For these reasons, we reject the view the NMAs merely represent unnatural disruption of actions caused by stimulating areas normally involved in positive movement generation. An alternative possibility remains open: negative motor responses might represent an artificial induction of a normal physiological process of action inhibition. In our view, the normal organization of complex (Gerloff et al., 1997) and fine movement (Fukaya et al., 2004) involves an element of inhibition. Hierarchical control is required to regulate the balance of activation and inhibition in several motor cortical areas, so that movements are neither hyperkinetic and impulsive, nor hypokinetic and ineffective.

Crucially, we suggest that there is some ‘functional truth’ in NMAs. We speculate that DES, albeit not ecological itself, produces negative motor responses by activating physiologically

inhibitory pathways that participate in signaling pathway normal action control. Crucially, negative motor responses are not simply an artifactual, unnatural disruption of ongoing movement, or an overloading of positive motor effects. The interesting observations reported by Swann et al. (2012) provide clear, and perhaps the first, evidence for a possible functional relevance of NMAs in action inhibition, as an important element of action control. The natural inhibitory function of NMAs could be important in action control for two distinct reasons. First, NMAs may reflect activation of an inhibitory mechanism for praxic control of fine details of action execution. PARP inhibitor Alternatively, NMAs may reflect artificial activation of an inhibitory mechanism for executive, decisional

control over whether actions occur or not. The data reviewed here cannot conclusively distinguish between these two alternatives, and future functional studies may shed light on this interesting question. Control of praxis has been strongly linked to lateral cortical pathways linking the inferior parietal cortex and the lateral premotor cortex (Tanji and Hoshi, 2008). In contrast, executive control of action has been linked to the prefrontal and medial frontal cortices (Badre and D’Esposito, 2009 and Stuss and Knight, 2002), and particular to the drive these areas receive from the basal ganglia (Heyder et al., Carnitine palmitoyltransferase II 2003). Our review shows two clear clusters of NMAs in the lateral frontal and dorsomedian frontal cortices. By analogy with the lateral/frontal division for positive motor function, we can thus speculate that the lateral frontal cluster of NMAs reflects a praxic mechanism for fine regulation of complex action sequences, while the medial frontal cluster represents an executive mechanism for regulating whether an action is executed or inhibited. From the evidence reviewed above, we suggest that NMAs are indeed truly inhibitory.