Neural networks related to observing symbolic and meaningless intransitive hand movements

In spite of the increasing literature on the neural system involved in the observation of biological movements1, few studies so far have dealt with the observation of meaningful hand movements2-4. Furthermore, they yielded partially conflicting results, possibly because of different techniques used, stimuli presented and tasks assigned to the subjects. Methods Twenty healthy right-handed volunteers (8 males, 12 females; mean age 26.6) took part in this study. An event-related paradigm was adopted. A continuous video was presented, showing a table with some common objects (glasses, cup, scissors, etc.). At intervals, an actor, of whom only the trunk and arms were visible, performed different kinds of hand movements: a) symbolic (OK, hello, etc.) (SY); b) meaningless (ML); c) grasping an object; b) simply touching an object. Only data regarding the first two conditions (i.e., intransitive movements) will be presented here. No movement was ever repeated during each experiment. Three runs were carried out for each subject. Six movements for each class were shown in each run, alternated in pseudorandom order. Functional imaging was performed on a 3T Philips Intera scanner. Twenty-four axial slices were acquired (in-plane matrix: 64x64; TR: 2515 ms; voxel size: 3.75x3.75x4 mm, with a 0.6 mm gap between contiguous slices). Data analysis was carried out using SPM5. Multi-subject analyses were performed using a random effect model. In particular, direct comparisons between SY and ML, and between ML and SY, were performed. Results Direct subtraction of SY vs. ML evoked a signal increase mainly in the left hemisphere (Fig. 1, top row) in a fronto-temporal circuit including the middle temporal gyrus/superior temporal sulcus (also on the right), dorsal premotor cortex and inferior frontal gyrus; in addition, activity increased in the dorsomedial prefrontal cortex (Talairach coordinates: x=4, y=48, z=23) and in the head of the left caudate nucleus. The contrast ML vs. SY (Fig. 1, bottom row) evoked increased activity bilaterally in the middle/inferior temporal gyrus; mainly on the right in the superior and inferior parietal lobules; and exclusively on the right in premotor cortex/inferior frontal gyrus. Conclusions The present findings show that observing symbolic actions involves a mainly left fronto-temporal pathway. A focus in the dorsomedial prefrontal cortex specifically active during SY observation could be related to a circuit involved in social interactions5. Observing meaningless movements activates bilateral temporal areas and predominantly right parietal and premotor areas. Some of these latter areas are part of the mirror system1. It appears therefore that mainly the right mirror system is involved in the analysis of the motor aspects of movement, when no semantic content is present. References 1) Rizzolatti and Craighero Annu. Rev. Neurosci. 27:169-192, 2004. 2) Decety et al. Brain 120:1763-1777, 1997. 3) Tanaka et al. Neuroreport 12:1171-1174, 2001. 4) Rumiati et al. J.Cogn.Neurosci. 17:1420-1431, 2005. 5) Iacoboni et al. Neuroimage 21:1167-1173, 2004.