Techniques and MethodsThe transcranial magnetic stimulation motor threshold depends on the distance from coil to underlying cortex: a replication in healthy adults comparing two methods of assessing the distance to cortex
Introduction
Transcranial magnetic stimulation (TMS) is a powerful tool for brain research George et al 1999, Ziemann and Hallett 2000. With TMS, an electromagnetic coil placed against a subject’s scalp sends a focal and rapidly changing magnetic pulse through the skull with only minimal discomfort to the subject. Upon reaching the cortex, the magnetic field induces an electrical current that depolarizes cortical neurons. Because the magnetic field declines exponentially with distance from the coil, any condition that would increase the distance between the scalp and the surface of the subject’s cortex theoretically might affect the intensity of magnetic stimulation (and thus induced electrical current) actually reaching the cortex.
Over the motor cortex, TMS causes movement in the contralateral limb or body, a visible effect. The motor threshold (MT) is defined as the lowest level of stimulation capable of causing a twitch in the contralateral thumb, specifically, the abductor pollicis brevis (APB; Pascual-Leone et al 1992). The MT varies widely across individuals but is relatively stable within individuals over time (Ziemann et al, 2000). Because of the wide range of absolute TMS intensity needed to produce comparable stimulation across individuals, most TMS researchers have attempted to correct for this variability by dosing TMS intensity as a proportion of each individual’s motor threshold.
Recently, Kozel et al (2000) examined the relationship between MT, age, and the distance from the scalp to both the motor cortex and the prefrontal cortex. In their cohort of 29 depressed subjects, MT strongly correlated with distance from scalp to motor cortex (p < .01). Interestingly, there was no significant correlation between MT and distance from scalp to prefrontal cortex, the site for repetitive TMS (rTMS) treatment of depression in their study. Kozel et al did not have a direct marker of the location of the motor cortex, however, and were forced to rely on an algorithm that derived the location of the motor cortex in relation to their more generally determined location of prefrontal cortex.
We questioned whether this relationship between MT and motor cortex distance was unique to depression and sought to test for this relationship in a group of healthy subjects stretching across a similarly broad range of ages. We also wondered whether a method of assessing distance from scalp to cortex that would sample the distance over a broad area of cortex (18 mm × 18 mm) rather than along a line (8 mm, as in Kozel’s study) would produce a more accurate assessment of this distance.
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Subjects
Seventeen healthy adults (9 men) between the ages of 19 and 75 (mean = 43, SD = 20) participated. Subjects were screened for health problems using a detailed history-taking and physical examination. Additionally, subjects over age 60 completed and passed mini-mental status exams to screen for dementia (Folstein et al 1975; mean score = 30, SD = 1, cutoff = 26). All subjects gave written informed consent as approved by the Medical University of South Carolina internal review board.
Transcranial magnetic stimulation and magnetic resonance method
As previously
Distance and MT
Confirming the prestudy hypothesis and replicating the prior Kozel study in this healthy cohort, MT significantly increased with increasing distance between scalp and motor cortex (Figure 2) (FisherR toZ, hypothesized correlation < 0,p = .0006,R2 = .491, slope = 5.261).
We found that MT did not significantly correlate with distance from scalp to prefrontal cortex (FisherR toZ, hypothesized correlation < 0,p = .0973, R2 = .111, slope = 1.923).
Motor cortex distance and prefrontal cortex distance
The distance from scalp to motor cortex significantly
Discussion
Our findings in this healthy and medication-free cohort are an important replication of Kozel’s work with depressed patients because we demonstrate that the relationship between motor threshold and distance from scalp to cortex is a feature of normal anatomy and physiology, not a pathologic consequence of depression.
This study also compares two methods of assessing distance from scalp to cortex using the same data. Both methods confirm the results from Kozel’s study that as distance increased
Acknowledgements
The National Alliance for Research on Schizophrenia and Depression and the Ted and Vada Stanley Foundation partially funded this work through research grants to Dr. George. Dr. Miller and Dr. Crouch of the Medical University of South Carolina Student Health Professions Summer Research Program provided salary support for KAM to perform this project.
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