Differential effect of linguistic and non-linguistic pen-holding tasks on motor cortex excitability

Writing and drawing are unique human activities. They are complex high-precision actions, which involve not only the motor system but also various cognitive systems, such as attention, short-term memory, action control, and language. In relation to motor control, the study of writing and drawing is of great interest as they provide insight in the interaction between motor control processes and the concurrent non-motor processes. Although sharing similar motor and mechanical demands, writing and drawing involve different levels of linguistic/semantic load and thus may be associated with different modulation of motor cortical excitability. Here, we have used transcranial magnetic stimulation to study separately activation of excitatory and inhibitory mechanisms of the motor cortex during performance of writing and drawing acts as well as during simple pen-squeezing task. While cortical excitatory mechanisms appeared to be saturated by the pure motor demands of the tasks, and thus not amenable to modulation by the tasks' linguistic load, variation in cortical inhibitory activity was the main vehicle for differential modulation of motor cortical excitability by linguistic demands of the tasks. The results of this study highlight the importance of cortical inhibitory mechanisms in the physiology of higher cognitive activities. They also provide further evidence that the task specific modulation of the excitability of the motor cortex goes beyond motor complexity of the task and is also dependant on associated cognitive components.