Planned movement is crucial in our everyday lives, yet it often necessitates deferred implementation. We stood crouched and ready as kids, but we waited for the “GO!” signal before running from the starting line. As adults, we wait for the light to turn green before making a turn. In both cases, the brain has planned our exact moves but has suppressed their execution until a particular trigger (e.g., a yell of “GO!” or a green light) is given. Scientists have now identified the brain network that converts plans into actions in response to this stimulus.
The finding, which was published in the scientific journal Cell, was made by scientists from the Max Planck Florida Institute for Neuroscience, the HHMI’s Janelia Research Campus, the Allen Institute for Brain Science, and others. The scientists set out to explore how stimuli in our surroundings might prompt planned movement, led by co-first authors Dr. Hidehiko Inagaki and Dr. Susu Chen, and senior author Dr. Karel Svoboda.
“The brain is like an orchestra,” Dr. Inagaki said. “Instruments in a symphony perform a variety of melodies with varying tempos and timbres. A musical phrase is formed by the combination of these sounds. Similarly, neurons in the brain are active in a variety of patterns and at different times. Specific features of human behavior are mediated by the ensemble of neural processes.”
The motor cortex, for example, is a brain region that governs movement. The patterns of activity in the motor cortex alter considerably between the planning and execution stages of movement. The transition between these patterns is necessary in order to initiate movement. However, the brain regions in charge of this shift were unclear. “There must be brain regions serving as conductors,” Dr. Inagaki said. “These regions are responsible for monitoring environmental inputs and orchestrating neuronal actions from one pattern to the next. The conductor ensures that plans are put into effect at the appropriate moment.”
The scientists monitored the activity of hundreds of neurons as a mouse conducted a cue-triggered movement challenge to discover the brain circuit that acts as the conductor to activate planned movement. Mice were taught to lick to the right if their whiskers were touched and to the left if their whiskers were not touched in this exercise. The animals were rewarded if they licked in the right direction. But there was a catch. The animals had to wait for a tone, or “go cue,” before moving. Only proper actions after the go cue would be rewarded. As a result, mice keep a plan for which direction they will lick until the go signal and then execute the planned lick.
The researchers then linked intricate neural activity patterns to specific phases of the behavioral task. The researchers discovered brain activation immediately after the go signal and throughout the transition between motor planning and execution. This activity in the brain was caused by a network of neurons in the midbrain, thalamus, and cortex.
The scientists employed optogenetics to determine if this circuit operated as a conductor. Using light, the scientists were able to activate or deactivate this circuit. Activating this circuit during the behavioral task’s planning phase moved the mouse’s brain activity from motor planning to execution, causing the mouse to lick. Turning off the circuit while playing the go cue, on the other hand, repressed the cued movement. The mice stayed in the motor planning stage as if they had not been given the go signal.
Dr. Inagaki and his colleagues discovered a neuronal circuit that is essential for activating movement in response to environmental signals. Dr. Inagaki shows how their results show generalizable behavioral control aspects. “We discovered a circuit that can switch motor cortex activity from motor planning to execution at the proper moment. This reveals how the brain orchestrates neural activity to achieve complicated behavior. Future research will look at how this circuit and others restructure neuronal activity across several brain areas.”
This research has substantial therapeutic implications in addition to making basic advancements in understanding how the brain works. Patients with motor abnormalities, such as Parkinson’s disease, have difficulties initiating self-initiated movement, including trouble walking. Adding contextual signals to stimulate movements, such as lines on the floor or auditory tones, may, on the other hand, significantly increase a patient’s mobility. This phenomenon, known as paradoxical kinesia, shows that separate brain pathways are activated for self-initiated and cue-triggered movement. The discovery of brain networks involved in cue-triggered movements, which are substantially spared in Parkinson’s disease, may aid in therapy optimization.