[Abstract Title]. - Society for Neuroscience

investigate the role of short and long latency intracortical inhibition in the previoiusly observed
interhemispheric modulation of motor cortical excitability. We delivered focal transcranial
magnetic stimulation (TMS) to healthy, right-handed subjects targeting either the extensor carpi
radialis (ECR) or first dorsal interosseous (FDI) motor representations. A paired-pulse protocol
was applied to the left primary motor cortex using interstimulus intervals of 3 ms (short interval
intracortical inhibition, SICI) and 100 ms (long interval intracortical inhibition, LICI). The
conditioning stimulus was set at 80% and 120% of resting motor threshold (MT) for the SICI and
LICI trials respectively. In both cases the test stimulus was suprathreshold at 120% MT. Motorevoked
potentials (MEPs), recorded using surface electrodes, were measured while the
homologous muscle groups contralateral to the right target hand were both active and at rest. In
the active condition, EMG activity from the initiation of a dynamic contraction (10% of the
maximal voluntary contraction) of either the ECR or FDI ipsilateral to the stimulated motor
cortex was used to trigger the TMS. Motor thresholds were adjusted in the active conditions. For
SICI and LICI conditions, the peak-to-peak MEP amplitude was averaged for each condition and
the resulting data was normalized as a percentage change from the average MEP for single pulse
stimulation. Results showed that discrete unilateral contractions of both ECR and FDI increased
the excitability of the contralateral homologous muscle representation compared to when the
muscle was at rest. Further, voluntary contraction of the contralateral ECR significantly reduced
SICI whereas contraction of the FDI had a marginal effect on SICI in the homologous motor
areas of the opposite hemisphere. Active conditions for both ECR and FDI had little effect on
LICI. Our preliminary findings suggest that the interhemispheric modulation of motor cortical
excitability between homologous muscle representations is primarily mediated through pathways
acting on GABA(A) mediated inhibitory intracortical interneurons.
Support Contributed By: NSERC
Disclosures: M.E. Goddard , None; W.R. Staines, None.
Poster
277. Voluntary Movement: Cortical Planning and Execution II
Time: Sunday, November 16, 2008, 1:00 pm - 5:00 pm
Program#/Poster#: 277.22/LL14
Topic: D.17.b. Cortical planning and execution
Support: NIH grant NS051825
NICHD center grant HD02528
Title: Comparison of output effects on EMG activity from primary motor cortex obtained with
different methods of microstimulation