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Brief interruption of voluntary EMG in a handAbbreviations BB, biceps brachii; CSP, cortical silent period; ECR, extensor carpi radialis; EIP, extensor indicis proprius; EMG, electromyographic; FDI, first dorsal interosseous; IHI, interhemispheric inhibition; ISP, ipsilateral silent period; L-IHI, long-interval interhemispheric inhibition; M1, primary motor cortex; MEP, motor-evoked potential; MSO, maximum stimulator output; RMS, root mean squares; RMT, resting motor threshold; RT, reaction time; S-IHI, short-interval interhemispheric inhibition; TA, tibialis anterior; TMS, transcranial magnetic stimulation. IntroductionIn humans, intricate and independent finger movements are enabled by a largely crossed system of fast-conducting axons that provides mono-synaptic connections between primary motor cortex (M1) and contralateral spinal motoneurones (Porter & Lemon, 1993). Execution of unimanual or asymmetric bilateral movements relies on a neural network that is capable of lateralising motor cortical output (Carson, 2005;Cincotta & Ziemann, 2008). While a full characterisation of this distributed network is still lacking, data from lesioned monkeys (Brinkman, 1984) and human patients (Chan & Ross, 1988) are in keeping with the view that it probably includes the supplementary motor area and the cingulate gyrus. Positron emission tomography (PET) findings (Sadato et al. 1997) transcranial magnetic stimulation (TMS) data in healthy human subjects (Meyer-Lindenberg et al. 2002;Cincotta et al. 2004;Giovannelli et al. 2006) suggest that the dorsal premotor cortex is also involved. This notion of a neuronal network for movement lateralisation upstream of M1 by no means rules out the possibility that movement lateralisation is supported, in addition, by active inhibition from the voluntarily active M1 to the opposite M1. TMS studies that examined interhemispheric inhibition (IHI) by a paired-pulse protocol with the conditioning stimulus delivered to one M1 and the test stimulus delivered to the other M1 support this hypothesis (Ferbert et al. 1992;Mochizuki et al. 2004;Duque et al. 2007;Hübers et al. 2008). In particular, volitional activity in the M1 receiving the conditioning pulse, e.g. slight unilateral contraction of the contralateral hand, facilitates inhibition of the motor-evoked potential (MEP) elicited by a test stimulus delivered 10 ms later to the opposite M1 (interhemispheric inhibition at short-interstimulus interval, S-IHI) when compared to the rest condition (Ferbert et al. 1992;Mochizuki et al. 2004;Talelli et al. 2008).Besides S-IHI of the MEP, interhemispheric inhibition can also be studied by a short attenuation or interruption of ongoing voluntary electromyographic (EMG) activity in hand muscles induced by focal TMS of the ipsilateral M1 (Wassermann et al. 1991;Ferbert et al. 1992;Meyer et al. 1995;Trompetto et al. 2004;Cincotta et al. 2006). This ipsilateral silent period (ISP) begins 30-40 ms after a single magnetic pulse and lasts, on average, 25 ms (Meyer et al. 1995). Studies in patients with ca...

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We used event-related potentials (ERPs) to tap the temporal dynamics of first impressions based on face appearance. Participants were asked to evaluate briefly presented faces for trustworthiness and political choice. Behaviorally, participants were better at discriminating faces that were pre-rated as untrustworthy. The ERP results showed that the P100 component was enhanced for untrustworthy faces, consistently with the view that signals of potential threat are given precedence in neural processing. The enhanced ERP responses to untrustworthy faces persisted throughout the processing sequence and the amplitude of early posterior negativity (EPN), and subsequent late positive potential (LPP) was increased with respect to trustworthy faces which, in contrast, elicited an enhanced positivity around 150 ms on frontal sites. These ERP patterns were found specifically for the trustworthiness evaluation and not for the political decision task. Political decision yielded an increase in the N170 amplitude, reflecting a more demanding and taxing structural encoding. Similar ERP responses, as previously reported in the literature for facial expressions processing, were found throughout the entire time course specifically elicited by faces explicitly judged as untrustworthy. One possibility might be that evolution has provided the brain with a 'special toolkit' for trust evaluation that is fast and triggers ERPs related to emotional processing.