Skill gained after a short period of practice in one motor task can be abolished if a second task is learned shortly afterwards, but not all motor activities cause interference. After all it is not necessary to remain completely still after practicing a task for learning to occur. Here we ask which mechanisms determine whether or not interference occurs. We hypothesised that interference requires the same neural circuits to be engaged in the two tasks and provoke competing processes of synaptic plasticity.
To test this, subjects learned a ballistic ankle plantarflexion task. Early motor memory was disrupted by subsequent learning of a precision tracking task with the same agonist muscle group, but not by learning involving antagonist muscles or by voluntary agonist contractions that did not require learning. If the competing task was learned with the same agonist muscle group 4 hours following learning of the primary task, no interference was observed.
Previous studies have suggested that primary motor cortex (M1) may be involved in early motor memory consolidation. 1Hz Repetitive Transcranial Magnetic Stimulation (rTMS) of corticospinal motor output at intensities below ankle movement threshold, which has been demonstrated to depress M1 excitability did however not abolish early motor memory. In contrast, 1Hz rTMS of M1 at a suprathreshold intensity (115% of motor threshold) which elicited motor evoked potentials did cause interference. This could indicate a role of sensory input in interference.
1Hz Repetitive electrical stimulation of the peripheral nerve to the plantarflexors (but not extensors) also caused interference. This was only the case if the stimulation intensity was sufficient to evoke a muscle response. We conclude that interference effects in motor learning are remarkably specific for circuits involved in activation of individual muscle groups and depends crucially on sensory error signals. We speculate, that this is because sensory feedback constitutes important error signals required for synaptic changes to occur during motor learning.