Paired-pulse transcranial magnetic stimulation of primary somatosensory cortex differentially modulates perception and sensorimotor transformations.

Intermodal selective attention is generally associated with facilitation of relevant information. However, recent studies demonstrate reduced activation of primary somatosensory cortex (S1) with continuous vibrotactile tracking during bimodal stimulation. Reduced activation has been hypothesized to reflect an interaction between the sensorimotor and intermodal requirements of the tracking task. Recently, it has been shown that transcranial magnetic stimulation (TMS) involving a supra-threshold test stimulus (TS) preceded by a sub-threshold conditioning stimulus (CS) adversely affects tactile perception by altering excitability of local intracortical circuits. The purpose of the current paper was to use TMS to assess the effects of differential sensorimotor requirements in the right sensorimotor cortex upon local intracortical networks and sensory processing in the left primary somatosensory cortex during constant multimodal stimulation. Single and paired-pulse TMS was used to probe intracortical networks in S1 and sensory processing during a sensorimotor task where a vibrotactile stimulus to the right index finder guided either continuous or discrete sensorimotor responses of the left hand. It was hypothesized that paired-pulse TMS would alter local intracortical networks and reduce performance during the discrete sensorimotor task, but that these effects would be mitigated during the continuous sensorimotor task, possibly a reflection of reduced S1 activation observed previously during a similar continuous sensorimotor task. Regardless of sensorimotor requirements, single-pulse TMS delivered over S1 decreased sensorimotor performance. Paired-pulse TMS further decreased sensorimotor performance only when the vibrotactile stimulus guided a discrete motor response but not when it was required to continuously guide the motor response. This effect disappeared when the TS was replaced by a sub-threshold stimulus. These results suggest that the CS facilitates sensory output neurons during perceptual detection but that differential responsiveness of local cortical networks in S1 suppresses the CS effects during continuous sensory-guided movement. This study highlights the importance of sensorimotor requirements in determining the net result of task-related sensory processing in S1.