potential (late MRP; [66,67]), which is likely to represent pyramidal neuron activity in the primary cortex (M1) at motor execution. MP amplitude modulation has been associated with the rapidness and precision of movement [65,68] and also with short- term training effects [66]. We expected higher MP amplitudes for the compatible condition, reflecting that sentence content facilitates the precision and quickness of a compatible action. The second component consistently observed was a peak over Cz after movement onset (200–300 ms) similar to a re-afferent potential (RAP). RAP is an index of movement-related sensory feedback to the primary sensory-motor cortex [67] and is considered an indicator of attention [66]. Higher RAP peaks were expected for the compatible condition, indicating that the facilitation of bottom-up attention on task-relevant information would optimize action performance. In the present work, we aim to introduce a novel perspective to the realm of action-language comprehension by supporting the bidirectionality hypothesis as an integrated method of understanding the motor-language interaction in comprehension. This hypothesis claims that action-language comprehension and motor processes share neural resources that co-operate mutually; that is to say that motor processes influences the comprehension of the action sentence, and action sentence comprehension influences the motor process. In a recent behavioral study, Kelly, O¨zy u¨rek and Maris [69] demonstrated for the first time the mutual influence of gesture and speech stimuli in language comprehension. We aim to extend this bidirectional interaction hypothesis to the realm of brain markers of actual actions and language processing. To test the bidirectionality hypothesis, we conducted an ACE study under the following prediction: if semantic content is shared by motor and linguistic processes, when both of them occur simultaneously, shared neural resources should co-operate bidi- rectionally. Specifically, sentence comprehension should facilitate actual movement when compatible, and at the same time, incompatible action should disrupt comprehension of the sentence. To prove this prediction, we utilized an ERP technique that measured both stimuli (sentence) and response (action) cortical processes. What is expected particularly is: (a) if motor response is facilitated by the compatible sentence comprehension, the motor potentials (MP and RAP) should be larger in this condition because of the quickness and precision of the facilitated movements. (b) if motor processes impact the semantics of the sentence, it would be expected that during cortical semantic processing, an incompatible motor process elicits a semantic incongruence manifested by the N400-like component. If the bidirectionality hypothesis is not fulfilled, any of the predictions (a or b) would not be present. Each prediction only accounts for evidence of a unidirectional effect. Hence, the satisfaction of only one of them is not sufficient to argue for their vice versa effect (if only (a) is accomplished, it could not be argued that motor action impact sentence comprehension; if only (b) is Neural Signatures of ACE accomplished, it could not be argued that semantics primes the motor process effect). Consequently, if no motor effect in the cortical processing of semantic processes is observed (no modulation of the N400-like component), the bidirectionality hypothesis fails. Also, if no semantic effect in the cortical processing of motor response is observed (no modulation of motor components, MP and RAP), the bidirectionality hypothesis fails. As a result, if both predictions fail, the bidirectionality hypothesis also fails. To the best of our knowledge, no single study of ACE neural signatures has been reported. This study attempts to provide temporal dynamics of detailed ACE brain markers during stimuli processing and motor responses to account for both directions of analysis (semantics-to-motor and motor-to-semantics). Thus, the current work addressed three issues: (1) a description of specific semantic and motor ERPs of ACE; (2) a comparison of the semantic and motor ERPs regarding the compatible and incompatible action-sentence effects; and (3) a comparison of the behavioral and neural markers of ACE to build a more precise understanding of the action-sentence interactions. Materials and Methods Ethics Statement All participants read and signed an informed consent in agreement with the Declaration of Helsinki before beginning the study. The ethical committee of the cognitive neuroscience laboratory approved the study. Participants Twenty-six native Spanish volunteers (14 females) aged 18 to 31 (M = 22.4 years, SD = 3.7) were included in this study. All participants were undergraduate students and right-handed as defined by the Edinburgh Inventory [70], with normal auditory acuity, normal or corrected-to normal vision, and no reported history of psychiatric or neurological illness. Regarding the response hand-shape (either open [OH] or closed [CH]), two randomly balanced groups were created: one group responded to the stimuli with an open hand (open hand-shape group [OHG]) and the other with the closed hand-shape [CHG]. No differences in age [F (1, 24) = 0.005, p = 0.94], gender [X2 = 0.05; p = 0.82] or educational level [X2 = 0.03; p = 0.94] between groups were found. Task The participants listened to auditory sentences (e.g., The show was praiseworthy, so Rocio applauded, see Table 1 for more examples) and indicated as quickly as possible once they understood each sentence. They made this judgment by pressing a button and, using a pre-assigned hand-shape, either a CH vertical to the button or an OH (see Figure 1.A and B). Similar CH and OH Table 1. Example of stimuli used in the experiment and their approximate English translation. Category Sentence OHS CHS NS El especta´culo era digno de alabanza, Roc´ıo aplaudio´. Ten´ıa que clavar el clavo muy derecho, Jose´ lo martillo´ Hace tiempo que quer´ıa ver a su abuela, Amaro la visito´ English Approximate Translation The show was praiseworthy, so Rocio applauded He needed to drive the nail correctly, so Joseph hammered it. After waiting a long time to see his grandmother, Amaro visited her The sentences were categorized according the hand-shape of the action encoded in OH sentences (OHS), CH sentences (CHS) and neutral sentences (NS). Final target verbs are underlined. doi:10.1371/journal.pone.0011751.t001 PLoS ONE | www.plosone.org 3 July 2010 | Volume 5 | Issue 7 | e11751