Movement priming of EEG/MEG brain responses for action-words characterizes the link between language and action

Abstract

Activation in sensorimotor areas of the brain following perception of linguistic stimuli referring to objects and actions has been interpreted as evidence for strong theories of embodied semantics. Although a large number of studies have demonstrated this "language-to-action" link, important questions about how activation in the sensorimotor system affects language performance ("action-to-language" link) are yet unanswered. As several authors have recently pointed out, the debate should move away from an "embodied or not" focus, and rather aim to characterize the functional contributions of sensorimotor systems to language processing in more detail. For this purpose, we here introduce a novel movement priming paradigm in combination with electro- and magnetoencephalography (EEG/MEG), which allows investigating effects of motor cortex pre-activation on the spatio-temporal dynamics of action-word evoked brain activation. Participants initiated experimental trials by either finger- or foot-movements before executing a two alternative forced choice task employing action-words. We found differential brain activation during the early stages of subsequent hand- and leg-related word processing, respectively, albeit in the absence of behavioral effects. Distributed source estimation based on combined EEG/MEG measurements revealed that congruency effects between effector type used for response initiation (hand or foot) and action-word category (hand- or foot-related) occurred not only in motor cortex, but also in a classical language comprehension area, posterior superior temporal cortex, already 150 msec after the visual presentation of the word stimulus. This suggests that pre-activation of hand- and leg-motor networks may differentially facilitate the ignition of semantic cell assemblies for hand- and leg-related words, respectively. Our results demonstrate the usefulness of movement priming in combination with neuroimaging to functionally characterize the link between language and sensorimotor systems.

Keywords: Embodiment; Motor cortex; Semantic somatotopy; Semantics; Word recognition.

Fig. 1

Trial structure for lexical decision (LD) and semantic decision (SD) tasks. Trials could be initiated by pressing a button either by finger or by foot. The “release” instructions refer to the lexical decision (LD, word/pseudoword) or semantic decision (SD, concrete/abstract) tasks, respectively. Target word categories in both tasks were arm-related (e.g., “stir”) and leg-related (“kick”) words.

Fig. 2

Time course of the EEG/MEG signals in sensor and source space. Source estimates for the average across all conditions at latencies derived for peaks in the time courses below. Activation is displayed on the right (top) and left (bottom) hemispheres of the inflated average cortical surface. Color scale is in nA. The time courses at the bottom represent root-mean-square (RMS) of signal-to-noise ratios (SNR) for all EEG and MEG sensors across time.

Fig. 3

Whole-brain activation for movement types and action-words. Source estimates at peak latencies for the finger (A) and foot (B) button presses that initiated each trial, as well as for all word stimuli 150 msec after stimulus onset (C). The ellipses indicate activation peaks used to define regions-of-interest in language comprehension areas for statistical analysis.

Fig. 4

Time-course of cortical activation in the source space ROI analyses. The ellipses indicate ROIs that showed significant congruency effects for effector type (finger and foot) and word category (arm- and leg-related). A and C present the time course of activation during the four experimental conditions [hand response (HR) to arm-related words (AW), HR to leg-related words (LW), Foot Response (FR) to AW and FR to LW] in pSTG and Hand area, respectively. The zero latency marks the onset of the written word. The bar graphs B and D show the mean activation values in the corresponding ROIs in the time interval around 150 msec. Error bars indicate standard errors of the mean after individual subject variance has been removed. Asterisks indicate a significance level of p < .05.