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Categorical clustering in the visual system is thought to have evolved as a function of intrinsic (intra-areal) and extrinsic (interareal) connectivity and experience. In the visual system, the extrastriate body area (EBA), an occipito-temporal region, responds to full body and body part images under the organizational principle of their functional/semantic meaning. Although frequency-specific modulations of neural activity associated with perceptive and cognitive functions are increasingly attracting the interest of neurophysiologists and cognitive neuroscientists, perceiving single body parts with different functional meaning and full body images induces time-frequency modulations over occipito-temporal electrodes are yet to be described. Here, we studied this issue by measuring EEG in participants who passively observed fingers, hands, arms and faceless full body images with four control plant stimuli, each bearing hierarchical analogy with the body stimuli. We confirmed that occipito-temporal electrodes (compatible with the location of EBA) show a larger event-related potential (ERP, N190) for body-related images. Furthermore, we identified a body part-specific (i.e. selective for hands and arms) theta event-related synchronization increase under the same electrodes. This frequency modulation associated with the perception of body effectors over occipito-temporal cortices is in line with recent findings of categorical organization of neural responses to human effectors in the visual system., (© 2017 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

Reading action-related verbs brings about sensorimotor neural activity, suggesting that the linguistic representation of actions impinges upon neural structures largely overlapping with those involved in actual action execution. While studies of direct action observation indicate that motor mirroring is inherently anticipatory, no information is currently available on whether deriving action-related knowledge from language also takes into account the temporal deployment of actions. Using transcranial magnetic stimulation, here we sought to determine whether reading action verbs conjugated in the future induced higher cortico-spinal activity with respect to when the same verbs were conjugated in the past tense. We recorded motor-evoked potentials (MEPs) from relaxed hand and leg muscles of healthy subjects who were reading silently hand- or leg-related action, sensorial (non-somatic) and abstract verbs conjugated either in future or past tense. The amplitude of MEPs recorded from the hand was higher during reading hand-related action verbs conjugated in the future than in the past. No future-related modulation of leg muscles activity was found during reading leg-related action verbs. In a similar vein, no future-related change of hand or leg muscles reactivity was found for abstract or sensorial verbs. These results indicate that the anticipatory mirroring of hand actions may be triggered by linguistic representations and not only by direct action observation.

Transcranial magnetic stimulation (TMS) studies report that viewing a given action performed by a model activates the neural representation of the onlooker's muscles that are activated during the actual execution of the observed action. Here we sought to determine whether this mirror observation-execution facilitation reflects only muscular specificity or whether it is also influenced by postural congruency between onlooker/model body parts. We recorded motor potentials evoked by single-pulse TMS from the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles during observation of the right index and little finger abduction/adduction movements of models who kept their hands in a palm-down or palm-up position. Moreover, in different experiments observers kept their right hand palm down or palm up. Selective motor facilitation was observed during observation of movements that map the motor function of the targeted muscles, regardless of the posture of the observed hand. Modulation of FDI, however, was obtained only when participants kept their hand palm down; by contrast, modulation of ADM was obtained only when participants kept their hand palm up. Interestingly, electromyographic recordings showed that FDI is mostly active when index abduction/adduction movements are performed in the palm-down position, whereas ADM is mostly active when little finger abduction/adduction movements are performed in the palm-up position. Results show that the influence of the onlooker's hand posture is comparable in action execution and observation, thus indicating a fine-grain functional correspondence between these two processes.