Elsevier

Biological Psychiatry

Volume 49, Issue 5, 1 March 2001, Pages 454-459
Biological Psychiatry

Techniques and Methods
The 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

https://doi.org/10.1016/S0006-3223(00)01039-8Get rights and content

Abstract

%Using transcranial magnetic stimulation (TMS), a handheld electrified copper coil against the scalp produces a powerful and rapidly oscillating magnetic field, which in turn induces electrical currents in the brain. The amount of electrical energy needed for TMS to induce motor movement (called the motor threshold [MT]), varies widely across individuals. The intensity of TMS is dosed relative to the MT. Kozel et al observed in a depressed cohort that MT increases as a function of distance from coil to cortex. This article examines this relationship in a healthy cohort and compares the two methods of assessing distance to cortex. mSeventeen healthy adults had their TMS MT determined and marked with a fiducial. Magnetic resonance images showed the fiducials marking motor cortex, allowing researchers to measure distance from scalp to motor and prefontal cortex using two methods: 1) measuring a line from scalp to the nearest cortex and 2) sampling the distance from scalp to cortex of two 18-mm-square areas. mConfirming Kozel’s previous finding, we observe that motor threshold increases as distance to motor cortex increased for both methods of measuring distance and that no significant correlation exists between MT and prefontal cortex distance. mDistance from TMS coil to motor cortex is an important determinant of MT in healthy and depressed adults. Distance to prefontal cortex is not correlated with MT, raising questions about the common practice of dosing prefontal stimulation using MT determined over motor 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.

Section snippets

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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