Verbal working memory modulates afferent circuits in motor cortex

Verbal instruction and strategies informed by declarative memory are key to performance and acquisition of skilled actions. We previously demonstrated that anatomically distinct sensory-motor inputs converging on the corticospinal neurons of motor cortex are differentially sensitive to visual attention load. However, how loading of working memory shapes afferent input to motor cortex is unknown. This study used short-latency afferent inhibition (SAI) to probe the effect of verbal working memory upon anatomically distinct afferent circuits converging on corticospinal neurons in the motor cortex. SAI was elicited by preceding a suprathreshold transcranial magnetic stimulus (TMS) with electrical stimulation of the median nerve at the wrist while participants mentally rehearsed a two- or six-digit numeric memory set. To isolate different afferent intracortical circuits in motor cortex SAI was elicited, using TMS involving posterior-anterior (PA) or anterior-posterior (AP) monophasic current. Both PA and AP SAI were significantly reduced during maintenance of the six-digit compared to two-digit memory set. The generalized effect of working memory across anatomically distinct circuits converging upon corticospinal neurons in motor cortex is in contrast to the specific sensitivity of AP SAI to increased attention load. The common response across the PA and AP SAI circuits to increased working memory load may reflect an indiscriminate perisomatic mechanism involved in the voluntary facilitation of desired and/or suppression of unwanted actions during action selection or response conflict.