Abstract
The aim of the present study was to investigate how the video speed of observed action affects the excitability of the primary motor cortex (M1), as assessed by the size of motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS). Twelve healthy subjects observed a video clip of a person catching a ball (Experiment 1: rapid movement) and another 12 healthy subjects observed a video clip of a person reaching to lift a ball (Experiment 2: slow movement task). We played each video at three different speeds (slow, normal and fast). The stimulus was given at two points of timing in each experiment. These stimulus points were locked to specific frames of the video rather than occurring at specific absolute times, for ease of comparison across different speeds. We recorded MEPs from the first dorsal interosseous muscle (FDI) and abductor digiti minimi muscle (ADM) of the right hand. MEPs were significantly different for different video speeds only in the rapid movement task. MEPs for the rapid movement task were higher when subjects observed an action played at slow speed than normal or fast speed condition. There was no significant change for the slow movement task. Video speed was effective only in the ADM. Moreover, MEPs in the ADM were significantly higher than in the FDI in a rapid movement task under the slow speed condition. Our findings suggest that the M1 becomes more excitable when subjects observe the video clip at the slow speed in a rapid movement, because they could recognize the elements of movement in others. Our results suggest the effects of manipulating the speed of the viewed task on the excitability of the M1 during passive observation differ depending on the type of movement task observed. It is likely that rehabilitation in the clinical setting will be more efficient if the video speed is changed to match the task’s characteristics.
Highlights
Recent years have seen great advances in brain imaging technology, enabling many researchers to elucidate brain mechanisms that were formerly ‘‘black boxes’’
In order to contribute to the development of the utility of action observation in rehabilitation and sports, we have previously investigated how the speed of observed action affects the excitability of M1, as assessed by the size of motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS)
Mean MEP Amplitude Compared to Control Condition A one-way analysis of variance (ANOVA) was performed and a main effect of SPEED was found in the abductor digiti minimi muscle (ADM) (F(3,33) = 7.521, p < 0.001, effect size = 0.975, power = 0.406), but not found in the first dorsal interosseous muscle (FDI) (F(3,33) = 0.402, non-significant, effect size = 0.115, power = 0.035)
Summary
Recent years have seen great advances in brain imaging technology, enabling many researchers to elucidate brain mechanisms that were formerly ‘‘black boxes’’. Researchers found that similar mirror systems existed in humans (action observation network; AON). They discovered that observing actions activates the same or related cortical motor areas that are involved in the performance of the actions (Fadiga et al, 1995; Grafton et al, 1996; Buccino et al, 2001; Rizzolatti and Craighero, 2004; Iacoboni and Mazziotta, 2007; Gatti et al, 2016). This neural system is the basis of action recognition (Gallese et al, 1996; Umiltà et al, 2001; Buccino et al, 2004; Sartori and Castiello, 2013; Naish et al, 2016), action understanding (Hari et al, 1998; Rizzolatti et al, 2001; Kilner, 2011; Jacquet et al, 2016), and automatic imitation (Nishitani and Hari, 2000; Iacoboni, 2005), and it is involved in motor learning (Mattar and Gribble, 2005; Stefan et al, 2005; Lago-Rodriguez et al, 2013)
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