Researchers from the newly established Gerald Choa Neuroscience Centre of the Faculty of Medicine at The Chinese University of Hong Kong (CUHK) recently published a study in Nature Communications, unveiling the intricate processes of motor memory formation taking place in the brain. The findings may help understand the malfunctions of the nervous system in Parkinson’s disease, stroke and many other motor disorders so that better therapies can be developed.
Uncovering the process of motor learning by training rats to perform repetitive task for seven consecutive days
The primary motor cortex (M1) is arguably one of the most important brain areas that contributes to motor learning. Not only for the execution of movement, researchers also suggest the possibility that M1 may be able to acquire and store motor memory.
M1 circuitry exhibits interlaminar specificity. Layer two and three provides excitatory input to layer 5a (L5a) and layer 5b (L5b), while L5a also relays feedforward excitatory drive to L5b. This study was aimed to measure the temporal dynamics of single-neuron and population activities in L5b.
In the experiment, food-restricted rats were trained with a forelimb reaching and grasping task for seven consecutive days. Simultaneous multi-channel single-unit recordings at L5b of the forelimb territory in M1 were put in place to measure the results. Each rat received six 10-min training sessions per day, with 5-min rest intervals between sessions.
“A very challenging aspect of the study is to track the activities of a large number of simultaneously recorded neurons faithfully for the entire period of training. Over a thousand neural signals per minute needed to be recorded while the recording period lasted more than one week,” remarked Associate Professor Ya Ke of the School of Biomedical Sciences at CUHK.
Over days of training, results showed that the rats’ forelimb trajectories in reaching the food became more uniform. The timing of reaching action also shortened significantly in day one and exhibited further decrease in day two and three, and remained steady thereafter. Additionally, rats responded to the provision of food with progressively shorter and less variable delays in first reach success trials.
Task-recruited L5b neurons may help drive increased uniformity and precision of movement during motor learning
Researchers also observed that as the training continued, a substantial proportion of L5b neurons have been progressively changing from being non-informative about forelimb velocity and trajectory to possessing similar mutual information about motor behavioural outputs as neurons that exhibited clear movement encoding firing at the beginning of training.
Not only the sub-population of task-recruited L5b neurons became more movement-encoding, but their activities were also found to be more structured and temporally aligned to motor execution with a timescale of refinement in tens-of-milliseconds. The dopamine-dependent recruitment of L5b neuronal ensembles via synaptic reorganization may allow the motor cortex to generate more temporally structured, movement-encoding output signal from M1 to downstream circuitry that drives increased uniformity and precision of movement during motor learning.
‘This study also revealed that the formation of motor memory is highly disrupted in the absence of dopamine in the motor cortex, a condition that could be present in some Parkinson’s disease patients, explaining deficient motor learning in the disease,’ explained Dr Owen Ho Ko, Clinical Lecturer, Department of Medicine and Therapeutics, Faculty of Medicine at CUHK. MIMS
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