Chapter 2 TMS and threshold hunting

Publisher Summary

Threshold is a general concept that is often used to describe a basic property of stimulus-response experiments. This chapter discusses a rigorous comparison of three types of threshold. This was achieved by measuring the complete relationship between stimulus strength, and success probability, in experiments on some hand and leg muscles, and to use these measured properties to perform Monte-Carlo simulations that allow a valid comparison of the different transcranial magnetic stimulation (TMS)-threshold-estimation strategies. The experimental results of this study confirm that TMS threshold behavior is similar to that of simple excitable membranes. In particular, the success probability increases monotonically with increasing stimulus strength in a sigmoidal fashion that may well be described by a cumulative Gaussian. Thus, the complete threshold behavior can be described by two parameters, the threshold and the threshold spread. The threshold spread parameter is believed to represent the number and the statistical properties of the sodium channels at the action potential initiating site, although for TMS experiments it cannot be excluded that synaptic transmission parameters at the interneuronal stages of the action–potential transmission influence this parameter as well.