Researchers at the Stanford School of Engineering have discovered how the brain plans for and executes movements in reaction to a “go” signal.

Their model allows us to predict with four times greater accuracy what the reaction time of any single arm motion is going to be, based on the neural activity observed prior to movement.

The researchers trained two rhesus monkeys to perform the task of touching a target on cue. They then neurosurgically implanted a four-millimeter-square electronic chip arrayed with 100 tiny electrodes on the surface of the monkeys’ brains.

The researchers then concentrated on the dorsal pre-motor cortical area, which shows high levels of activity during the period of time when arm movement planning takes place. Activity in this region varies depending upon the direction, distance, and speed of a pending movement.

The existing hypothesis, known as “rise-to-threshold,” held that in anticipation of a “go” cue, our brains begin to plan the motions necessary to satisfactorily complete the movement by simply increasing the activity of neurons.

Neurons do begin to fire, but not enough to cause the movement to take place, the researchers said. Upon the “go” signal, the brain accelerates this neural firing until it crosses a “threshold” initiating the motion. According to the theory, the longer a preparatory period one has, the greater the neural activity will be and, thus, the faster the reaction time.

The researchers were able to document a process based less on the amount of activity and more on the trajectory of the neural activity through the brain. In graphs of neural activity prior to display of the target, the monkeys’ neural activity appears somewhat scattered. The moment a target is displayed, however, the neural activity concentrates in an activity region that the researchers dubbed the “optimal sub-space.”

The key to reaction time, the researchers found, is the relationship between where the neural activity is and its speed along the ideal trajectory just prior to the “go” cue. If the neural activity is closer to the final destination, the reaction time will be shorter; if farther away, longer.

A fundamental understanding of planning and movement is a central question in building electronic interfaces that convert neural activity into signals that can control computer cursors and prosthetic arms, the researchers said.

Ref.: Afsheen Afshar, et al., Single-Trial Neural Correlates of Arm Movement Preparation, Neuron, 11 August 2011; 71(3) pp. 555 – 564 [DOI: 10.1016/j.neuron.2011.05.047]