Comparison between different methods to determine motor threshold to transcranial magnetic stimulation

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Motor is a crucial parameter for determination of intensities of transcranial magnetic stimulation. However, the definition of this measure has been heterogeneous in the literature. This study aims at investigating different technical aspects related to motor measurement and their impact on the design of experimental protocols of transcranial magnetic stimulation. A total of 256 measurements were performed in sixteen subjects aged 23 to 39 years. A figure-of-eight coil was used to perform transcranial magnetic stimulation. Implications of the following issues were evaluated: 1) defining the stimulated for motor measurement according to evaluation of amplitudes of motor evoked potentials (optimal position) or according to positions arbitrarily determined, considering reference positions on the skull as fiducial markers; 2) defining according to motor evoked potentials registered in the abductor pollicis brevis muscle with surface electromyography or according to visualization of hand movements; 3) using different coil positions on of the first interosseus dorsalis and biceps brachialis muscles; 4) using different definitions of motor and different coil positions to determine stimulus intensities of transcranial magnetic stimulation and to measure amplitudes of motor evoked potentials; 5) using different numbers of stimuli to define motor thresholds. There were statistically significant differences between thresholds measured with different methods. We found significant differences between motor thresholds of the abductor pollicis brevis measured with stimulation of the position or with stimulation of an arbitrary on the skull (difference 3.6 ′ 1.3%, p = 0.017). Significant differences were also found between motor thresholds measured with recording of motor evoked potentials or with observation of movements (p = 0.031). Intensities of magnetic stimulation were significantly higher (difference, 5.3%, p = 0.04) when was measured with stimulation of an arbitrary position. Amplitudes of motor evoked potentials were not significantly different (p = 0.92) with stimulation of the position or with stimulation of an arbitrary position, when intensities of stimulation were adjusted according to the measured in each position. There were no significant differences between measurements performed with evaluation of six motor evoked potentials at each intensity of stimulation, compared to ten potentials (p = 0,70). In order to measure motor threshold, it is important to search for the target muscle optimal position. We discourage usage of the term 'motor threshold' interchangeably for thresholds measured with MEP evaluation and with visualization of movement. We propose that the term be used to designate the former, while movement threshold be used for the latter. Differences between the two techniques should be considered in design and comparison of TMS protocols.