The results do, however,

suggest that the rules governing the effect of plasticity-inducing interventions, and especially interactions between them, are complex, and depend on what type of data is considered to be indicative of plasticity (e.g. behavioural vs. neurophysiological). A similar dissociation between changes of excitability and behavioural measures has been described for the SI following PAS (Litvak et al., 2007). In these experiments, a gain in tactile acuity depended on whether TMS applied to the SI was near-synchronous to afferent signals containing either mechanoreceptive or proprioceptive information. In the latter case, acuity remained unchanged despite changes in excitability, which questions a simple relation Selleckchem PD-1/PD-L1 inhibitor between enhancement of synaptic efficacy and behavioural gain. In another study, facilitative PAS has been reported to inhibit motor learning (homeostatic interaction), only if 90 min were allowed

PLX3397 to elapse between PAS and motor practice (Jung & Ziemann, 2009). If motor practice was carried out immediately after PAS, then PAS actually improved learning (non-homeostatic interaction). In contrast, studies that explore homeostatic plasticity using MEPs as an indicator often find that such effects develop immediately. Furthermore, the time window during which homeostatic plasticity can be demonstrated using this paradigm appears to be relatively short, as revealed by studies in which short priming interventions were used. In such cases, even a 5- or 10-min interval between interventions

is sufficient to abolish homeostatic interaction 3-mercaptopyruvate sulfurtransferase (Huang et al., 2010; Iezzi et al., 2011). The lack of significant influence of iHFS on tactile acuity when applied after rTMS contrasts with the results previously reported by Ragert et al. (2003), in which the two types of stimuli produced an additive effect. This shows that the manner in which the two interventions interact might be dependent on their timing. In a previous study (Nitsche et al., 2007), it was shown that the same two plasticity-inducing techniques (tDCS and PAS) interact homeostatically when applied simultaneously and synergistically when applied in succession. This, as the authors point out, contradicts previous results combining tDCS and rTMS (Lang et al., 2004; Siebner et al., 2004), which showed a homeostatic interaction after sequential application. This indicates that the mode of interaction between two interventions (i.e. homeostatic or synergistic) may also depend on the specific form of stimulation used. However, once a certain plasticity process is underway, it may exhibit a degree of immunity to further changes induced by additional interventions.