuous jolts of electric currents, causing audio monitors to signal an increase in brain activity. And when the animal started to lick the spout of its choice, these same cells reduced their firing.
The increase in activity of these cells only during the time the animal was choosing between two spouts hinted that they might be holding onto the memory of which spout contained water, says Cohen.
As a reality check, Bari then silenced the same prefrontal cortex nerve cells using a drug. He saw an immediate change in the mouse’s behavior: It could not decide between the two spouts and kept licking the same waterspout over and over again, unable to make the right decision if the spout that carried water was suddenly switched. Cohen and Bari recently published their findings on the role of the prefrontal cortex cells in decision-making in the journal Neuron.
Having taken the initial steps in understanding how the prefrontal cortex may be driving decision-making and behavior, Cohen wants to take a deeper dive into how neuromodulators change the level of activity of prefrontal cortex neurons and consequently the way memories are stored for decision-making. Studying the effects of neuromodulators may have a practical application in psychiatry since many drugs target neuromodulators to reduce depression and mood-related symptoms, he says.
But his ultimate goal is to understand how different neuromodulators interact to have a broader understanding of how the human brain actually functions.
“My long-term goal is in expanding the understanding of behavior beyond the restricted set of conditions that we're studying as a field,” says Cohen. “We tend to study different types of nerve cells in isolation, but it’s clear that’s an extreme oversimplification. We need to put them all together to build a unified model about how different parts of the brain, containing different types of cells, work together to make us who we are.”