Transcranial magnetic stimulation and action observation: exploring methodological issues

Loporto, Michela (2012) Transcranial magnetic stimulation and action observation: exploring methodological issues. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

This thesis explored a number of methodological issues present in motor cognition research using transcranial magnetic stimulation (TMS). The facilitatory effect of the corticospinal pathway during observation of simple hand actions was also investigated. TMS was applied to the motor cortex during action observation and the resulting MEP peak-to-peak amplitudes were analysed. A series of four studies were conducted to test whether a motor facilitation effect specific to the muscles involved in the observed actions were obtained, while simultaneously investigating five prominent methodological concerns in TMS research. In Study 1 the issue of choosing the optimal control condition was investigated. The MEP facilitation obtained during action observation (ball pinch) was compared to two commonly used control conditions (fixation cross and static image). Consistent with published literature, the action condition resulted in larger MEP amplitudes than the controls. There was no statistical difference in MEP amplitude between the two resting conditions. It was argued, however, that the static image allows for more accurate comparison with the action condition by providing meaningful visual cues without the associated action. In Study 2, the effect of short-term physical execution on the relationship between observed actions and neural activity was explored. The motor facilitation effect was present during action observation. This was not enhanced following execution of the observed action which is in contrast with the literature that shows the observation-execution matching system tuned to familiarity with an action. In TMS studies, different stimulation timings are included in order to reduce anticipatory effects of the TMS pulse. While the different timings are usually analysed together, in Studies 1 and 2, the two stimulation timings were analysed separately. As a consequence, a motor facilitation effect was only evident for the earlier stimulation timing of 6250ms in Study 1. When participants executed the action prior to observing it in Study 2, there was no effect of stimulation timing, leading to speculation that the prior execution may have had some effect on the attentional demands during the subsequent observation. Studies 3 and 4 explored two general methods concerns regarding the motor hotspot and stimulation intensity. In Study 3, the muscle- vi specificity notion was explored via observation of index finger and little finger movements versus observation of a static hand, with the corresponding muscles tested at their individual hotspots. This was a novel approach as one hotspot is typically used for all muscles under investigation. The choice of motor hotspot, however, did not significantly affect the muscle-specific findings, providing further support for the muscle-specific motor facilitation findings reported in the literature. Finally, Study 4 investigated the concept of stimulation intensity. TMS action observation studies differ in the stimulation intensities used, typically ranging from 110% to 130% of resting motor threshold. Since the motor response obtained through TMS may be affected depending on the stimulation intensity used, two stimulation intensities were employed (high vs. low) during observation of finger movements. A motor facilitation effect was reported in the low intensity stimulation, which was expected given that near threshold intensities are more representative of the ongoing level of cortical excitability. No motor facilitation effect was shown in the high intensity stimulation, possibly due to the nature of high stimulation intensities on the corticospinal pathway, or simply because the low intensity stimulations were always delivered before the high intensity stimulations. In light of the stimulation timing findings of Study 1, this may have resulted in participants getting distracted or fatigued, focussing their attention elsewhere (and therefore lowering MEP amplitudes) during the latter high stimulations. From the results presented in these studies, it is clear that there is a muscle specific motor facilitation during action observation and its characteristics are influenced by many procedural, technical and cognitive and attentional factors. This thesis provides a much needed critical analysis into the methods and methodologies commonly adopted in this area of research. It is essential to continue to explore the methods employed in TMS motor cognition studies, making them accepted universally and scientifically rigorous.