Question
The mechanism of LTP or LTD-like effect in paired associative stimulation (PAS) is explained on spike-timing dependent plasticity. In facilitatory PAS, however, it is not clear why one pairing of PAS similar with short-latency afferent inhibition can induce LTP-like effect. The present data aim to provide a hypothesis about this.
Methods and Results
First step: There is a hypothesis that PAS is induced by summation of network activity (Thickbroom WG, 2007). If it is true, LTP-like effect is induced similarly when interstimulus interval (ISI) is varied between "N20 peak latency+ 2" to 25 ms. Seven healthy volunteers showed LTP-like effect with ISI of 25 ms and "N20 + 2" ms. But the effect was disappeared when ISI was randomly intermingled. This result denies the summation of network activity.
Second Step: In animal studies the somatosensory input generates excitatory postsynaptic potentials (EPSPs) and the stimulated nerves in the motor cortex generate back-propagating action potential (BPAP). These two potentials interact with each other, leading to LTP-like effect. We hypothesized that the somatosensory input may increase the probability of two interactions. If it is true, the jitter of peak latency is less when somatosensory input comes. In thirteen healthy subjects, the MCD was less when median nerve stimulation was applied 25 ms before TMS, compared with single pulse TMS (p = 0.02). Thus the somatosensory input may decrease the variation of BPAP.
Third step: If it is true, the LTP-like effect may be negatively correlated with jitter in some parameter. In effect, during PAS intervention, the effect was negatively correlated with the CoV of MEP amplitude between two serial MEPs during the last pairing from 120 to 180 stimuli (r= -0.59) in 18 healthy subjects.
Conclusion
It may be plausible that the somatosensory input may play a role in increasing the probability of interaction between EPSPs by somatosensory input and back-propagating action potential.