Modulation of corticospinal excitability by reward depends on task framing

Area of Science:

• Neuroscience
• Cognitive Neuroscience
• Motor Control

Background:

• Previous transcranial magnetic stimulation (TMS) studies indicate primary motor cortex (M1) sensitivity to environmental reward.
• The precise influence of reward conditions on M1 activity remains unclear.
• Dopamine neuron responses encode reward probability and outcome uncertainty, crucial for decision-making.

Purpose of the Study:

• To investigate how reward probability and uncertainty independently modulate corticospinal excitability in M1.
• To determine if M1 activity reflects reward probability or outcome uncertainty.
• To examine the effect of task framing (seeking vs. avoiding) on M1 response to reward.

Main Methods:

• Single-pulse transcranial magnetic stimulation (TMS) was used to measure corticospinal excitability via motor evoked potential (MEP) amplitude.
• Participants (n=40) performed a computer task involving choices with varying reward probabilities and uncertainty levels.
• Paired-pulse TMS was administered to a subset to assess short-intracortical inhibition (SICI).

Main Results:

• Motor evoked potential (MEP) amplitude increased with rising reward probability specifically when participants were instructed to "find" a target.
• No significant effect of reward uncertainty on MEP amplitude was observed.
• Response times were influenced by the number of choices presented in the task.

Conclusions:

• M1 modulation by reward is primarily driven by reward probability or perceived utility, not outcome uncertainty.
• The conceptual framing of a task (seeking vs. avoiding) significantly alters M1's response to reward probability.
• Short-intracortical inhibition (SICI) did not appear to be modulated by the task conditions.