Response variability in a rhythmic tapping task during sub- and suprathreshold TMS over motor cortex.
Konkle, Verstynen, & Ivry
During repetitive finger tapping, transcranial magnetic stimulation (TMS) over the primary motor cortex (M1) increases temporal variability. Using the two-component model of Wing and Kristofferson (1973), we have shown that this increase is specific to the estimate of motor implementation variability and has no effect on the estimate of central noise. In the current study, we simultaneously varied the stimulation intensity and recorded from active muscles in order to understand the specific mechanisms behind this variability. Each trial consisted of 10 paced taps (metronome every 450 ms), followed by 30 unpaced taps. During the unpaced phase, random TMS pulses were delivered to the M1 hand area or a control site (medial occipital-parietal junction). TMS stimulation intensity for a given trial was set to 85%, 105%, or 125% of active motor threshold, and EMG activity of the first dorsus interosseous (FDI) and extensor indicis proprius (EIP) muscles was recorded. Subthreshold TMS did not affect performance. At the two suprathreshold levels, stimulation over M1 led to an increase in temporal variability, and increased with the magnitude of stimulation intensity. This increase was specific to the estimate of motor implementation variability. At a finer-level of analysis, the TMS-induced noise was only apparent during certain phases of FDI and EIP activity and resulted in delaying the tap after the pulse. Moreover, the effect of the TMS pulse was not limited to the current interval, but also extended to subsequent intervals. These results suggest that a) TMS-induced motor noise only occurs when the descending corticospinal tracts are stimulated, b) there is a critical window in which M1 stimulation affects motor responses, and c) the effects of a single TMS pulse can persist into subsequent responses.
Konkle, T., Verstynen, T., & Ivry, R. B. Response variability in a rhythmic tapping task during sub- and suprathreshold TMS over motor cortex. Program No. 180.10. 2004 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2004. Online.