Brain function overlaps when people observe emblems, speech, and grasping

A hand grasping a cup or gesturing "thumbs-up", while both manual actions, have different purposes and effects. Grasping directly affects the cup, whereas gesturing "thumbs-up" has an effect through an implied verbal (symbolic) meaning. Because grasping and emblematic gestures ("emblems") are both goal-oriented hand actions, we pursued the hypothesis that observing each should evoke similar activity in neural regions implicated in processing goal-oriented hand actions. However, because emblems express symbolic meaning, observing them should also evoke activity in regions implicated in interpreting meaning, which is most commonly expressed in language. Using fMRI to test this hypothesis, we had participants watch videos of an actor performing emblems, speaking utterances matched in meaning to the emblems, and grasping objects. Our results show that lateral temporal and inferior frontal regions respond to symbolic meaning, even when it is expressed by a single hand action. In particular, we found that left inferior frontal and right lateral temporal regions are strongly engaged when people observe either emblems or speech. In contrast, we also replicate and extend previous work that implicates parietal and premotor responses in observing goal-oriented hand actions. For hand actions, we found that bilateral parietal and premotor regions are strongly engaged when people observe either emblems or grasping. These findings thus characterize converging brain responses to shared features (e.g., symbolic or manual), despite their encoding and presentation in different stimulus modalities.