Effects of parietal TMS on visual and auditory processing at the primary cortical level -- a concurrent TMS-fMRI study

The potential of interleaved TMS-fMRI for linking stimulation-induced changes in task-related activity with behavioral modulations

The simultaneous combination of TMS with fMRI has emerged as a promising means to investigate the direct interaction between stimulation-induced changes at the behavioral and neural activity level. This enables the investigation of whole brain neurobehavioral interactions underlying cognitive disruption or facilitation. Yet to date, the literature on interleaved TMS-fMRI in cognitive neuroscience is sparse and neuromodulatory patterns of different TMS protocols are still poorly understood. Here, we synthesize interleaved TMS-fMRI studies on the relationship between direct stimulation-induced changes on task related neural activity and behavior. The following main findings are discussed. First, approximately half of the studies report a relationship between neural activity and behavioral changes as a marker for network excitation or inhibition. Secondly, task difficulty and stimulation timing are crucial factors that impact the interaction between neural activity changes and behavior. Third, stimulation-induced changes in remote, connected areas seem to be stronger associated with facilitation effects at the behavioral level. A better understanding of the relationship between stimulation-induced changes at the neural and behavioral level will increase the current understanding of the neuromodulatory potential of TMS at different levels and may help to develop more efficient stimulation protocols for basic and applied research.

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.

Excitatory Crossmodal Input to a Widespread Population of Primary Sensory Cortical Neurons

Crossmodal information processing in sensory cortices has been reported in sparsely distributed neurons under normal conditions and can undergo experience- or activity-induced plasticity. Given the potential role in brain function as indicated by previous reports, crossmodal connectivity in the sensory cortex needs to be further explored. Using perforated whole-cell recording in anesthetized adult rats, we found that almost all neurons recorded in the primary somatosensory, auditory, and visual cortices exhibited significant membrane-potential responses to crossmodal stimulation, as recorded when brain activity states were pharmacologically down-regulated in light anesthesia. These crossmodal cortical responses were excitatory and subthreshold, and further seemed to be relayed primarily by the sensory thalamus, but not the sensory cortex, of the stimulated modality. Our experiments indicate a sensory cortical presence of widespread excitatory crossmodal inputs, which might play roles in brain functions involving crossmodal information processing or plasticity.

TMS Does Not Increase BOLD Activity at the Site of Stimulation: A Review of All Concurrent TMS-fMRI Studies

Transcranial magnetic stimulation (TMS) is widely used for understanding brain function in neurologically intact subjects and for the treatment of various disorders. However, the precise neurophysiological effects of TMS at the site of stimulation remain poorly understood. The local effects of TMS can be studied using concurrent TMS-functional magnetic resonance imaging (fMRI), a technique where TMS is delivered during fMRI scanning. However, although concurrent TMS-fMRI was developed over 20 years ago and dozens of studies have used this technique, there is still no consensus on whether TMS increases blood oxygen level-dependent (BOLD) activity at the site of stimulation. To address this question, here we review all previous concurrent TMS-fMRI studies that reported analyses of BOLD activity at the target location. We find evidence that TMS increases local BOLD activity when stimulating the primary motor (M1) and visual (V1) cortices but that these effects are likely driven by the downstream consequences of TMS (finger twitches and phosphenes). However, TMS does not appear to increase BOLD activity at the site of stimulation for areas outside of the M1 and V1 when conducted at rest. We examine the possible reasons for such lack of BOLD signal increase based on recent work in nonhuman animals. We argue that the current evidence points to TMS inducing periods of increased and decreased neuronal firing that mostly cancel each other out and therefore lead to no change in the overall BOLD signal.

Visual cortex responds to sound onset and offset during passive listening

Sounds enhance our ability to detect, localize, and respond to co-occurring visual targets. Research suggests that sounds improve visual processing by resetting the phase of ongoing oscillations in visual cortex. However, it remains unclear what information is relayed from the auditory system to visual areas and if sounds modulate visual activity even in the absence of visual stimuli (e.g., during passive listening). Using intracranial electroencephalography (iEEG) in humans, we examined the sensitivity of visual cortex to three forms of auditory information during a passive listening task: auditory onset responses, auditory offset responses, and rhythmic entrainment to sounds. Because some auditory neurons respond to both sound onsets and offsets, visual timing and duration processing may benefit from each. Additionally, if auditory entrainment information is relayed to visual cortex, it could support the processing of complex stimulus dynamics that are aligned between auditory and visual stimuli. Results demonstrate that in visual cortex, amplitude-modulated sounds elicited transient onset and offset responses in multiple areas, but no entrainment to sound modulation frequencies. These findings suggest that activity in visual cortex (as measured with iEEG in response to auditory stimuli) may not be affected by temporally fine-grained auditory stimulus dynamics during passive listening (though it remains possible that this signal may be observable with simultaneous auditory-visual stimuli). Moreover, auditory responses were maximal in low-level visual cortex, potentially implicating a direct pathway for rapid interactions between auditory and visual cortices. This mechanism may facilitate perception by time-locking visual computations to environmental events marked by auditory discontinuities.

Cerebral Cortical Activity During Academic Stress Amongst Undergraduate Medical Students at Kampala International University (Uganda)

BACKGROUND

Stress among medical students is related to their academic lifespan; however, information on brain health among medical students from developing countries continues to be scarce. The objective of this study was to establish perceived academic stress levels, assess the ability to cope with stress, and investigate its effects on the visual reaction time (VRT), audio reaction time (ART), and tactile reaction time (TRT) in the somatosensory cortex among medical students of Uganda.

METHODS

This was a cross-sectional study conducted among preclinical (n = 88) and clinical (n = 96) undergraduate medical students at Kampala International University Western Campus. A standard Perceived Stress Scale (PSS) was used to categorize stress into low, moderate, and severe while the ability to cope with stress was categorized into below average, average, above average, and superior stresscoper (SS). Data on reaction time were acquired through VRT, ART, and TRT using the catch-a-ruler experiment, and this was analyzed using SPSS version 20.

RESULTS

This study shows that preclinical students are more stressed than clinical students (PSS prevalence for low stress = preclinical; clinical: 40, 60%). Moderate stress was 48.4 and 51.6% while high perceived stress was 75 and 25% among preclinical and clinical students. Among male and female students in preclinical years, higher TRT and VRT were found in clinical students showing that stress affects the tactile and visual cortical areas in the brain, although the VRT scores were only significantly (P = 0.0123) poor in male students than female students in biomedical sciences. Also, highly stressed individuals had higher TRT and ART and low VRT. SS had high VRT and ART and low TRT in preclinical students, demonstrating the importance of the visual cortex in stress plasticity. Multiple regression showed a close relationship between PSS, ability to cope with stress, age, and educational level (P < 0.05), demonstrating the importance of social and psychological support, especially in the biomedical sciences.

CONCLUSION

Preclinical students suffer more from stress and are poorer SS than clinical students. This strongly impairs their cortical regions in the brain, thus affecting their academic productivity.

Concurrent TMS-fMRI: Technical Challenges, Developments, and Overview of Previous Studies

Transcranial magnetic stimulation (TMS) is a promising treatment modality for psychiatric and neurological disorders. Repetitive TMS (rTMS) is widely used for the treatment of psychiatric and neurological diseases, such as depression, motor stroke, and neuropathic pain. However, the underlying mechanisms of rTMS-mediated neuronal modulation are not fully understood. In this respect, concurrent or simultaneous TMS-fMRI, in which TMS is applied during functional magnetic resonance imaging (fMRI), is a viable tool to gain insights, as it enables an investigation of the immediate effects of TMS. Concurrent application of TMS during neuroimaging usually causes severe artifacts due to magnetic field inhomogeneities induced by TMS. However, by carefully interleaving the TMS pulses with MR signal acquisition in the way that these are far enough apart, we can avoid any image distortions. While the very first feasibility studies date back to the 1990s, recent developments in coil hardware and acquisition techniques have boosted the number of TMS-fMRI applications. As such, a concurrent application requires expertise in both TMS and MRI mechanisms and sequencing, and the hurdle of initial technical set up and maintenance remains high. This review gives a comprehensive overview of concurrent TMS-fMRI techniques by collecting (1) basic information, (2) technical challenges and developments, (3) an overview of findings reported so far using concurrent TMS-fMRI, and (4) current limitations and our suggestions for improvement. By sharing this review, we hope to attract the interest of researchers from various backgrounds and create an educational knowledge base.

Resolving multisensory and attentional influences across cortical depth in sensory cortices

In our environment, our senses are bombarded with a myriad of signals, only a subset of which is relevant for our goals. Using sub-millimeter-resolution fMRI at 7T, we resolved BOLD-response and activation patterns across cortical depth in early sensory cortices to auditory, visual and audiovisual stimuli under auditory or visual attention. In visual cortices, auditory stimulation induced widespread inhibition irrespective of attention, whereas auditory relative to visual attention suppressed mainly central visual field representations. In auditory cortices, visual stimulation suppressed activations, but amplified responses to concurrent auditory stimuli, in a patchy topography. Critically, multisensory interactions in auditory cortices were stronger in deeper laminae, while attentional influences were greatest at the surface. These distinct depth-dependent profiles suggest that multisensory and attentional mechanisms regulate sensory processing via partly distinct circuitries. Our findings are crucial for understanding how the brain regulates information flow across senses to interact with our complex multisensory world.

EPI distortion correction for concurrent human brain stimulation and imaging at 3T

BACKGROUND

Transcranial magnetic stimulation (TMS) can be paired with functional magnetic resonance imaging (fMRI) in concurrent TMS-fMRI experiments. These multimodal experiments enable causal probing of network architecture in the human brain which can complement alternative network mapping approaches. Critically, merely introducing the TMS coil into the scanner environment can sometimes produce substantial magnetic field inhomogeneities and spatial distortions which limit the utility of concurrent TMS-fMRI.

METHOD AND RESULTS

We assessed the efficacy of point spread function corrected echo planar imaging (PSF-EPI) in correcting for the field inhomogeneities associated with a TMS coil at 3 T. In phantom and brain scans, we quantitatively compared the coil-induced distortion artifacts measured in EPI scans with and without PSF correction. We found that the application of PSF corrections to the EPI data significantly improved signal-to-noise and reduced distortions. In phantom scans with the PSF-EPI sequence, we also characterized the temporal profile of dynamic artifacts associated with TMS delivery and found that image quality remained high as long as the TMS pulse preceded the RF excitation pulses by at least 50 ms. Lastly, we validated the PSF-EPI sequence in human brain scans involving TMS and motor behavior as well as resting state fMRI scans.

CONCLUSIONS

Our collective results demonstrate the potential benefits of PSF-EPI for concurrent TMS-fMRI when coil-related artifacts are a concern. The ability to collect high quality resting state fMRI data in the same session as the concurrent TMS-fMRI experiment offers a unique opportunity to interrogate network architecture in the human brain.

Single pulse TMS to the DLPFC, compared to a matched sham control, induces a direct, causal increase in caudate, cingulate, and thalamic BOLD signal

BACKGROUND

In the 20 years since our group established the feasibility of performing interleaved TMS/fMRI, no studies have reported direct comparisons of active prefrontal stimulation with a matched sham. Thus, for all studies there is concern about what is truly the TMS effect on cortical neurons.

OBJECTIVE

After developing a sham control for use within the MRI scanner, we used fMRI to test the hypothesis of greater regional BOLD responses for active versus control stimulation.

METHODS

We delivered 4 runs of interleaved TMS/fMRI with a limited field of view (16 slices, centered at AC-PC) to the left DLPFC (2 active, 2 control; counterbalanced) of 20 healthy individuals (F3; 20 pulses/run, interpulse interval:10-15sec, TR:1sec). In the control condition, 3 cm of foam was placed between the TMS coil and the scalp. This ensured magnetic field decay, but preserved the sensory aspects of each pulse (empirically evaluated in a subset of 10 individuals).

RESULTS

BOLD increases in the cingulate, thalamus, insulae, and middle frontal gyri (p < 0.05, FWE corrected) were found during both active and control stimulation. However, relative to control, active stimulation caused elevated BOLD signal in the anterior cingulate, caudate and thalamus. No significant difference was found in auditory regions.

CONCLUSION(S)

This TMS/fMRI study evaluated a control condition that preserved many of the sensory features of TMS while reducing magnetic field entry. These findings support a relationship between single pulses of TMS and activity in anatomically connected regions, but also underscore the importance of using a sham condition in future TMS/fMRI studies.

Comparing TMS perturbations to occipital and parietal cortices in concurrent TMS-fMRI studies-Methodological considerations

Neglect and hemianopia are two neuropsychological syndromes that are associated with reduced awareness for visual signals in patients’ contralesional hemifield. They offer the unique possibility to dissociate the contributions of retino-geniculate and retino-colliculo circuitries in visual perception. Yet, insights from patient fMRI studies are limited by heterogeneity in lesion location and extent, long-term functional reorganization and behavioural compensation after stroke. Transcranial magnetic stimulation (TMS) has therefore been proposed as a complementary method to investigate the effect of transient perturbations on functional brain organization. This concurrent TMS-fMRI study applied TMS perturbation to occipital and parietal cortices with the aim to ‘mimick’ neglect and hemianopia. Based on the challenges and interpretational limitations of our own study we aim to provide tutorial guidance on how future studies should compare TMS to primary sensory and association areas that are governed by distinct computational principles, neural dynamics and functional architecture.

Improved SNR for combined TMS-fMRI: A support device for commercially available body array coil

BACKGROUND

Transcranial magnetic stimulation (TMS) is a noninvasive brain stimulation tool extensively used in clinical and cognitive neuroscience research. TMS has been applied during functional magnetic resonance imaging (i.e., concurrent/interleaved TMS-fMRI) to understand neural mechanisms underlying cognitive functions. However, no advanced commercial multi-channel whole-brain array MR coils can fit the large TMS coil. We developed a low-cost and easy-to-configure setup that takes advantage of the superior signal-to-noise ratio (SNR) performance of commercially available flexible body array coils that can accommodate the TMS coil.

NEW METHOD

Two flexible MRI body array coils (i.e., the Combo coil) were fitted on a simple coil support with a TMS-coil holder. Phantom and in vivo images acquired using the Combo coil with and without a TMS coil were compared with those from a product 12-channel (12CH) form-fit head array coil.

RESULTS

Relative to the 12CH head coil, images acquired using the Combo coil were of similar quality, but with increased noise levels, leading to moderately reduced temporal SNR values.

COMPARISON WITH EXISTING METHOD

A previous study reported that the temporal SNR of a product 12CH head coil was twice that of a transmit/receive volume birdcage coil commonly used in combined TMS-fMRI. Together with the results of the present work, they indicate that the Combo-coil setup improves SNR performance for combined TMS-fMRI acquisition.

CONCLUSION

The inexpensive and easy-to-configure Combo-coil setup offers an effective and likely superior alternative to transmit/receive birdcage coil for combined TMS-fMRI.

Stimulation-Based Control of Dynamic Brain Networks

The ability to modulate brain states using targeted stimulation is increasingly being employed to treat neurological disorders and to enhance human performance. Despite the growing interest in brain stimulation as a form of neuromodulation, much remains unknown about the network-level impact of these focal perturbations. To study the system wide impact of regional stimulation, we employ a data-driven computational model of nonlinear brain dynamics to systematically explore the effects of targeted stimulation. Validating predictions from network control theory, we uncover the relationship between regional controllability and the focal versus global impact of stimulation, and we relate these findings to differences in the underlying network architecture. Finally, by mapping brain regions to cognitive systems, we observe that the default mode system imparts large global change despite being highly constrained by structural connectivity. This work forms an important step towards the development of personalized stimulation protocols for medical treatment or performance enhancement.

Brain stimulation is increasingly used in clinical settings to treat neurological disorders, but much remains unknown about how stimulation to a single brain region impacts large-scale, brain network activity. Using structural neuroimaging scans, we create computational models of brain dynamics for eight participants to explore how structure-function relationships constrain the effect of stimulation to a single region on the brain as a whole. Our results show that network control theory can be used to predict if the effects of stimulation remain focal or spread globally, and structural connectivity differentially constrains the effects of regional stimulation. Additionally, we study how stimulation of different cognitive systems spreads throughout the brain and find that stimulation of regions within the default mode network provide a mechanism to impart large change in overall brain dynamics through a densely connected structural network. By revealing how the stimulation of different brain regions and cognitive systems spreads differently through the brain, we provide a modeling framework to develop stimulation protocols to personalize medical treatments, enable performance enhancements, and facilitate cortical plasticity.

The role of the ventro-lateral prefrontal cortex in idiom comprehension: An rTMS study

Previous research is equivocal with respect to the neural substrates of idiom processing. Particularly elusive is the role of the left ventro-lateral prefrontal cortex (VLPFC), a region implicated in semantic control generally. Although fMRI studies have shown that the left VLPFC is active during idiom processing (see Rapp et al. (2012), for review), rTMS studies have failed to corroborate a clear role of this prefrontal region (e.g., Oliveri et al., 2004). We investigated this issue using a semantic meaningfulness judgment task that compared idiom comprehension following rTMS-stimulation to the left VLPFC relative to a control site (vertex). We also investigated whether individual differences in general cognitive capacity among comprehenders modulated the effects of rTMS. The results indicate that left VLPFC stimulation particularly affected the processing of low-familiar idioms, possibly because these items involve a maximal semantic conflict between a salient literal and less-known figurative meaning. Of note, this pattern only emerged for comprehenders with higher cognitive control capacity, possibly because they were more likely to activate or maintain multiple semantic representations during idiom processing, which required VLPFC integrity. Taken together, the results support the importance of the left VLPFC to idiom processing.

Potential applications of concurrent transcranial magnetic stimulation and functional magnetic resonance imaging in acquired brain injury and disorders of consciousness

BACKGROUND

Diagnostic assessment, prognosis and treatment monitoring in patients with disorders of consciousness (DoC) rest largely on behaviorally-based procedures. This approach can lead to misdiagnosis, inaccurate outcome prediction and inappropriate judgements regarding the effectiveness of treatment interventions. Concurrent transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) may provide a biological measure of conscious awareness, aid clinicians in clinical decision-making and provide a treatment alternative for DoC.

STUDY

This paper reviews the use of TMS and fMRI in the assessment of patients with DoC and suggests potential applications for concurrent use of these procedures.

Modelling of the Electric Field Distribution in Deep Transcranial Magnetic Stimulation in the Adolescence, in the Adulthood, and in the Old Age

In the last few years, deep transcranial magnetic stimulation (dTMS) has been used for the treatment of depressive disorders, which affect a broad category of people, from adolescents to aging people. To facilitate its clinical application, particular shapes of coils, including the so-called Hesed coils, were designed. Given their increasing demand and the lack of studies which accurately characterize their use, this paper aims to provide a picture of the distribution of the induced electric field in four realistic human models of different ages and gender. In detail, the electric field distributions were calculated by using numerical techniques in the brain structures potentially involved in the progression of the disease and were quantified in terms of both amplitude levels and focusing power of the distribution. The results highlight how the chosen Hesed coil (H7 coil) is able to induce the maxima levels of E mainly in the prefrontal cortex, particularly for the younger model. Moreover, growing levels of induced electric fields with age were found by going in deep in the brain, as well as a major capability to penetrate in the deepest brain structures with an electric field higher than 50%, 70%, and 90% of the peak found in the cortex.

Concurrent TMS-fMRI Reveals Interactions between Dorsal and Ventral Attentional Systems

Adaptive behavior relies on combining bottom-up sensory inputs with top-down control signals to guide responses in line with current goals and task demands. Over the past decade, accumulating evidence has suggested that the dorsal and ventral frontoparietal attentional systems are recruited interactively in this process. This fMRI study used concurrent transcranial magnetic stimulation (TMS) as a causal perturbation approach to investigate the interactions between dorsal and ventral attentional systems and sensory processing areas. In a sustained spatial attention paradigm, human participants detected weak visual targets that were presented in the lower-left visual field on 50% of the trials. Further, we manipulated the presence/absence of task-irrelevant auditory signals. Critically, on each trial we applied 10 Hz bursts of four TMS (or Sham) pulses to the intraparietal sulcus (IPS). IPS-TMS relative to Sham-TMS increased activation in the parietal cortex regardless of sensory stimulation, confirming the neural effectiveness of TMS stimulation. Visual targets increased activations in the anterior insula, a component of the ventral attentional system responsible for salience detection. Conversely, they decreased activations in the ventral visual areas. Importantly, IPS-TMS abolished target-evoked activation increases in the right temporoparietal junction (TPJ) of the ventral attentional system, whereas it eliminated target-evoked activation decreases in the right fusiform. Our results demonstrate that IPS-TMS exerts profound directional causal influences not only on visual areas but also on the TPJ as a critical component of the ventral attentional system. They reveal a complex interplay between dorsal and ventral attentional systems during target detection under sustained spatial attention.

SIGNIFICANCE STATEMENT

Adaptive behavior relies on combining bottom-up sensory inputs with top-down attentional control. Although the dorsal and ventral frontoparietal systems are key players in attentional control, their distinct contributions remain unclear. In this TMS-fMRI study, participants attended to the left visual field to detect weak visual targets presented on half of the trials. We applied brief TMS bursts (or Sham-TMS) to the dorsal intraparietal sulcus (IPS) 100 ms after visual stimulus onset. IPS-TMS abolished the visual induced response suppression in the ventral occipitotemporal cortex and the response enhancement to visual targets in the temporoparietal junction. Our results demonstrate that IPS causally influences neural activity in the ventral attentional system 100 ms poststimulus. They have important implications for our understanding of the neural mechanisms underlying attentional control.

A novel coil array for combined TMS/fMRI experiments at 3 T

Purpose

To overcome current limitations in combined transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) studies by employing a dedicated coil array design for 3 Tesla.

Methods

The state‐of‐the‐art setup for concurrent TMS/fMRI is to use a large birdcage head coil, with the TMS between the subject's head and the MR coil. This setup has drawbacks in sensitivity, positioning, and available imaging techniques. In this study, an ultraslim 7‐channel receive‐only coil array for 3 T, which can be placed between the subject's head and the TMS, is presented. Interactions between the devices are investigated and the performance of the new setup is evaluated in comparison to the state‐of‐the‐art setup.

Results

MR sensitivity obtained at the depth of the TMS stimulation is increased by a factor of five. Parallel imaging with an acceleration factor of two is feasible with low g‐factors. Possible interactions between TMS and the novel hardware were investigated and were found negligible.

Conclusion

The novel coil array is safe, strongly improves signal‐to‐noise ratio in concurrent TMS/fMRI experiments, enables parallel imaging, and allows for flexible positioning of the TMS on the head while ensuring efficient TMS stimulation due to its ultraslim design. Magn Reson Med 74:1492–1501, 2015. © 2014 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.

Electrocorticography Reveals Enhanced Visual Cortex Responses to Visual Speech

Human speech contains both auditory and visual components, processed by their respective sensory cortices. We test a simple model in which task-relevant speech information is enhanced during cortical processing. Visual speech is most important when the auditory component is uninformative. Therefore, the model predicts that visual cortex responses should be enhanced to visual-only (V) speech compared with audiovisual (AV) speech. We recorded neuronal activity as patients perceived auditory-only (A), V, and AV speech. Visual cortex showed strong increases in high-gamma band power and strong decreases in alpha-band power to V and AV speech. Consistent with the model prediction, gamma-band increases and alpha-band decreases were stronger for V speech. The model predicts that the uninformative nature of the auditory component (not simply its absence) is the critical factor, a prediction we tested in a second experiment in which visual speech was paired with auditory white noise. As predicted, visual speech with auditory noise showed enhanced visual cortex responses relative to AV speech. An examination of the anatomical locus of the effects showed that all visual areas, including primary visual cortex, showed enhanced responses. Visual cortex responses to speech are enhanced under circumstances when visual information is most important for comprehension.

Combined neurostimulation and neuroimaging in cognitive neuroscience: past, present, and future

Modern neurostimulation approaches in humans provide controlled inputs into the operations of cortical regions, with highly specific behavioral consequences. This enables causal structure–function inferences, and in combination with neuroimaging, has provided novel insights into the basic mechanisms of action of neurostimulation on distributed networks. For example, more recent work has established the capacity of transcranial magnetic stimulation (TMS) to probe causal interregional influences, and their interaction with cognitive state changes. Combinations of neurostimulation and neuroimaging now face the challenge of integrating the known physiological effects of neurostimulation with theoretical and biological models of cognition, for example, when theoretical stalemates between opposing cognitive theories need to be resolved. This will be driven by novel developments, including biologically informed computational network analyses for predicting the impact of neurostimulation on brain networks, as well as novel neuroimaging and neurostimulation techniques. Such future developments may offer an expanded set of tools with which to investigate structure–function relationships, and to formulate and reconceptualize testable hypotheses about complex neural network interactions and their causal roles in cognition.

Auditory-driven phase reset in visual cortex: human electrocorticography reveals mechanisms of early multisensory integration

Findings in animal models demonstrate that activity within hierarchically early sensory cortical regions can be modulated by cross-sensory inputs through resetting of the phase of ongoing intrinsic neural oscillations. Here, subdural recordings evaluated whether phase resetting by auditory inputs would impact multisensory integration processes in human visual cortex. Results clearly showed auditory-driven phase reset in visual cortices and, in some cases, frank auditory event-related potentials (ERP) were also observed over these regions. Further, when audiovisual bisensory stimuli were presented, this led to robust multisensory integration effects which were observed in both the ERP and in measures of phase concentration. These results extend findings from animal models to human visual cortices, and highlight the impact of cross-sensory phase resetting by a non-primary stimulus on multisensory integration in ostensibly unisensory cortices.

Efficient and robust identification of cortical targets in concurrent TMS-fMRI experiments

Transcranial magnetic stimulation (TMS) can be delivered during fMRI scans to evoke BOLD responses in distributed brain networks. While concurrent TMS-fMRI offers a potentially powerful tool for non-invasively investigating functional human neuroanatomy, the technique is currently limited by the lack of methods to rapidly and precisely localize targeted brain regions - a reliable procedure is necessary for validly relating stimulation targets to BOLD activation patterns, especially for cortical targets outside of motor and visual regions. Here we describe a convenient and practical method for visualizing coil position (in the scanner) and identifying the cortical location of TMS targets without requiring any calibration or any particular coil-mounting device. We quantified the precision and reliability of the target position estimates by testing the marker processing procedure on data from 9 scan sessions: Rigorous testing of the localization procedure revealed minimal variability in coil and target position estimates. We validated the marker processing procedure in concurrent TMS-fMRI experiments characterizing motor network connectivity. Together, these results indicate that our efficient method accurately and reliably identifies TMS targets in the MR scanner, which can be useful during scan sessions for optimizing coil placement and also for post-scan outlier identification. Notably, this method can be used generally to identify the position and orientation of MR-compatible hardware placed near the head in the MR scanner.

Virtual lesion of angular gyrus disrupts the relationship between visuoproprioceptive weighting and realignment

Posterior parietal cortex is thought to be involved in multisensory processes such as sensory weighting (how much different modalities are represented in sensory integration) and realignment (recalibrating the estimates given by unisensory inputs relative to each other, e.g., when viewing the hand through prisms). Sensory weighting and realignment are biologically independent but can be correlated such that the lowest-weighted modality realigns most. This is important for movement precision because it results in the brain's estimate of hand position favoring the more reliable (higher-weighted) modality. It is unknown if this interaction is an emergent property of separate neural pathways for weighting and realignment or if it is actively mediated by a common substrate. We applied disruptive TMS to the angular gyrus near the intraparietal sulcus (PGa) before participants performed a task with misaligned visual and proprioceptive information about hand position. Visuoproprioceptive weighting and realignment were unaffected. However, the relationship between weighting and realignment, found in control conditions, was absent after TMS in the angular gyrus location. This suggests that a specific region in the angular gyrus actively mediates the interaction between visuoproprioceptive weighting and realignment and may thus play a role in the decreased movement precision associated with posterior parietal lesions.