Acquisition and extinction of conditioned action tendencies in the motor cortex

Routine behaviours can become habitual, persisting even when task goals have changed. Thus, it is important that we understand how these behaviours, and their underlying neurophysiological mechanisms, can be modulated. In the current experiment, participants were trained across multiple sessions to associate specific stimuli on a computer monitor with a key-press response. This learned action tendency, developed through repeated stimulus-response pairings, was then reduced using an extinction procedure in which the stimulus was repeatedly presented but no response was to be made. The underlying neurophysiological mechanisms of the learned response and its subsequent extinction were investigated using motor-evoked potentials (MEP) elicited by transcranial magnetic stimulation (TMS) delivered to the primary motor cortex (M1). We observed that exposure to the conditioned stimulus increased corticospinal excitability in M1, and this effect was modulated by the extinction procedure. We also found evidence that stimulation of M1 using TMS can trigger the release of a cued motor response that may have otherwise been withheld. This novel finding supports the notion that an associated motor plan is generated automatically in M1 when perceiving a conditioned stimulus associated with responding. Implications of these results in the context of conditioned action tendencies are discussed.