The researchers suspected that one part of the brain stem in particular, the dorsal raphe nucleus, might be crucial to behaviors that control effort. This cluster of cells is a production hub for serotonin – a chemical messenger that changes the firing behavior of other cells. Serotonin is associated with mood modulation; many antidepressant drugs, for instance, may act by increasing serotonin concentration in the brain.
When the pathway between the prefrontal cortex and the dorsal raphe nucleus was stimulated, rodents facing a challenge in the lab showed an immediate and dramatic surge in motivation.
Curiously, however, when the rodents were relaxing in their home environment, the same stimulation had no effect. The pathway was not merely linked to any action, or to agitation; it was, more specifically, helping to set the effort that the organism was willing to put forth to meet a challenge.
Researchers were also able to produce the opposite effect – reduced effort in response to challenge – by stimulating prefrontal neurons that project to the lateral habenula, a region perched atop the brain stem that is thought to play a role in depression. When this region was getting signals driven optogenetically from the prefrontal cortex, rodents put forward less effort.