Reply: Plasticity and intracortical inhibition in dystonia--methodological reconsiderations

Abstract

Sir, We would like to thank Dr Rosenkrantz for her interest in our recent paper on physiological differences between patients with psychogenic and organic forms of limb dystonia. We are also very grateful for the opportunity to discuss in detail the connection between input–output relationships and effects of paired associative stimulation (PAS). It is a topic of some confusion among those who use transcranial magnetic stimulation and it is worthwhile exploring in detail possible connections between these two phenomena. In the usual PAS experimental design, a standard transcranial magnetic stimulus is used to evoke motor evoked potentials in the abductor pollicis brevis muscle. Application of the PAS25 protocol increases the amplitude of motor evoked potentials evoked by this standard pulse for the following 30 min or so. In contrast, the input–output curve uses a range of transcranial magnetic stimulation intensities to plot the relationship between intensity and size of evoked motor evoked potential. As Dr Rosenkranz points out, if the effect of PAS25 is equivalent to increasing the intensity of the transcranial magnetic stimulation pulse, then subjects who have a steeper input–output curve will achieve a greater increase in amplitude of motor evoked potential than those with a shallow input–output curve. However, first it is important to underline that proposing PAS25 to be the same as increasing the intensity of the transcranial magnetic stimulation pulse is not equivalent as saying that the effect is the ‘same’ as increasing transcranial magnetic stimulation intensity. Measurements of the strength-duration properties of transcranial magnetic stimulation indicate that it stimulates axons; turning up stimulation intensity increases response size because more axons are recruited at higher intensities. For PAS25 to have the ‘same’ effect we would have to propose that PAS25 increases the excitability of axonal membranes, so that more axons are stimulated by the standard …