Distribution of brainstem somatosensory evoked potentials following upper and lower limb stimulation

Influence of parameter settings on paired-pulse-suppression in somatosensory evoked potentials: a systematic analysis

OBJECTIVE

Paired-pulse somatosensory evoked potentials (SEPs) are a common tool to investigate excitability in the human somatosensory cortex. Comparing literature about paired-pulse SEP, there is no standard set of stimulation parameters, while little is known about the influence of stimulation parameters on paired-pulse suppression.

METHODS

We analyzed changes of paired-pulse ratios by varying repetition rates from 1 to 9Hz, and using stimulus intensities of 250% of the sensory threshold and 100%, 120%, and 140% of the motor threshold, which are most frequently used in studies using paired-pulse SEPs.

RESULTS

We found a significant effect of repetition rate on paired-pulse suppression with increasing paired-pulse ratios from 1 to 9Hz, which is mainly caused by a change of single pulse amplitudes. We found no difference in paired-pulse suppression at the tested stimulation intensities.

CONCLUSIONS

The extent of paired-pulse ratios across different studies should be interpreted with caution due to the high dependence on repetition rate, while the results at the commonly used stimulus intensities are comparable.

SIGNIFICANCE

For an optimized parameter setting with sufficient paired-pulse suppression, we suggest a stimulation rate of 1 or 3Hz and a stimulation intensity of 250% of sensory threshold or slightly above motor threshold.

Multichannel SEP-recording after paired median nerve stimulation suggests origin of paired-pulse inhibition rostral of the brainstem

Paired-pulse techniques are a common tool to investigate the excitability of the cerebral cortex. Whereas in the motor system short interval intracortical inhibition assessed by paired-pulse transcranial magnetic stimulation clearly could be demonstrated to be generated within the motor cortex, the mechanism of paired-pulse inhibition measured over the somatosensory cortex after paired-pulse median nerve stimulation is less clear. The aim of this study was to further investigate the level of somatosensory processing where this paired-pulse inhibition is generated. We applied single and paired electrical stimulation of the median nerve with an interstimulus interval of 30ms. Somatosensory evoked potentials were recorded over the brachial plexus, the cranial cervical medulla and the primary somatosensory cortex. We analyzed peak-to-peak amplitudes evoked by the second stimulus of paired-pulse stimulation after digital subtraction of a single pulse (A2s), and referred it to the first response before linear subtraction (A1). Paired-pulse inhibition was expressed as a ratio (A2s/A1) of the amplitudes of the second (A2s) and the first (A1) peaks. We found a significant reduction of A2s as compared to A1 over S1, but no significant difference between A1 and A2s over brachial plexus and cranial medulla. In addition, the cortical amplitude ratio A2s/A1 was significantly reduced compared to the amplitude ratios over cranial medulla and brachial plexus. These results suggest that the underlying inhibitory mechanisms are generated rostral to the brainstem nuclei, probably due to the activity of thalamic or intracortical inhibitory interneurons.