Henri Hannula

Preoperative Functional Mapping for Rolandic Brain Tumor Surgery: Comparison of Navigated Transcranial Magnetic Stimulation to Direct Cortical Stimulation

BACKGROUND:Transcranial magnetic stimulation (TMS) is the only noninvasive method for presurgical stimulation mapping of cortical function. Recent technical advancements have significantly increased the focality and usability of the method. OBJECTIVE:To compare the accuracy of a 3-dimensional magnetic resonance imaging-navigated TMS system (nTMS) with the gold standard of direct cortical stimulation (DCS). METHODS:The primary motor areas of 20 patients with rolandic tumors were mapped preoperatively with nTMS at 110% of the individual resting motor threshold. Intraoperative DCS was available from 17 patients. The stimulus locations eliciting the largest electromyographic response in the target muscles (“hotspots”) were determined for both methods. RESULTS:The nTMS and DCS hotspots were located on the same gyrus in all cases. The mean ± SEM distance between the nTMS and DCS hotspots was 7.83 ± 1.18 mm for the abductor pollicis brevis (APB) muscle (n = 15) and 7.07 ± 0.88 mm for the tibialis anterior muscle (n = 8). When a low number of DCS stimulations was performed, the distance between the nTMS and DCS hotspots increased substantially (r = −0.86 for APB). After the exclusion of the cases with < 15 DCS APB responses, the mean ± SEM distance between the hotspots was only 4.70 ± 1.09 mm for APB (n = 8). CONCLUSION:Peritumoral mapping of the motor cortex by nTMS agreed well with the gold standard of DCS. Thus, nTMS is a reliable tool for preoperative mapping of motor function.

A novel approach for documenting naming errors induced by navigated transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is widely used both in basic research and in clinical practice. TMS has been utilized in studies of functional organization of speech in healthy volunteers. Navigated TMS (nTMS) allows preoperative mapping of the motor cortex for surgical planning. Recording behavioral responses to nTMS in the speech-related cortical network in a manner that allows off-line review of performance might increase utility of nTMS both for scientific and clinical purposes, e.g., for a careful preoperative planning. Four subjects participated in the study. The subjects named pictures of objects presented every 2-3s on a computer screen. One-second trains of 5 pulses were applied by nTMS 300ms after the presentation of pictures. The nTMS and stimulus presentation screens were cloned. A commercial digital camera was utilized to record the subjects performance and the screen clones. Delays between presentation, audio and video signals were eliminated by carefully tested combination of displays and camera. An experienced neuropsychologist studied the videos and classified the errors evoked by nTMS during the object naming. Complete anomias, semantic, phonological and performance errors were observed during nTMS of left fronto-parieto-temporal cortical regions. Several errors were detected only in the video classification. nTMS combined with synchronized video recording provides an accurate monitoring tool of behavioral TMS experiments. This experimental setup can be particularly useful for high-quality cognitive paradigms and for clinical purposes.

Somatotopic blocking of sensation with navigated transcranial magnetic stimulation of the primary somatosensory cortex

We demonstrate that spatially accurate and selective stimulation is crucial when cortical functions are studied by the creation of temporary lesions with transcranial magnetic stimulation (TMS). Previously, the interpretation of the TMS results has been hampered by inaccurate knowledge of the site and strength of the induced electric current in the brain. With a Navigated Brain Stimulation (NBS) system, which provides real‐time magnetic resonance image (MRI)‐guided targeting of the TMS‐induced electric field, we found that TMS of a spatially restricted cortical S1 thenar area is sufficient to abolish sensation from a weak electric stimulation of the corresponding skin area. We demonstrate that with real‐time navigation, TMS can be repeatably directed at millimeter‐level precision to a target area defined on the MRI. The stimulation effect was temporally and spatially specific: the greatest inhibition of sensation occurred when TMS was applied 20 ms after the cutaneous test stimulus and the TMS effect was sensitive to 8–13 mm displacements of the induced electric field pattern. The results also indicate that TMS selectively to S1 is sufficient to abolish perception of cutaneous stimulation of the corresponding skin area. Hum Brain Mapp, 2005.

Non-primary motor areas in the human frontal lobe are connected directly to hand muscles

Structural studies in primates have shown that, in addition to the primary motor cortex (M1), premotor areas are a source of corticospinal tracts. The function of these putative corticospinal neuronal tracts in humans is still unclear. We found frontal non-primary motor areas (NPMAs), which react to targeted non-invasive magnetic pulses and activate peripheral muscles as fast as or even faster than those in M1. Hand muscle movements were observed in all our subjects about 20 ms after transcranial stimulation of the superior frontal gyrus (Brodmann areas 6 and 8). Stimulation of NPMA could activate both proximal and distal upper limb muscles with the same delay as a stimulation of the M1, indicating converging motor representations with direct functional connections to the hand. We suggest that these non-primary cortical motor representations provide additional capacity for the fast execution of movements. Such a capacity may play a role in motor learning and in recovery from motor deficits.

Increasing top-down suppression from prefrontal cortex facilitates tactile working memory

Navigated transcranial magnetic stimulation (TMS) combined with diffusion-weighted magnetic resonance imaging (DW-MRI) and tractography allows investigating functional anatomy of the human brain with high precision. Here we demonstrate that working memory (WM) processing of tactile temporal information is facilitated by delivering a single TMS pulse to the middle frontal gyrus (MFG) during memory maintenance. Facilitation was obtained only with a TMS pulse applied to a location of the MFG with anatomical connectivity to the primary somatosensory cortex (S1). TMS improved tactile WM also when distractive tactile stimuli interfered with memory maintenance. Moreover, TMS to the same MFG site attenuated somatosensory evoked responses (SEPs). The results suggest that the TMS-induced memory improvement is explained by increased top-down suppression of interfering sensory processing in S1 via the MFG-S1 link. These results demonstrate an anatomical and functional network that is involved in maintenance of tactile temporal WM.

Navigated transcranial magnetic stimulation of the primary somatosensory cortex impairs perceptual processing of tactile temporal discrimination

Previous studies indicate that transcranial magnetic stimulation (TMS) with biphasic pulses applied approximately over the primary somatosensory cortex (S1) suppresses performance in vibrotactile temporal discrimination tasks; these previous results, however, do not allow separating perceptual influence from memory or decision-making. Moreover, earlier studies using external landmarks for directing biphasic TMS pulses to the cortex do not reveal whether the changes in vibrotactile task performance were due to action on S1 or an adjacent area. In the present study, we determined whether the S1 area representing a cutaneous test site is critical for perceptual processing of tactile temporal discrimination. Electrical test pulses were applied to the thenar skin of the hand and the subjects attempted to discriminate single from twin pulses. During discrimination task, monophasic TMS pulses or sham TMS pulses were directed anatomically accurately to the S1 area representing the thenar using magnetic resonance image-guided navigation. The subjects capacity to temporal discrimination was impaired with a decrease in the delay between the TMS pulse and the cutaneous test pulse from 50 to 0 ms. The result indicates that S1 area representing a cutaneous test site is involved in perceptual processing of tactile temporal discrimination.

Protocol for motor and language mapping by navigated TMS in patients and healthy volunteers; workshop report

IntroductionNavigated transcranial magnetic stimulation (nTMS) is increasingly used for preoperative mapping of motor function, and clinical evidence for its benefit for brain tumor patients is accumulating. In respect to language mapping with repetitive nTMS, literature reports have yielded variable results, and it is currently not routinely performed for presurgical language localization. The aim of this project is to define a common protocol for nTMS motor and language mapping to standardize its neurosurgical application and increase its clinical value.MethodsThe nTMS workshop group, consisting of highly experienced nTMS users with experience of more than 1500 preoperative nTMS examinations, met in Helsinki in January 2016 for thorough discussions of current evidence and personal experiences with the goal to recommend a standardized protocol for neurosurgical applications.ResultsnTMS motor mapping is a reliable and clinically validated tool to identify functional areas belonging to both normal and lesioned primary motor cortex. In contrast, this is less clear for language-eloquent cortical areas identified by nTMS. The user group agreed on a core protocol, which enables comparison of results between centers and has an excellent safety profile. Recommendations for nTMS motor and language mapping protocols and their optimal clinical integration are presented here.ConclusionAt present, the expert panel recommends nTMS motor mapping in routine neurosurgical practice, as it has a sufficient level of evidence supporting its reliability. The panel recommends that nTMS language mapping be used in the framework of clinical studies to continue refinement of its protocol and increase reliability.

Facilitation of tactile working memory by top-down suppression from prefrontal to primary somatosensory cortex during sensory interference.

Tactile working memory (WM) is improved by increasing top-down suppression of interfering sensory processing in S1 via a link from the middle frontal gyrus (MFG) to S1. Here we studied in healthy subjects whether the efficacy of top-down suppression varies with submodality of sensory interference. Navigated stimulation of the MFG-S1 link significantly improved tactile WM performance when accompanied by tactile but not visual interference of memory maintenance.

Navigated Transcranial Magnetic Stimulation: Principles and Protocol for Mapping the Motor Cortex

Transcranial magnetic stimulation (TMS) is a unique method for studying the human brain. Whereas the majority of imaging tools detect and map all the brain areas that participate during a given task (both primary and secondary network activations), TMS, when used to evoke a measurable physiological response, maps only those areas that are mandatory for the observed reaction. As such, TMS is particularly suitable for mapping cortical motor areas and for assessing the functional status of the motor tracts, both in normal subjects and in patients. In this chapter, we explore the physical and mechanistic background of using TMS to map the motor cortex. In addition, we outline a detailed protocol for mapping the cortical representation of various muscles—a protocol which can be used in basic research or as part of a clinical diagnostic or treatment procedure.

Basic Principles of Navigated TMS

Navigated transcranial magnetic stimulation (nTMS) has been developed to make the induced electric field visible to the operator when targeting anatomical loci in the cortex. Since its introduction in 1985, TMS has offered the scientist a unique way to stimulate the brain, but the lack of accurate knowledge about the stimulated cortical spot and the stimulation intensity at the target area has compromised reproducibility and reliability, therefore limiting the clinical value of the technique. The majority of functional brain imaging tools indicate all brain areas that participate in a given task. Especially when there are structural and vascular changes in the brain after disease or trauma, indirect neuroimaging methods are sensitive to artifacts limiting their reliability in clinical routine work. TMS can evoke directly measurable physiological responses that produce maps from only those cortical spots that are mandatory for the observed reaction. The technological development that has enabled the stimulating electric field of the nTMS system to be calculated and visualized online for the operator has made nTMS particularly suitable for mapping the cortical motor and language areas for assessing the functional status of the cortical areas and tracts both in normal subjects and in patients.