Our brain is constantly making decisions about
movement: Should I cross the street now or at the intersection? When you're presented with
two options, your brain's motor neurons prep for both possibilities
before you've decided which action to take, say researchers in a study
published in the journal Cell Reports.
"The brain is continuously translating visual targets into actions
that can be performed on those targets," says study co-author Jason
Gallivan, a neuroscientist at Queen's University in Ontario, Canada.
"Even outside your conscious awareness, your motor system appears to
always be operating in the background, coming up with these potential
‘When you are presented with two options, your brain's motor neurons prep for both possibilities before you've decided which action to take.’
For example, imagine a hockey forward speeding across the ice
towards the goal. As the forward approaches, he must dodge the other
team's defense and find an opening to shoot the puck past the goalie.
The forward sees two openings. Within a split second, neurons in the
hockey player's motor cortex fire and encode the muscle commands needed
to take both of the two possible shots. Both plans of attack are primed
and ready to go. The forward decides on one target, but suddenly, one of
the other team's defenders appears out of nowhere, blocking the shot.
Without missing a beat, the forward's sensorimotor system pivots to the
already-encoded plan B. He takes the shot.
"Because you've already specified these two plans in the brain, you
can readily switch between and implement each one more quickly if you
need to," says Gallivan. "This makes your reaction time quicker. So if
the goalie were to move one way or the other, you could more quickly
launch the alternative plan."
There has been a long debate over which comes first - the decision
about which target to act on or the movement plan. Although previous
studies have shown activations for multiple potential targets in the
sensorimotor regions of the brain, this activity could either encode the
visual locations of the targets or the motor plans required to act on
the targets. On a hockey rink (and in everyday life), motor decisions
happen so fast that it's proved extremely difficult to disentangle these
However, Gallivan and his colleagues devised a task that separated
visual targets from the movements needed to reach them. In the
experiment, 16 volunteers were asked to steer a cursor towards one of
two targets, but the catch was that they had to start the movement
before finding out which of the two targets they'd have to pick. "When
you're forced to launch an action without knowing which target is going
to be selected, people simply launch actions that are right down the
middle, between the targets," says Gallivan. The question was: Was the
motor cortex averaging the distance between targets or splitting the
difference between two potential movement plans?
Unknown to the volunteers, there was a critical hidden feature
to the task. At first, the position of the cursor matched the position
of the hand exactly, but with each repetition of the task, the cursor
slipped a little bit more out of sync with the controller. Because the
change was so gradual and because the controller was covered so that the
volunteers couldn't see their hand, people unconsciously compensated
for the controller-cursor mismatch by appropriately altering their hand
movement. By the end of the experiment, the difference between the
movement path needed to reach the target and the trajectory of the
cursor on the screen was 30 degrees.
When the researchers analyzed the data, they found that the
volunteers' "down the middle" hand movements were the average of the
movement paths needed to reach the two potential targets, not the
average between the two target positions on the screen. "The faithful
relationship between the two really surprised us," says Gallivan. "The
spatial averaging behavior is not strategic or deliberate, and it's not
linked to target locations."
This finding supports the idea that the brain perceives the world as
a series of possible actions and objects to interact with. Having
immediately available backup plans likely has tangible benefits, but
researchers are still looking into what those benefits are. Gallivan's
lab also plans to follow up with fMRI studies to see what motor encoding
looks like in the brain.