Summary: Motor system neurons not only control movement; they also incite action.
Motor system neurons not only control movement, but stimulate it. This is the surprising discovery made by the UCLouvain Cognition and Action Laboratory.
Have you ever made a decision too quickly, resulting in a poor choice? The answer is certainly yes: the propensity to make good choices systematically decreases as the speed at which decisions are made increases. What explains this? The “speed-accuracy trade-off,” which is universal in the animal kingdom.
There’s no escaping this trade-off, but it’s possible to voluntarily regulate it according to the context, by favoring either decision-making speed (to the detriment of the propensity to make good choices) or caution (to the detriment of decision-making speed).
In a recent study published in PLOS Biology, researchers from the Cognition and Actions Lab (UCLouvain Institute of Neuroscience, IoNS) made a surprising discovery. Dr. Gérard Derosière and Prof. Julie Duqué, in collaboration with Dr. David Thura (INSERM, Lyon) and Prof. Paul Cisek (University of Montreal), demonstrated that the activity of neurons projecting to the muscles involved in the execution of a chosen action was strongly amplified when subjects favored quick decisions.
More important, they discovered that this amplification is present in other groups of neurons that project to muscles that are not at all involved in the execution of the chosen action.
Another discovery was that the activity of a third type of neuron is rapidly reduced during the decision, which not only allows us to decide quickly but also to contract specific muscles quickly and thus move more quickly.
The motor system is involved in our choices
“We’ve succeeded in demonstrating that motor system neurons not only control movement but also incite action,” explains FNRS scientific collaborator Dr. Derosière.
“While decision-making is commonly associated with the brain’s prefrontal structures, located just above the eyes, our work shows the importance of the motor system in the speed of our choices and in impulsivity.”
Recent research on decision-making seemed to suggest that rapidly made choices were based on global changes in the activity of these neurons.
However, this hypothesis remained speculative and it remained unclear how decision speed was regulated, because the tools used by scientists did not, until now, allow the activity of these neurons to be recorded accurately.
The UCLouvain team had the idea of using an existing tool, transcranial magnetic stimulation, to establish more precise measurements. The subject is asked to perform a task that requires decision-making; at the same time, neurons in the motor cortex are stimulated and potentials in several muscles (up to nine!) are measured.
“Typically, transcranial magnetic stimulation is used to measure potentials in a single muscle,” explains Dr. Derosière. “Here, we stimulated several locations within the skull, which enabled us to measure potentials in many muscles and to establish a detailed map of changes in neuron activity with very good spatial resolution.”
The result: the researchers were able to demonstrate, for the first time, an amplification of motor cortex activity in a context of impulsivity, but also an amplification of the potentials in the legs of a subject who moves his hands, even though his lower limbs perform no movement!
What’s the point of this research? Thanks to this study, we know that motor system neurons are directly involved in the regulation of decision speed in healthy subjects. But in addicts (alcoholics, for example), this regulation is disrupted: they decide impulsively and make bad choices. Studying the motor system’s role in this context makes sense in the light of this discovery.
Hasty sensorimotor decisions rely on an overlap of broad and selective changes in motor activity
Humans and other animals are able to adjust their speed–accuracy trade-off (SAT) at will depending on the urge to act, favoring either cautious or hasty decision policies in different contexts.
An emerging view is that SAT regulation relies on influences exerting broad changes on the motor system, tuning its activity up globally when hastiness is at premium. The present study aimed to test this hypothesis.
A total of 50 participants performed a task involving choices between left and right index fingers, in which incorrect choices led either to a high or to a low penalty in 2 contexts, inciting them to emphasize either cautious or hasty policies.
We applied transcranial magnetic stimulation (TMS) on multiple motor representations, eliciting motor-evoked potentials (MEPs) in 9 finger and leg muscles. MEP amplitudes allowed us to probe activity changes in the corresponding finger and leg representations, while participants were deliberating about which index to choose. Our data indicate that hastiness entails a broad amplification of motor activity, although this amplification was limited to the chosen side.
On top of this effect, we identified a local suppression of motor activity, surrounding the chosen index representation. Hence, a decision policy favoring speed over accuracy appears to rely on overlapping processes producing a broad (but not global) amplification and a surround suppression of motor activity.
The latter effect may help to increase the signal-to-noise ratio of the chosen representation, as supported by single-trial correlation analyses indicating a stronger differentiation of activity changes in finger representations in the hasty context.