A model of the effect of MEP amplitude variation on the accuracy of TMS mapping
Introduction
Transcranial magnetic stimulation (TMS) has been widely used for mapping the corticomotor projection to a number of target muscles, and while such maps are only indirectly related to the origins of the corticomotor projection, they are nevertheless, a valuable tool for investigating the motor cortex in a range of physiological and pathological conditions (Wassermann et al., 1992, Wilson et al., 1993, Pascual-Leone et al., 1995, Thompson et al., 1997). Of particular interest is the use of TMS mapping to investigate reorganizational changes in the motor cortex, such as a shift in the position of a map or a change in other map parameters under certain conditions (Byrnes et al., 1998). In interpreting such studies, an understanding of the factors which contribute to the accuracy of TMS maps is important.
A TMS map is generated by measuring motor evoked potential (MEP) amplitude as a function of scalp site stimulated, and quantitating such parameters as map position and map area based upon these data. MEP amplitude can fluctuate significantly, even under carefully controlled conditions, and therefore, a major factor likely to influence the reliability of TMS maps is the reliability of the estimate of MEP amplitude at each scalp site. In the present study, we have modelled the effect of MEP amplitude variation on TMS maps.
Section snippets
Methods
Studies were carried out using mapping data from 5 control subjects (two F, R-handed, 23–44 years of age). Post-TMS MEPs were measured from the left abductor pollicis brevis muscle via surface electrodes taped over the motor point and the metacarpophalangeal joint. The electromyographic signal (EMG) was amplified by 1000, filtered between 10–2000 Hz, and digitised at intervals of 0.5 ms for 500 ms following each stimulus. Stimulation was carried out using a 5 cm diameter figure-of-8 coil
Results
At the centre site (the site with the largest measured MEP amplitude) the 2 standard error range over which the simulated MEP amplitude was varied had a mean (across subjects) of 21% of the measured MEP amplitude. The percentage range generally increased with distance from the centre site, as the measured MEP amplitude decreased and became more variable. The actual ranges used for each subject for the central sites are presented in Table 1.
The variability in mapping parameters was estimated
Discussion
These simulations have demonstrated that in the presence of random changes in the MEP amplitude value assigned to each scalp site, TMS map position and area are quite stable with variations of the order of only 1 mm for map position and less than 5% for map area.
While it may have been expected that to maximise the reliability of TMS maps, a large number of stimuli should be delivered at each site in order to accurately measure mean MEP amplitude, the present simulations suggest that this is not
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