Impact of coil position and electrophysiological monitoring on determination of motor thresholds to transcranial magnetic stimulation Adriana Bastos Conforto a,b , Werner J. Z’Graggen a , Alexandra S. Kohl a , Kai M. Ro ¨sler a , Alain Kaelin-Lang a, * a Neurology Department, Inselspital, University of Berne, Berne, CH-3010 Berne, Switzerland b Neurology Department, Sa ˜o Paulo University, Sa ˜o Paulo, Brazil Accepted 16 November 2003 Abstract Objective: We compared motor and movement thresholds to transcranial magnetic stimulation (TMS) in healthy subjects and investigated the effect of different coil positions on thresholds and MEP (motor-evoked potential) amplitudes. Methods: The abductor pollicis brevis (APB) ‘hot spot’ and a standard scalp position were stimulated. APB resting motor threshold (APB MEP-MT) defined by the ‘5/10’ electrophysiological method was compared with movement threshold (MOV-MT), defined by visualization of movements. Additionally, APB MEP-MTs were evaluated with the ‘3/6 method,’ and MEPs were recorded at a stimulation intensity of 120% APB MEP-MT at each position. Results: APB MEP-MTs were significantly lower by stimulation of the ‘hot spot’ than of the standard position, and significantly lower than MOV-MTs (n ¼ 15). There were no significant differences between the ‘3/6’ and the ‘5/10’ methods, or between APB MEP amplitudes by stimulating each position at 120% APB MEP-MT. Conclusions: Coil position and electrophysiological monitoring influenced motor threshold determinations. Performing 6 instead of 10 trials did not produce different threshold measurements. Adjustment of intensity according to APB MEP-MT at the stimulated position did not influence APB MEP amplitudes. Significance: Standardization of stimulation positions, nomenclature and criteria for threshold measurements should be considered in design and comparison of TMS protocols. q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Movement threshold; Hot spot; Transcranial magnetic stimulation; Motor cortex; Human 1. Introduction In biology, threshold represents the minimal level of stimulation at which a distinct response is produced (Mills and Nithi, 1997). For transcranial magnetic stimulation (TMS), resting motor threshold (MT) has been defined (Rossini et al., 1994) as the stimulation intensity in which reliable motor-evoked potentials (MEPs) are recorded with surface electromyography (EMG) “in about 50% of 11 – 20 consecutive stimuli.” MTs have substantial inter-subject variability but less intra-subject variability (Chen et al., 1998; Wassermann, 2002). The possibility to non-inva- sively obtain an objective, quantitative and relatively stable measure has prompted the use of MT as an indicator of cortical sensitivity to TMS and therefore for calibration of stimulation intensities, comparison of intensities across individuals, and establishment of recommendations for safe administration of repetitive TMS (rTMS) (Pascual-Leone et al., 1992, 1993, 1994; Wassermann, 1998; Wassermann et al., 1996). However, EMG recording may be difficult in some situations, such as inside a magnetic resonance imaging (MRI) scanner, because of the scanner’s high magnetic field. In studies performing TMS and MRI or functional MRI (fMRI) simultaneously (Baudewig et al., 2001; Bohning et al., 2000; McConnell et al., 2001; Nahas et al., 2001), thresholds have been defined by observation of TMS-evoked hand movements (MOV-MTs) (Pridmore et al., 1998). This procedure to define threshold has also been increasingly performed in a number of studies focusing Clinical Neurophysiology 115 (2004) 812–819 www.elsevier.com/locate/clinph 1388-2457/$30.00 q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2003.11.010 * Corresponding author. Tel.: þ41-31-632-2111; fax: þ 41-31-632-9679. E-mail address: alain.kaelin@dkf6.unibe.ch (A. Kaelin-Lang).