Neural Signatures of ACE Figure 1. Response button and pre-assigned hand-shapes. A) OH motor response during sentence comprehension task. B) CH motor response during sentence comprehension task. C) Custom-made response button. A commonly available USB joystick with analogue sticks was adapted to detect when a participant initiated a response. doi:10.1371/journal.pone.0011751.g001 responses have been reported previously to be modulated by sentence content [50]. Stimuli One hundred fifty-six sentences in Spanish, previously validated (see below), were considered as stimuli (see Stimuli S1). One hundred and four sentences encoded hand actions; half contained verbs encoding an action with an OH and half encoded a CH. In addition, 52 neutral sentences encoding no action or an action different to a hand action were included. Sentences were categorized as OH sentences (OHS), CH sentences (CHS) and neutral sentences (NS) (Table 1). All sentences were third-person whose critical verb was in undefined preterit tense (English’s Simple Past Tense) and was always placed as the last word of the sentence. Relevant linguistic variables were matched between lists (such as Transitivity, Situation Aspect, and content of the clauses; see Methods S1). Also, the number of syllables of the final target word (M = 2.46 syllables, SD = 0.08 in OHS; M = 2.53 syllables, SD = 0.08 in CHS; and M = 2.53 syllables, SD = 0.08 in NS; F(2, 153) = 0.25, p = 0.77), as well as the frequency of use (moderate levels) for those targets, was controlled. The overall length in time of sentences was 4.57 s (SD = 0.06 s). Audio files of each trial were edited with 400 ms of silence at the beginning and 200 ms at the end. The onset of the target verb within the sentence was 4.05 s (SD = 0.06; 2.92 s minimum, 5.64 s maximum). Stimuli validation A rating study was designed to validate stimuli (see Methods S1). Lists of CHS, OHS and NS were controlled for sentence length, final word length and frequency (moderate). In addition, predictability, prototypicality and the degree of manual specificity of hand sentences were controlled. Predictability was defined as how easily the final verb was to determine from the previous sentential context. Prototypicality was defined as how representa- tive of the pertinent hand-shape (CH or OH) was the manual action encoded by the sentence. The degree of manual specificity was related to manual aperture or closure. In other words, the degree of manual specificity defined how open or closed the hand- shape must be to perform the determined action encoded by the sentence. Only highly predictable sentence endings were considered. NS presented statistically enhanced levels of predictability compared with OHS and CHS, but no difference between OHS and CHS was observed. All hand actions encoded by sentences (OHS and CHS) were highly prototypical of their shape. However, CHS was reported as more difficult to perform with the incompatible movement (OH) compared to OHS. Consequently, we expected more accentuated ACE motor responses in the CHG. Higher prototypicality implies an enhancement of facilitation for the compatible movement because the information about the hand- shape of movement is strongly reinforced by the very specific hand-shape content of the sentence. The detailed process of validation can be found in Methods S1. Measurement of OH and CH responses Responses were captured using a custom-made response button (Figure 1) that could be depressed 4 cm and was mounted on a spring sufficiently stiff to hold the button and hand in the normal position. The button size allowed the hand to rest comfortably in either a closed or an open position. A USB joystick with analog sticks was mechanically adapted (see Figure 1.C) so that one axis of one stick would measure how deep the button was depressed to detect when a participant initiated a response. This design allowed a relatively long range of the button that emphasized the hand action characteristic of the response. At 8 bits of resolution per axis, the stick measured values between 2128 and 127 with 0 being the center and 2128 being fully depressed (half the range of the stick was unused). When a measurement was more negative than some pre-established threshold (to rule out spurious responses due to measurement noise), a response was detected. In this way, both short and complete depressions could be detected, despite the long run of the button. Procedure The experiment was conducted in an electrically- and sound- shielded room under dim lighting conditions. The subjects were comfortably seated behind a desk facing a computer display. The button was located on the right side of the desk so that participants could easily press it. Participants were instructed to listen to the sentences and indicate by button press as quickly as possible the moment of comprehension. They were asked to keep both hands in the pre-assigned hand-shape throughout the experiment (see part A and B of Figure 1); the right hand was placed over the button and the left hand over the desk. Also, to ensure that participants were attending to the stimuli, they were told that they would be asked about the content of the last word of each sentence at the end of the experiment. Subjects had a training session (5 trials) for familiarization them with the task. PLoS ONE | www.plosone.org 4 July 2010 | Volume 5 | Issue 7 | e11751