Our results indicate that even in the absence of explicit stimulus motion, observation of static photographs of human actions with implied motion produces a clearly greater cortical activation than observation of static images of less dynamic actions. ERP analysis showed that the motor content of the photographs exerted a strong effect, with a much larger positivity at all scalp sites (ranging from 350 to 600 ms in latency) in response to dynamic/effortful than to less effortful (static) actions. Since the pictures were balanced for other types of perceptual parameters except the degree of implicit biological motion represented, and the observers were engaged in a secondary target detection task, the data suggest a strong effect of implied motion on the amplitude of the ERPs.
The swLORETA linear inverse solution computed on the dynamic-minus-static difference wave in the time window corresponding to the ascending phase of late positivity (380–430 ms) showed activation of a series of regions belonging to the action and motion representation systems, namely: V5/MT, EBA, STS, premotor and motor areas, and cingulate IF cortex. In the next temporal window (430–480) V5/MT, motor and premotor areas were no longer activated (more to dynamic than static pictures), while cingulate activation was increased along with the inferior frontal and orbitofrontal cortices, possibly suggesting a switch from a sensory-motor code for action representation to a more abstract cognitive/affective code for representing visual information.
The activation of motor areas when viewing implicit biological motion is fully consistent with previous literature 
on implied body actions, providing evidence of the brain's ability to extract motion information from static images. In particular, Urgesi and coworkers, using single-pulse transcranial magnetic stimulation, found that the mere observation of static snapshots of hands suggesting a pincer grip action induced greater corticospinal excitability than observation of resting, relaxed hands, or hands suggesting a completed action.
In our study, the greater activation of BA19 for dynamic than for static actions indicates involvement of area V5/MT, which has been shown to respond not only to real motion 
but also to implicit 
and illusory 
motion. Similarly, the greater activation of the STG, cingulate and extrastriate body area (EBA) regions for processing more dynamic actions suggests that they have a role in biological motion and action representation, as indicated by many studies on the mirror neuron system (MNS) in humans 
The EBA, located in the lateral occipitotemporal cortex (BA37), was first reported to respond selectively to visual images of human bodies or body parts 
. Later it was shown that the EBA responds not only during the perception of other people's body parts, but also during goal-directed movements of the observer's body parts 
. fMRI studies using point-light animations of biological motion showed that viewing biological motion selectively activates the posterior superior temporal sulcus (STSp) 
. The role of the EBA in the perception of body motion is not really clear, since for example in one of these studies 
it was found that BOLD responses to whole bodies in motion and to images of stationary, headless bodies were equivalent in the EBA but not STS regions, which were selectively driven by the dynamics of the human form.
In our study, the finding of brain regions devoted to motion and action processing supports the hypothesis that implied motion was the crucial factor in determining a difference in brain activation between the two conditions. However, it cannot be excluded that other possible confounding factors such as attraction, emotional valence, preference and interest might have contributed to determine a greater amplitude of LP to dynamic vs. static pictures, or to induce specific sex differences. At this regard, two additional LORETAs performed separately in women and men relative Tailarach coordinates of significant activations are reported as Supplementary material in Fig. S1
and Table S1
. Overall, the results suggest that men exert a higher degree of neuronal mirror activity to movement related pictures (either for evolutionary and/or cultural reasons), whereas women showed a strong visual interest for all human figures 
Overall, the present electrophysiological and swLORETA source reconstruction findings provide evidence of greater activation of the EBA, V5/MT, STG (BA38), motor (BA4) and premotor (BA 6) areas in response to effortful (dynamic) than static actions, thus suggesting a stronger response of the corresponding mirror neurons to implicit motion 
. This pattern of results may provide the cortical counterpart to the autonomic response described in previous physiological literature 
, suggesting an interesting correlation in perfectly still observers between greater effortfulness of observed actions and increased respiration and heart rates.
The sex difference in cortical sensitivity to dynamic actions, consisting in a finer discriminative response of the LP in males than females, might possibly be ascribed to a difference of cultural origin (but probably not entirely). In any case, it might reflect a men preference for highly dynamic actions depicting intense muscular activity, or a sporty context.