Figure 2. RTs of ACE for OHG and CHG. In the CHG, the compatible sequence trials comprised CHS and the incompatible comprised OHS. In the OHG, the compatible and incompatible effects were opposite. Vertical bars denote 0.95 confidence intervals. doi:10.1371/journal.pone.0011751.g002 In brief, behavioral effects regarding stimulus content suggest that more predictable sentences (NS) elicit shorter responses compared with OHS and CHS. The CHG presented a trend towards a more rapid response. More importantly, ACEs was present in both groups, although it was accentuated in the CHG. ERPs analysis Effects related to stimulus (N400-like). As illustrated in Figure 3, incompatible stimuli exhibited an N400-like component around Cz in both OHG and CHG. An ANOVA with category as the within-subject factor and group as the between-subject factor yielded an effect of category (F (2, 48) = 65.27, p,0.001). The incompatible category (M = 23.04 mV, SD = 0.33) presented more negative values compared with the compatible (M = 0.06 mV, SD = 0.38) and neutral categories (M = 0.01 mV, SD = 0.14). Post hoc comparisons showed statistical differences between compatible and incompatible categories (p,0.001), as well as between neutral and incompatible categories (p,0.001). However, no differences between neutral and compatible categories were observed (p = 0.99). Neither an effect of group (F (1, 24) = 0.79; p = 0.38) nor a group6category interaction effect was found (F (2, 48) = 0.84; p = 0.43). To evaluate whether stimulus content affected the N400-like component, an ANOVA of stimulus content 6 group was per- formed. As stated in Table S1, sentences with hand content (OHS and CHS) elicited enhanced N400-like amplitudes compared with NS, but this effect was affected by the type of hand-shape response (content6group interaction: (F(2, 48) = 48.31, p,0.001; see post hoc effects in Table S1). Because those NS were rated more predictable, the effect can be explained by predictability, as one robust modulator of the N400 component [75]. The overall results of N400-like amplitude suggest an ACE which discriminated incompatible stimuli from compatible and neutral stimuli. In addition, this component seemed to be affected by the predictability of the sentences, because OHS and CHS elicited more negative amplitudes than NS. No group differences were found in this component. Effects related to response: Motor potential (290 to 50 ms). In the early time-window related to the response, a category effect was observed (F (2,48) = 32.85; p,0.001). Conversely to the N400-like components, post hoc effects Neural Signatures of ACE (MS = 28.01, df = 48) indicated an enhanced amplitude for the compatible category (M = 221.39 mV, SD = 1.75; p,0.001), compared with the incompatible (M = 211.14 mV, SD = 1.36; p,0.001) and neutral categories (M = 211.03 mV, SD = 0.91). No differences were observed between these categories (p = 0.99). Figure 4.A shows the compatibility effects and the difference waveforms and Figure 4.B illustrates the voltages maps. In addition, a category 6 group interaction was obtained (F (2, 48) = 8.21, p,0.001). Although both groups presented the same pattern, with the compatible category eliciting a greater amplitude, this effect was more pronounced in the CHG (see Figure 4.C). Post hoc comparisons performed over this last interaction confirmed that the compatibility effects were larger in the CHG (see Table S2). In this group, the post hoc effects of compatibility yielded statistical significance. In brief, the early ERPs elicited by the motor response were modulated by compatibility effects, suggesting action-sentence compatibility facilitation. Moreover, these effects were accentuated in the CHG. Effects related to response: Late motor response RAP (200–300 ms). The analysis of late motor effects yielded similar results to MP; compatibility modulation was observed (F (2, 48) = 44.56, p,0.001). Post hoc comparisons (MS = 12.11, df = 48) of this effect indicated that compatible stimuli elicited enhanced positivity (M = 13.79 mV, SD = 1.007) compared with incompatible (M = 5.92 mV, SD = 0.86; p,0.001) and neutral stimuli (M = 5.88 mV, SD = 0.56; p,0.001). A group6category interaction effect was also found (F (2, 48) = 12.7; p,0.001). Post hoc comparisons (MS = 13.47, df = 70.54) revealed similar effects of CHG and OHG as the early motor response (see Table S3). In conclusion, RAP seemed to be affected by compatibility, suggesting an enhanced motor facilitation in response to compatible stimuli. Nevertheless, when group comparisons were considered, this effect was only statistically significant in the CHG. Discussion The present study sought to reveal an interaction between action sentence comprehension and motor processes by analyzing the impact of each on RTs and neurophysiological correlates of the semantic process and action performance. The present work was performed with the aim to test the bidirectionality hypothesis and results confirmed the needed predictions to fulfill it. Behaviorally, the effects of action-sentence compatibility were consistent with previous findings [10,50,51,54,76–78]. In this design, the ACE was based on the interaction between response hand-shape (CH or OH) and hand-shape actions encoded in the sentence. As expected, subjects were quicker to press the response button when the hand-shape required to respond was compatible with the hand-shape implied by the sentence. Similarly, RTs were significantly slower on the incompatible action sentences. This can be interpreted as evidence that action-language comprehension has a multimodal priming effect on action performance and to this level of specific motor attribute of movements (hand-shape) and with third-person sentences (see also [50]). As addressed previously, behavioral data indicated that the NS elicited a shorter response than both manual sentences. The best explanation of the rapidity of response in the neutral condition might be the facilitation by predictability effect. ERPs analyses, an excellent tool to investigate the temporal and functional mechanisms supporting language comprehension, refined the behavioral understanding of the ACE. In this study, N400 from semantic processing was time-locked to the target final PLoS ONE | www.plosone.org 6 July 2010 | Volume 5 | Issue 7 | e11751