The voluntary movements we make must be "prepared" in our brain before they are executed. However, be it perfect timing, a false-start, or a delayed reaction, the neural circuitry underlying movement preparation is not well understood. Now a new study provides intriguing insight into how a neural circuit forms a motor plan. The research, published by Cell Press in the August 11 issue of the journal Neuron, uses a new type of analysis to assess the moment-by-moment firing rate of neurons in the brain to accurately predict the reaction time for making an arm movement.
We often prepare movements in anticipation of events, such as pressing the accelerator in the car when a traffic light turns green, but our reaction time can be frustratingly variable. Sometimes we are too slow to react, while other times we perform the movement before we are fully ready. "What is the cause of this imprecision?" asks senior study author, Dr. Krishna V. Shenoy from Stanford University. "Presumably, it is related to the neural operation of planning and executing movements." Based on earlier observation that neural activity follows a movement-dependent trajectory during preparation, Dr. Shenoy's group in collaboration with co-senior author Dr. Maneesh Sahani from University College London developed a hypothesis called the "initial condition hypothesis." It states that during movement preparation, the network firing activity in the motor system is brought to a suitable initial condition from which the sequence of neural commands that underlies a movement may be efficiently generated.
To test their new hypothesis, the researchers simultaneously recorded from tens to hundreds of neurons in the monkey premotor cortex, a part of the brain associated with the control of movement, while the animals were performing delayed-reach movements. The monkeys had extensive training in the movement and exhibited stereotypical neural activity trajectories, making it possible for the researchers to compare subtle differences in activity. "We found that the degree to which the neural activity had advanced, and the speed with which it had been advancing, along this trajectory at the time of the 'go' cue contributed substantially to determining reaction time," explains Dr Shenoy. "To our knowledge, the initial condition hypothesis leads to the best known trial-by-trial predictor of fluctuations in reaction time. Future studies are needed to determine whether reaction time can be predicted with similar accuracy under less-stereotypical conditions with untrained subjects."