BOLD fMRI response to direct stimulation (transcranial magnetic stimulation) of the motor cortex shows no decline with age

Previous studies using BOLD fMRI to examine age-related changes in cortical activation used tasks that relied on peripheral systems to activate the brain. They were unable to distinguish between alterations due to age-related changes in the periphery and actual changes in cortical physiology. Transcranial magnetic stimulation (TMS), which allows direct, noninvasive stimulation of cortical neurons, was interleaved with BOLD fMRI to study 6 young and 5 old subjects. Three different tasks were compared: direct stimulation by TMS, indirect active stimulation produced by a motor task, and indirect passive stimulation produced by hearing the TMS coil discharge. Direct neuronal stimulation by TMS produced similar fMRI signal increases in both groups, suggesting that cortical physiology itself may not necessarily decline with age.

Unilateral left prefrontal transcranial magnetic stimulation (TMS) produces intensity-dependent bilateral effects as measured by interleaved BOLD fMRI

Transcranial magnetic stimulation (TMS) administered over the prefrontal cortex has been shown to subtly influence neuropsychological tasks, and has antidepressant effects when applied daily for several weeks. Prefrontal TMS does not, however, produce an immediate easily observable effect, making it hard to determine if one has stimulated the cortex. Most prefrontal TMS studies have stimulated using intensity relative to the more easily determined motor threshold (MT) over motor cortex. Five healthy adults were studied in a 1.5 T MRI scanner during short trains of 1 Hz TMS delivered with a figure eight MR compatible TMS coil followed by rest epochs. In a randomized manner, left prefrontal TMS was delivered at 80%, 100% and 120% of MT interleaved with BOLD fMRI acquisition. Compared to rest, all TMS epochs activated auditory cortex, with 80% MT having no other areas of significant activation. 100% MT showed contralateral activation and 120% MT showed bilateral prefrontal activation. Higher intensity TMS, compared to lower, in general produced more activity both under the coil and contralaterally. Higher prefrontal TMS stimulation intensity produces greater local and contralateral activation. Importantly, unilateral prefrontal TMS produces bilateral effects, and TMS at 80% MT produces only minimal prefrontal cortex activation.

A low-cost system for monitoring skin conductance during functional MRI

We built a low-cost system for monitoring human skin conductance responses (SCRs) in a clinical magnetic resonance (MR) scanner during functional imaging. The average scanner-induced conductance noise level was suppressed sufficiently to allow SCR measurements over the full range of SCR amplitudes, and functional image signal-to-noise ratio was unaffected by the skin conductance apparatus. The system may be useful for a variety of imaging studies.

Feasibility of vagus nerve stimulation-synchronized blood oxygenation level-dependent functional MRI

RATIONALE AND OBJECTIVES

Left cervical vagus nerve stimulation (VNS) by use of an implanted neurocybernetic prosthesis (NCP) system is effective in treating epilepsy, with open data suggesting effectiveness in depression, yet the mechanisms of action are unknown. Our objective was to develop a methodology for performing VNS-synchronized functional magnetic resonance imaging (VNS-fMRI) and then to demonstrate its feasibility for studying VNS effects.

METHODS

In nine patients implanted for treatment of intractable depression, a Macintosh computer was used to detect the signal from the implanted VNS stimulator and then to synchronize fMRI image acquisition with its regular firing.

RESULTS

With our VNS-fMRI methodology, the blood oxygenation level-dependent response to VNS was shown in brain regions regulated by the vagus nerve: orbitofrontal and parieto-occipital cortex bilaterally, left temporal cortex, the hypothalamus, and the left amygdala.

CONCLUSIONS

Vagus nerve stimulation pulses from an NCP system can be detected externally to determine its firing pattern, thus allowing VNS-fMRI studies of VNS-induced brain activity.

Structural and functional neuroimaging of electroconvulsive therapy and transcranial magnetic stimulation

Neuroimaging has long been utilized to provide a measure of the effects of electroconvulsive therapy (ECT) on brain structure and function as well as to better understand its mechanisms of action. In a similar fashion, functional neuroimaging may provide the means to elucidate both the underlying neurobiological effects and therapeutic potential of transcranial magnetic stimulation (TMS). This article will review findings of neuroimaging studies of both TMS and ECT, concentrating on how such studies may help guide treatment.

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

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. Seventeen 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. Confirming 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. Distance 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.

Motor cortex brain activity induced by 1-Hz transcranial magnetic stimulation is similar in location and level to that for volitional movement

RATIONALE AND OBJECTIVES

The relatively high temporal and spatial resolution of functional MR imaging was used to compare the blood oxygenation level dependent (BOLD) response associated with movement induced by transcranial magnetic stimulation (TMS) with that for a similar movement executed volitionally (VOL).

METHODS

Seven healthy adults were studied in a 1.5-T MR scanner. One hertz TMS at 110% of motor threshold was applied over the motor cortex for the thumb in 21-pulse trains in alternation with VOL every 63 seconds and interleaved with functional MR imaging.

RESULTS

BOLD increases in motor cortex associated with TMS and VOL movement were similar (2%-3%). Mean separation of their centers of activity was 3.7 + 1.9 mm (mean displacement: left/right = 0.3 +/- 4.1 mm; superior/inferior = 0.7 +/- 1.9 mm). There was no indication of supraphysiological brain activity.

CONCLUSIONS

Motor cortex BOLD response associated with thumb movement induced by 1-Hz TMS at 110% motor threshold is similar in both location and level to that caused by a similar movement executed volitionally.

BOLD-f MRI response to single-pulse transcranial magnetic stimulation (TMS)

Five healthy volunteers were studied using interleaved transcranial magnetic stimulation/functional magnetic resonance imaging (TMS/fMRI) and an averaged single trial (AST) protocol. Blood oxygenation level-dependent (BOLD)-fMRI response to single TMS pulses over the motor cortex was detectable in both the ipsilateral motor cortex under the TMS coil and the contralateral motor cortex, as well as bilaterally in the auditory cortex. The associated BOLD signal increase showed the typical fMRI hemodynamic response time course. The brain's response to a single TMS pulse over the motor cortex at 120% of the level required to induce thumb movement (1.0%-1.5% signal increase) was comparable in both level and duration to the auditory cortex response to the sound accompanying the TMS pulse (1.5% -2.0% signal increase).

A combined TMS/fMRI study of intensity-dependent TMS over motor cortex

BACKGROUND

Transcranial magnetic stimulation (TMS) allows noninvasive stimulation of neurons using time-varying magnetic fields. Researchers have begun combining TMS with functional imaging to simultaneously stimulate and image brain activity. Recently, the feasibility of interleaving TMS with functional magnetic resonance imaging (fMRI) was demonstrated. This study tests this new method to determine if TMS at different intensities shows different local and remote activation.

METHODS

Within a 1.5 Tesla (T) MRI scanner, seven adults were stimulated with a figure-eight TMS coil over the left motor cortex for thumb, while continuously acquiring blood oxygen level dependent (BOLD) echoplanar images. TMS was applied at 1 Hz in 18-second long trains delivered alternately at 110% and 80% of motor threshold separated by rest periods.

RESULTS

Though the TMS coil caused some artifacts and reduced the signal to noise ratio (SNR), higher intensity TMS caused greater activation than lower, both locally and remotely. The magnitude (approximately 3% increase) and temporal onset (2 to 5 sec) of TMS induced blood flow changes appear similar to those induced using other motor and cognitive tasks.

CONCLUSIONS

Though work remains in refining this potentially powerful method, combined TMS/fMRI is both technically feasible and produces measurable dose-dependent changes in brain activity.

Echoplanar BOLD fMRI of brain activation induced by concurrent transcranial magnetic stimulation

RATIONALE AND OBJECTIVES

The authors demonstrate the feasibility of combining transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) inside an MR scanner to noninvasively stimulate and image regional brain activity.

METHODS

Echoplanar blood oxygen level dependent (BOLD)-based fMRI studies of TMS response were performed on three human volunteers inside a standard 1.5 T MR scanner using independent computer control to interleave echoplanar image acquisition and stimulation of right thumb primary motor cortex with a nonferromagnetic TMS coil.

RESULTS

Significant (P< 0.001) response was observed in motor cortex under the TMS coil during stimulation compared to rest, as well in auditory cortex, the latter presumably due to the loud "snap" when the coil was pulsed.

CONCLUSIONS

Concurrent TMS stimulation and echoplanar BOLD fMRI imaging is possible. This method has potential for tracing neural circuits with brain imaging, as well as investigating the effects of TMS.