Low or High-Level Motor Coding? The Role of Stimulus Complexity

Transcranial magnetic stimulation (TMS) studies have shown that observing an action induces activity in the onlooker's motor system. In light of the muscle specificity and time-locked mirroring nature of the effect, this motor resonance has been traditionally viewed as an inner automatic replica of the observed movement. Notably, studies highlighting this aspect have classically considered movement in isolation (i.e., using non-realistic stimuli such as snapshots of hands detached from background). However, a few recent studies accounting for the role of contextual cues, motivational states, and social factors, have challenged this view by showing that motor resonance is not completely impervious to top-down modulations. A debate is still present. We reasoned that motor resonance reflects the inner replica of the observed movement only when its modulation is assessed during the observation of movements in isolation. Conversely, the presence of top-down modulations of motor resonance emerges when other high-level factors (i.e., contextual cues, past experience, social, and motivational states) are taken into account. Here, we attempt to lay out current TMS studies assessing this issue and discuss the results in terms of their potential to favor the inner replica or the top-down modulation hypothesis. In doing so, we seek to shed light on this actual debate and suggest specific avenues for future research, highlighting the need for a more ecological approach when studying motor resonance phenomenon.

No differences in CSE during AO vs baseline.
OP CSE: ↑ during closing movement of the hand, to close the normal plier vs opening movement of the hand to open the magnetic plier; FDI CSE: -↑ during closing movement of the hand, to close the normal plier vs opening movement of the hand to close the reverse plier and vs opening movement of the hand to open the magnetic plier; -↓ during hand opening movement with magnetic plier (no effects of movement phase for the reverse plier). EMG recording during AO of reaching to grasp a small or a big object: -with online action correction from large to small object; -when information on action goal was provided; -when information on action goal was not provided.
Increased CSE for static hand vs AO conditions; EXP 2 (action goal information provided) FDI CSE: -↑ for action directed to small vs large object (consistent with AE activation), especially at handobject time contact; -↑ only after online correction from large to small object at 80% of movement completion; ADM CSE: no modulation; EXP 3 (action goal information not provided) FDI or ADM: no differences between to be grasped small object and online (toward small object correction) before movement completion. Hand-squeezing actions after being primed in one out of 3 possible conditions: -low-power: participants wrote an essay about an experience where someone had power over them; -high-power: they wrote about an experience where they had power over someone else; -neutral: they wrote about what happened the day before.
APB CSE: -↓ in the high-power group; -no significant differences between neutral condition and low-and high-power conditions. Pouring sugar action with a spoon performed by using a precision grip (PG) or pouring coffee action with a thermos using a whole-hand grip (WHG) into different coffee cups/mugs. Critically, the 3 first cups/mugs where located on the table near the model performing the actions, while the fourth one was located far away, near the observer. Anyone on that side of the table wanting to pick up the cup/mug would need to use a PG/WHG.

Fixation cross
Ratio from baseline FDI ADM SpTMS (~110% MT). At five time points: T1, first contact with the object (i.e., spoon or thermos) around 1550ms; T2, when pouring sugar/coffee into the third cup (~6530ms); T3, when the hand get away from the cup (~6680ms); T4, when model's wrist trajectory move towards the 4th cup in the social condition (or not, in the non-social one, ~6950ms); T5, the model's arm reaches the 4th cup (or not, ~7160ms).
ADM CSE: ↑ for the social than for the non-social condition at T4 and T5 (i.e., timing of the complementary request) when the request demanded a WHG and ↓ when the request demanded a PG.
Grasp modulation ADM CSE: ↓ at T1, T2, and T3 for actions performed with PG (i.e., grasping the spoon) indicating emulation (mimicking AO and ignoring the action goal). ↑ at T4 and T5 for actions requesting a WHG (i.e., model pouring coffee in a mug), indicating motor resonance in terms of reciprocity.
Social context modulation (i.e., shift from emulation to reciprocity) on CSE.