Chronometric TMS-fMRI of personalized left dorsolateral prefrontal target reveals state-dependency of subgenual anterior cingulate cortex effects

Transcranial magnetic stimulation (TMS) applied to a left dorsolateral prefrontal cortex (DLPFC) area with a specific connectivity profile to the subgenual anterior cingulate cortex (sgACC) has emerged as a highly effective non-invasive treatment option for depression. However, antidepressant outcomes demonstrate significant variability among therapy plans and individuals. One overlooked contributing factor is the individual brain state at the time of treatment. In this study we used interleaved TMS-fMRI to investigate the influence of brain state on acute TMS effects, both locally and remotely. TMS was performed during rest and during different phases of cognitive task processing. Twenty healthy participants were included in this study. In the first session, imaging data for TMS targeting were acquired, allowing for identification of individualized targets in the left DLPFC based on highest anti-correlation with the sgACC. The second session involved chronometric interleaved TMS-fMRI measurements, with 10 Hz triplets of TMS administered during rest and at distinct timings during an N-back task. Consistent with prior findings, interleaved TMS-fMRI revealed significant BOLD activation changes in the targeted network. The precise timing of TMS relative to the cognitive states during the task demonstrated distinct BOLD response in clinically relevant brain regions, including the sgACC. Employing a standardized timing approach for TMS using a task revealed more consistent modulation of the sgACC at the group level compared to stimulation during rest. In conclusion, our findings strongly suggest that acute local and remote effects of TMS are influenced by brain state during stimulation. This study establishes a basis for considering brain state as a significant factor in designing treatment protocols, possibly improving TMS treatment outcomes.

Concurrent TMS/fMRI reveals individual DLPFC dose-response pattern

BACKGROUND

TMS is a valuable tool in both research and clinical settings, playing a crucial role in understanding brain-behavior relationships and providing treatment for various neurological and psychiatric conditions. Importantly, TMS over left DLPFC is an FDA approved treatment for MDD. Despite its potential, response variability to TMS remains a challenge, with stimulation parameters, particularly the stimulation intensity, being a primary contributor to these differences.

OBJECTIVE

The objective of this study was to establish dose-response relationships of TMS stimulation in DLPFC by means of concurrent TMS/fMRI.

METHODS

Here, we stimulated 15 subjects at different stimulation intensities of 80, 90, 100 and 110 % relative to the motor threshold during concurrent TMS/fMRI. The experiment comprised two sessions: one session to collect anatomical data in order to perform neuronavigation and one session dedicated to dose-response mapping. We calculated GLMs for each intensity level and each subject, as well as at a group-level per intensity.

RESULTS

On a group level, we show that the strongest BOLD-response was at 100 % stimulation. However, investigating individual dose response-relationships showed differences in response patterns across the group: subjects that responded to subthreshold stimulation, subjects that required above threshold stimulation in order to show a significant BOLD-response and atypical responders.

CONCLUSIONS

We observed qualitative inter-subject variability in terms of dose-response relationship to TMS over left DLPFC, which hints towards the motor threshold not being directly transferable to the excitability of the DLPFC. Concurrent TMS/fMRI might have the potential to improve response rates to rTMS applications. As such, it may be valuable in the future to consider implementing this approach prior to clinical TMS or validating more cost-effective methods to determine dose and target with respect to changes in clinical symptoms.

Improved brain stimulation targeting by optimising image acquisition parameters

Functional connectivity analysis from rs-fMRI data has been used for determining cortical targets in therapeutic applications of non-invasive brain stimulation using transcranial magnetic stimulation (TMS). Reliable connectivity measures are therefore essential for every rs-fMRI-based TMS targeting approach. Here, we examine the effect of echo time (TE) on the reproducibility and spatial variability of resting-state connectivity measures. We acquired multiple runs of single-echo fMRI data with either short (TE = 30 ms) or long (TE = 38 ms) echo time to investigate inter-run spatial reproducibility of a clinically relevant functional connectivity map, i.e., originating from the sgACC. We find that connectivity maps obtained from TE = 38 ms rs-fMRI data are significantly more reliable than those obtained from TE = 30 ms data sets. Our results clearly show that optimizing sequence parameters can be beneficial for ensuring high-reliability resting-state acquisition protocols to be used for TMS targeting. The differences between reliability in connectivity measures for different TEs could inform future clinical research in optimising MR sequences.

Functional connectivity explains how neuronavigated TMS of posterior temporal subregions differentially affect language processing

BACKGROUND

"Wernicke's area" is most often used to describe the posterior superior temporal gyrus (STG) and refers to a region traditionally thought to support language comprehension. However, the posterior STG additionally plays a critical role in language production. The purpose of the current study was to determine to what extent regions within the posterior STG are selectively recruited during language production.

METHODS

23 healthy right-handed participants completed an auditory fMRI localizer task, resting-state fMRI and underwent neuronavigated TMS language mapping. We applied repetitive TMS bursts during a picture naming paradigm to probe speech disruptions of different categories (anomia, speech arrest, semantic paraphasia and phonological paraphasia). We combined an in-house built high precision stimulation software suite with E-field modeling to map the naming errors to cortical regions and revealed a dissociation of language functions within the temporal gyrus. Resting state fMRI was used to explain how E-field peaks of different categories differentially affected language production.

RESULTS

Peaks for phonological and semantic errors were found in the STG while those for anomia and speech arrest were located in the MTG. Seed-based connectivity analysis revealed a local connectivity pattern for phonological and semantic errors, while anomia and speech arrest seeds resulted in a larger network between IFG and posterior MTG.

CONCLUSIONS

Our study provides important insights into the functional neuroanatomy of language production and might help to increase the current understanding of specific language production difficulties on a causal level.

Comparison of Stimulus Types for Retinotopic Cortical Mapping of Macular Disease

Purpose

Retinotopic maps acquired using functional magnetic resonance imaging (fMRI) provide a valuable adjunct in the assessment of macular function at the level of the visual cortex. The present study quantitatively assessed the performance of different visual stimulation approaches for mapping visual field coverage.

Methods

Twelve patients with geographic atrophy (GA) secondary to age-related macular degeneration (AMD) were examined using high-resolution ultra-high field fMRI (Siemens Magnetom 7T) and microperimetry (MP; Nidek MP-3). The population receptive field (pRF)-based coverage maps obtained with two different stimulus techniques (moving bars, and rotating wedges and expanding rings) were compared with the results of MP. Correspondence between MP and pRF mapping was quantified by calculating the simple matching coefficient (SMC).

Results

Stimulus choice is shown to bias the spatial distribution of pRF centers and eccentricity values with pRF sizes obtained from wedge/ring or bar stimulation showing systematic differences. Wedge/ring stimulation results show a higher number of pRF centers in foveal areas and strongly reduced pRF sizes compared to bar stimulation runs. A statistical comparison shows significantly higher pRF center numbers in the foveal 2.5 degrees region of the visual field for wedge/ring compared to bar stimuli. However, these differences do not significantly influence SMC values when compared to MP (bar <2.5 degrees: 0.88 ± 0.13; bar >2.5 degrees: 0.88 ± 0.11; wedge/ring <2.5 degrees: 0.89 ± 0.12 wedge/ring; >2.5 degrees: 0.86 ± 0.10) for the peripheral visual field.

Conclusions

Both visual stimulation designs examined can be applied successfully in patients with GA. Although the two designs show systematic differences in the distribution of pRF center locations, this variability has minimal impact on the SMC when compared to the MP outcome.

Intrasession and Intersession Reproducibility of Artificial Scotoma pRF Mapping Results at Ultra-High Fields

Functional magnetic resonance imaging (fMRI) combined with population receptive field (pRF) mapping allows for associating positions on the visual cortex to areas on the visual field. Apart from applications in healthy subjects, this method can also be used to examine dysfunctions in patients suffering from partial visual field losses. While such objective measurement of visual deficits (scotoma) is of great importance for, e.g., longitudinal studies addressing treatment effects, it requires a thorough assessment of accuracy and reproducibility of the results obtained. In this study, we quantified the reproducibility of pRF mapping results within and across sessions in case of central visual field loss in a group of 15 human subjects. We simulated scotoma by masking a central area of 2° radius from stimulation to establish ground-truth conditions. This study was performed on a 7T ultra-high field MRI scanner for increased sensitivity. We found excellent intrasession and intersession reproducibility for the pRF center position (Spearman correlation coefficients for x, y: >0.95; eccentricity: >0.87; polar angle: >0.98), but only modest reproducibility for pRF size (Spearman correlation coefficients around 0.4). We further examined the scotoma detection performance using an automated method based on a reference dataset acquired with full-field stimulation. For the 2° artificial scotoma, the group-averaged scotoma sizes were estimated at between 1.92° and 2.19° for different sessions. We conclude that pRF mapping of visual field losses yields robust, reproducible measures of retinal function and suggest the use of pRF mapping as an objective method for monitoring visual deficits during therapeutic interventions or disease progression.

Combining stimulus types for improved coverage in population receptive field mapping

Retinotopy experiments using population receptive field (pRF) mapping are ideal for assigning regions in the visual field to cortical brain areas. While various designs for visual stimulation were suggested in the literature, all have specific shortcomings regarding visual field coverage. Here we acquired high-resolution 7 Tesla fMRI data to compare pRF-based coverage maps obtained with the two most commonly used stimulus variants: moving bars; rotating wedges and expanding rings. We find that stimulus selection biases the spatial distribution of pRF centres. In addition, eccentricity values and pRF sizes obtained from wedge/ring or bar stimulation runs show systematic differences. Wedge/ring stimulation results show lower eccentricity values and strongly reduced pRF sizes compared to bar stimulation runs. Statistical comparison shows significantly higher pRF centre numbers in the foveal 2° region of the visual field for wedge/ring compared to bar stimuli. We suggest and evaluate approaches for combining pRF data from different visual stimulus patterns to obtain improved mapping results.