Summary: A new study reports both planned and spontaneous movements have the same neural activity during the action, but the preceding brain activity differs.
The runners are lined up at the starting line, patiently awaiting the start signal for the 1000-meter race. In the second turn a runner falls in front of the one next to him. He dodges his falling neighbor and continues to sprint towards the finish line. Whilst awaiting the start signal, the runner had time to plan his first steps, whilst dodging, he had to react immediately.
Until now, the difference between the brain activity of planned and spontaneous movements have been unknown. Scientists from the German Primate Center – Leibniz Institute for Primate Research (DPZ) have been able to show in their recently published study of two rhesus monkeys that planned and spontaneous gripping movements have the same brain activity during the movement but that the preceded brain activity differs. This helps us to understand what happens in the brain when we plan a movement and not execute it immediately – an important finding that could be helpful for clinical rehabilitation measures (The Journal of Neuroscience).
The neuroscientists Benjamin Dann and Jonathan Michaels of the German Primate Center have trained two rhesus monkeys to perform a gripping movement when a certain signal appears. Depending on whether a green or white circle appeared on a screen, the animals had to make a power grip with the whole hand or a precision grip with two fingers. However, they were only allowed to execute the movement when a red circle disappeared from the monitor. In the brain, the first decision was to choose the type of movement (power or precision grip) and then wait for the signal to indicate the actual start of the movement. This waiting period varied from 0 to 1300 milliseconds.
In order to systematically study the interplay of planning and movement in the brain, the scientists measured the activity of populations of neurons responsible for generating and executing grasping movements in two different brain regions. Depending on the length of the waiting period before the animals were allowed to perform the indicated movement, the original activity of the neuronal populations of both areas changed to a state of planning. “Our results show that planning a movement not only retains the brain activity necessary to perform a movement, but it also shows that a new state of activity for movements from short-term memory exists,” says Benjamin Dann, one of the two lead authors of the study.
These findings could be helpful for developing rehabilitation measures for patients who, for example after a stroke or a tumor operation, have difficulties with planning and initializing of movements. “If we understand how exactly our brain works when planning a movement, we could treat motoric handicaps more specific in future,” says Benjamin Dann.
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to Benjamin Dann.
Original Research: Abstract for “Neural dynamics of variable grasp movement preparation in the macaque fronto-parietal network” by Jonathan A Michaels, Benjamin Dann, Rijk W Intveld and Hansjörg Scherberger in Journal of Neuroscience. Published May 24 2018
Neural dynamics of variable grasp movement preparation in the macaque fronto-parietal network
Our voluntary grasping actions lie on a continuum between immediate action and waiting for the right moment, depending on the context. Therefore, studying grasping requires investigating how preparation time affects this process. Two macaque monkeys (Macaca mulatta – 1 male, 1 female) performed a grasping task with a short instruction followed by an immediate or delayed go cue (0-1300 ms) while we recorded in parallel from neurons in the grasp preparation relevant areas F5 of the ventral premotor cortex and the anterior intraparietal area (AIP). Initial population dynamics followed a fixed trajectory in the neural state space unique to each grip type, reflecting unavoidable movement selection, then diverged depending on the delay, reaching unique states not achieved for immediately cued movements. Population activity in AIP was less dynamic, whereas F5 activity continued to evolve throughout the delay. Interestingly, both areas contained a readout tracking subjective anticipation of the go cue that predicted single trial reaction time, however the prediction of reaction time was better from F5 activity. Intriguingly, activity during movement initiation clustered into two trajectory groups, corresponding to movements that were either ‘as fast as possible’ or withheld movements, demonstrating a widespread state shift in the fronto-parietal grasping network when movements must be withheld. Our results reveal how dissociation between immediate and delay-specific preparatory activity, as well as differentiation between cortical areas, is possible through population level analysis.
Many of our movements must occur with no warning, while others are planned in advance. Yet, it’s unclear how preparation for movements along the spectrum between these two situations differs in the brain. Two macaque monkeys made reach to grasp movements after varying amounts of preparation time while we recorded from premotor and parietal cortex. We found that the initial response to a grasp instruction was specific to the required movement, but not to the preparation time, reflecting required movement selection. However, when more preparation time was given, neural activity achieved unique states that likely related to withholding movements and anticipation of movement, shedding light on the roles of premotor and parietal cortex in grasp planning.