Abstract
Movement observation (MO) has been shown to activate the motor cortex of the observer as indicated by an increase of corticomotor excitability for muscles involved in the observed actions. Moreover, behavioral work has strongly suggested that this process occurs in a near-automatic manner. Here we further tested this proposal by applying transcranial magnetic stimulation (TMS) when subjects observed how an actor lifted objects of different weights as a single or a dual task. The secondary task was either an auditory discrimination task (experiment 1) or a visual discrimination task (experiment 2). In experiment 1, we found that corticomotor excitability reflected the force requirements indicated in the observed movies (i.e. higher responses when the actor had to apply higher forces). Interestingly, this effect was found irrespective of whether MO was performed as a single or a dual task. By contrast, no such systematic modulations of corticomotor excitability were observed in experiment 2 when visual distracters were present. We conclude that interference effects might arise when MO is performed while competing visual stimuli are present. However, when a secondary task is situated in a different modality, neural responses are in line with the notion that the observers motor system responds in a near-automatic manner. This suggests that MO is a task with very low cognitive demands which might be a valuable supplement for rehabilitation training, particularly, in the acute phase after the incident or in patients suffering from attention deficits. However, it is important to keep in mind that visual distracters might interfere with the neural response in M1.
Highlights
Movement Observation (MO) activates the motor system of the observer in a similar way as movement execution
Corticomotor excitability of the opponens pollicis (OP) was modulated by MO and in a weight-dependent manner, such that z-transformed mean MEP amplitudes (zMEPamp) were smallest for the baseline condition and highest for the HEAVY movie condition
This modulation was similar when the movies were observed as a single or a dual task, as indicated by a significant main effect of Movie (F(2, 26) = 4.5534, p = .021) in the absence of a significant Movie x Task interaction (F(2, 26) = .28428, p = .75). This result was further confirmed by pre-planned comparisons revealing that the zMEPamp were significantly larger for the HEAVY than the LIGHT movies irrespective of whether MO was performed as single (t(13) = 1.82323, p = .047) or dual task (t(13) = 2.39015, p = .017)
Summary
Movement Observation (MO) activates the motor system of the observer in a similar way as movement execution. Behavioral studies showed consistently that responses to stimuli depicting motor actions are faster when the executed and observed movement are congruent than when they are incongruent [25,26,27,28,29,30,31,32,33]. This so-called ‘‘automatic imitation’’ effect was demonstrated even when the observed action was irrelevant to the participants’ response or when participants attended to an orthogonal stimulus dimension (e.g., to the brightness of the shown limb than to the observed movement, [32])
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