Transcranial magnetic stimulation of the human frontal eye field facilitates visual awareness

Same, Same but Different? A Multi-Method Review of the Processes Underlying Executive Control

Attention, working memory, and executive control are commonly considered distinct cognitive functions with important reciprocal interactions. Yet, longstanding evidence from lesion studies has demonstrated both overlap and dissociation in their behavioural expression and anatomical underpinnings, suggesting that a lower dimensional framework could be employed to further identify processes supporting goal-directed behaviour. Here, we describe the anatomical and functional correspondence between attention, working memory, and executive control by providing an overview of cognitive models, as well as recent data from lesion studies, invasive and non-invasive multimodal neuroimaging and brain stimulation. We emphasize the benefits of considering converging evidence from multiple methodologies centred on the identification of brain mechanisms supporting goal-driven behaviour. We propose that expanding on this approach should enable the construction of a comprehensive anatomo-functional framework with testable new hypotheses, and aid clinical neuroscience to intervene on impairments of executive functions.

Transcranial magnetic stimulation on the right dorsal attention network modulates the center-surround profile of the attentional focus

Psychophysical observations indicate that the spatial profile of visuospatial attention includes a central enhancement around the attentional focus, encircled by a narrow zone of reduced excitability in the immediate surround. This inhibitory ring optimally amplifies relevant target information, likely stemming from top-down frontoparietal recurrent activity modulating early visual cortex activations. However, the mechanisms through which neural suppression gives rise to the surrounding attenuation and any potential hemispheric specialization remain unclear. We used transcranial magnetic stimulation to evaluate the role of two regions of the dorsal attention network in the center-surround profile: the frontal eye field and the intraparietal sulcus. Participants performed a psychophysical task that mapped the entire spatial attentional profile, while transcranial magnetic stimulation was delivered either to intraparietal sulcus or frontal eye field on the right (Experiment 1) and left (Experiment 2) hemisphere. Results showed that stimulation of right frontal eye field and right intraparietal sulcus significantly changed the center-surround profile, by widening the inhibitory ring around the attentional focus. The stimulation on the left frontal eye field, but not left intraparietal sulcus, induced a general decrease in performance but did not alter the center-surround profile. Results point to a pivotal role of the right dorsal attention network in orchestrating inhibitory spatial mechanisms required to limit interference by surrounding distractors.

Effects of contextualized emotional conflict control on domain-general conflict control: fMRI evidence of neural network reconfiguration

Domain-general conflict control refers to the cognitive process in which individuals suppress task-irrelevant information and extract task-relevant information. It supports both effective implementation of cognitive conflict control and emotional conflict control. The present study employed functional magnetic resonance imaging and adopted an emotional valence conflict task and the arrow version of the flanker task to induce contextualized emotional conflicts and cognitive conflicts, respectively. The results from the conjunction analysis showed that the multitasking-related activity in the pre-supplementary motor area, bilateral dorsal premotor cortices, the left posterior intraparietal sulcus (IPS), the left anterior IPS and the right inferior occipital gyrus represents common subprocesses for emotional and cognitive conflict control, either in parallel or in close succession. These brain regions were used as nodes in the domain-general conflict control network. The results from the analyses on the brain network connectivity patterns revealed that emotional conflict control reconfigures the domain-general conflict control network in a connective way as evidenced by different communication and stronger connectivity among the domain-general conflict control network. Together, these findings offer the first empirical-based elaboration on the brain network underpinning emotional conflict control and how it reconfigures the domain-general conflict control network in interactive ways.

Functional Connectivity Alterations of Cognitive Flexibility in Aging: Different Patterns of Global and Local Switch Costs

OBJECTIVES

Cognitive flexibility declines with aging and is usually indicated by task switch costs including global and local switch costs. Cognitive flexibility in aging is associated with alterations in functional connectivity. However, whether different task-modulated connectivity mechanisms underlying global and local switch costs remain unclear.

METHODS

Here we use the support vector machine to identify age-related functional connectivity in global and local switch costs between older (n = 32) and young adults (n = 33). Participants completed a cued task-switching task during the functional magnetic resonance imaging scan.

RESULTS

Results show an age-related decline behaviorally in global but not in local switch costs. Moreover, distinct patterns of age-related alterations of connectivity were observed for each cost. Specifically, only multivariate changes in connectivity patterns were observed for local switch cost, whereas specific age-related connections were revealed for global switch cost. In older adults, the task-modulated left dorsal premotor cortex-left precuneus connectivity decreased, and the left inferior frontal junction-left inferior parietal sulcus connectivity correlated with decreased global switch cost.

DISCUSSION

This study provides novel evidence for different neural patterns in global and local switch costs by illuminating connectivity mechanisms underlying cognitive flexibility in aging.

Functional characterization of correct and incorrect feature integration

Our sensory system constantly receives information from the environment and our own body. Despite our impression to the contrary, we remain largely unaware of this information and often cannot report it correctly. Although perceptual processing does not require conscious effort on the part of the observer, it is often complex, giving rise to errors such as incorrect integration of features (illusory conjunctions). In the present study, we use functional magnetic resonance imaging to study the neural bases of feature integration in a dual task that produced ~30% illusions. A distributed set of regions demonstrated increased activity for correct compared to incorrect (illusory) feature integration, with increased functional coupling between occipital and parietal regions. In contrast, incorrect feature integration (illusions) was associated with increased occipital (V1-V2) responses at early stages, reduced functional connectivity between right occipital regions and the frontal eye field at later stages, and an overall decrease in coactivation between occipital and parietal regions. These results underscore the role of parietal regions in feature integration and highlight the relevance of functional occipito-frontal interactions in perceptual processing.

Effects of Transcranial Direct Current Stimulation Over the Right Dorsolateral Prefrontal Cortex on Fairness-Related Decision-Making

Neuroimaging studies suggest that the right dorsolateral prefrontal cortex (rDLPFC) is an important brain area involved in fairness-related decision-making. In the present study, we used transcranial direct current stimulation (tDCS) over the rDLPFC to investigate the effects of changed cortical excitability on fairness norm enforcement in social decision-making. Participants received anodal, cathodal, or sham stimulation before performing a modified ultimatum game (UG) task, in which participants were asked to accept or reject the proposer's offer and self-rate the intensity of their anger at offers on a 7-point scale. The results showed that the rejection rate of unfair offers and anger level were higher in the anodal compared to the sham and cathodal groups, and that the level of anger at unfair offers can predict the rejection rate. Furthermore, the fairness effect of RTs was more prominent in the anodal group than in the sham and cathodal groups. Our findings validate the causal role of the rDLPFC in fairness-related decision-making through tDCS, suggesting that strengthening the rDLPFC increases individuals' reciprocal fairness in social decision-making, both in subjective rating and behaviors.

Functional Segregation within the Dorsal Frontoparietal Network: A Multimodal Dynamic Causal Modeling Study

Discrimination and integration of motion direction requires the interplay of multiple brain areas. Theoretical accounts of perception suggest that stimulus-related (i.e., exogenous) and decision-related (i.e., endogenous) factors affect distributed neuronal processing at different levels of the visual hierarchy. To test these predictions, we measured brain activity of healthy participants during a motion discrimination task, using electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). We independently modeled the impact of exogenous factors (task demand) and endogenous factors (perceptual decision-making) on the activity of the motion discrimination network and applied Dynamic Causal Modeling (DCM) to both modalities. DCM for event-related potentials (DCM-ERP) revealed that task demand impacted the reciprocal connections between the primary visual cortex (V1) and medial temporal areas (V5). With practice, higher visual areas were increasingly involved, as revealed by DCM-fMRI. Perceptual decision-making modulated higher levels (e.g., V5-to-Frontal Eye Fields, FEF), in a manner predictive of performance. Our data suggest that lower levels of the visual network support early, feature-based selection of responses, especially when learning strategies have not been implemented. In contrast, perceptual decision-making operates at higher levels of the visual hierarchy by integrating sensory information with the internal state of the subject.

Human Premotor Corticospinal Projections Are Engaged in Motor Preparation at Discrete Time Intervals: A TMS-Induced Virtual Lesion Study

Objectives: The significance of pre-motor (PMC) corticospinal projections in a frontoparietal motor network remains elusive. Temporal activation patterns can provide valuable information about a region's engagement in a hierarchical network. Navigated transcranial magnetic stimulation (nTMS)-induced virtual lesions provide an excellent method to study cortical physiology by disrupting ongoing activity at high temporal resolution and anatomical precision. We use nTMS-induced virtual lesions applied during an established behavioral task demanding pre-motor activation to clarify the temporal activation pattern of pre-motor corticospinal projections. Materials and Methods: Ten healthy volunteers participated in the experiment (4 female, mean age 24 ± 2 years, 1 left-handed). NTMS was used to map Brodmann areae 4 and 6 for primary motor (M1) and PMC corticospinal projections. We then determined the stimulator output intensity required to elicit a 1 mV motor evoked potential (1 mV-MT) through M1 nTMS. TMS pulse were randomly delivered at distinct time intervals (40, 60, 80, 100, 120, and 140 ms) at 1 mV-MT intensity to M1, PMC and the DLPFC (dorsolateral pre-frontal cortex; control condition) before participants had to perform major changes of their trajectory of movement during a tracing task. Each participant performed six trials (20 runs per trial). Task performance and contribution of regions under investigation was quantified through calculating the tracing error induced by the stimulation. Results: A pre-motor stimulation hotspot could be identified in all participants (16.3 ± 1.7 mm medial, 18.6 ± 1.4 mm anterior to the M1 hotspot). NTMS over studied regions significantly affected task performance at discrete time intervals (F(10, 80) = 3.25, p = 0.001). NTMS applied over PMC 120 and 140 ms before changes in movement trajectory impaired task performance significantly more than when applied over M1 (p = 0.021 and p = 0.003) or DLPFC (p = 0.017 and p < 0.001). Stimulation intensity did not account for error size (β = -0.0074, p = 1). Conclusions: We provide novel evidence that the role of pre-motor corticospinal projections extends beyond that of simple corticospinal motor output. Their activation is crucial for task performance early in the stage of motor preparation suggesting a significant role in shaping voluntary movement. Temporal patterns of human pre-motor activation are similar to that observed in intracortical electrophysiological studies in primates.

Top-down control of visual cortex by the frontal eye fields through oscillatory realignment

Voluntary allocation of visual attention is controlled by top-down signals generated within the Frontal Eye Fields (FEFs) that can change the excitability of lower-level visual areas. However, the mechanism through which this control is achieved remains elusive. Here, we emulated the generation of an attentional signal using single-pulse transcranial magnetic stimulation to activate the FEFs and tracked its consequences over the visual cortex. First, we documented changes to brain oscillations using electroencephalography and found evidence for a phase reset over occipital sites at beta frequency. We then probed for perceptual consequences of this top-down triggered phase reset and assessed its anatomical specificity. We show that FEF activation leads to cyclic modulation of visual perception and extrastriate but not primary visual cortex excitability, again at beta frequency. We conclude that top-down signals originating in FEF causally shape visual cortex activity and perception through mechanisms of oscillatory realignment.

Causal contributions of human frontal eye fields to distinct aspects of decision formation

Several theories propose that perceptual decision making depends on the gradual accumulation of information that provides evidence in favour of one of the choice-options. The outcome of this temporally extended integration process is thought to be categorized into the 'winning' and 'losing' choice-options for action. Neural correlates of corresponding decision formation processes have been observed in various frontal and parietal brain areas, among them the frontal eye-fields (FEF). However, the specific functional role of the FEFs is debated. Recent studies in humans and rodents provide conflicting accounts, proposing that the FEF either accumulate the choice-relevant information or categorize the outcome of such evidence integration into discrete actions. Here, we used transcranial magnetic stimulation (TMS) on humans to interfere with either left or right FEF activity during different timepoints of perceptual decision-formation. Stimulation of either FEF affected performance only when delivered during information integration but not during subsequent categorical choice. However, the patterns of behavioural changes suggest that the left-FEF contributes to general evidence integration, whereas right-FEF may direct spatial attention to the contralateral hemifield. Taken together, our results indicate an FEF involvement in evidence accumulation but not categorization, and suggest hemispheric lateralization for this function in the human brain.

Using Transcranial Magnetic Stimulation to Test a Network Model of Perceptual Decision Making in the Human Brain

Previous research has suggested that the lateral occipital cortex (LOC) is involved with visual decision making, and specifically with the accumulation of information leading to a decision. In humans, this research has been primarily based on imaging and electroencephalography (EEG), and as such only correlational. One line of such research has led to a model of three spatially distributed brain networks that activate in temporal sequence to enable visual decision-making. The model predicted that disturbing neural processing in the LOC at a specific latency would slow object decision-making, increasing reaction time (RT) in a difficult discrimination task. We utilized transcranial magnetic stimulation (TMS) to test this prediction, perturbing LOC beginning at 400 ms post-stimulus onset, a time in the model corresponding to LOC activation at a particular difficulty level, with the expectation of increased RT. Thirteen healthy adults participated in two TMS sessions in which left and right LOC were stimulated separately utilizing neuronavigation and robotic coil guidance. Participants performed a two-alternative forced-choice task selecting whether a car or face was present on each trial amidst visual noise pre-tested to approximate a 75% accuracy level. In an effort to disrupt processing, pairs of TMS pulses separated by 50 ms were presented at one of five stimulus onset asynchronies (SOAs): -200, 200, 400, 450, or 500 ms. Behavioral performance differed systematically across SOAs for RT and accuracy measures. As predicted, TMS at 400 ms resulted in a significant slowing of RT. TMS delivered at -200 ms resulted in faster RT, indicating early stimulation may result in priming and performance enhancement. Use of TMS thus causally demonstrated the involvement of LOC in this task, and more broadly with perceptual decision-making; additionally, it demonstrated the role of TMS in testing well-developed neural models of perceptual processing.

Right frontal eye field has perceptual and oculomotor functions during optokinetic stimulation and nystagmus

The right frontal eye field (rFEF) is associated with visual perception and eye movements. rFEF is activated during optokinetic nystagmus (OKN), a reflex that moves the eye in response to visual motion (optokinetic stimulation, OKS). It remains unclear whether rFEF plays causal perceptual and/or oculomotor roles during OKS and OKN. To test this, participants viewed a leftward-moving visual scene of vertical bars and judged whether a flashed dot was moving. Single pulses of transcranial magnetic stimulation (TMS) were applied to rFEF on half of trials. In half of blocks, to explore oculomotor control, participants performed an OKN in response to the OKS. rFEF TMS, during OKN, made participants more accurate on trials when the dot was still, and it slowed eye movements. In separate blocks, participants fixated during OKS. This not only controlled for eye movements but also allowed the use of EEG to explore the FEF's role in visual motion discrimination. In these blocks, by contrast, leftward dot motion discrimination was impaired, associated with a disruption of the frontal-posterior balance in alpha-band oscillations. None of these effects occurred in a control site (M1) experiment. These results demonstrate multiple related yet dissociable causal roles of the right FEF during optokinetic stimulation.NEW & NOTEWORTHY This study demonstrates causal roles of the right frontal eye field (FEF) in motion discrimination and eye movement control during visual scene motion: previous work had only examined other stimuli and eye movements such as saccades. Using combined transcranial magnetic stimulation and EEG and a novel optokinetic stimulation motion-discrimination task, we find evidence for multiple related yet dissociable causal roles within the FEF: perceptual processing during optokinetic stimulation, generation of the optokinetic nystagmus, and the maintenance of alpha oscillations.

Entrainment of local synchrony reveals a causal role for high-beta right frontal oscillations in human visual consciousness

Prior evidence supports a critical role of oscillatory activity in visual cognition, but are cerebral oscillations simply correlated or causally linked to our ability to consciously acknowledge the presence of a target in our visual field? Here, EEG signals were recorded on humans performing a visual detection task, while they received brief patterns of rhythmic or random transcranial magnetic stimulation (TMS) delivered to the right Frontal Eye Field (FEF) prior to the onset of a lateralized target. TMS entrained oscillations, i.e., increased high-beta power and phase alignment (the latter to a higher extent for rhythmic high-beta patterns than random patterns) while also boosting visual detection sensitivity. Considering post-hoc only those participants in which rhythmic stimulation enhanced visual detection, the magnitude of high-beta entrainment correlated with left visual performance increases. Our study provides evidence in favor of a causal link between high-beta oscillatory activity in the Frontal Eye Field and visual detection. Furthermore, it supports future applications of brain stimulation to manipulate local synchrony and improve or restore impaired visual behaviors.

Increased Noise in Cortico-Cortical Integration After Mild TBI Measured With the Equivalent Noise Technique

The bulk of deficits accompanying mild traumatic brain injury (mTBI) is understood in terms of cortical integration—mnemonic, attentional, and cognitive disturbances are believed to involve integrative action across brain regions. Independent of integrative disturbances, mTBI may increase cortical noise, and this has not been previously considered. High-level integrative deficits are exceedingly difficult to measure and model, motivating us to utilize a tightly-controlled task within an established quantitative model to separately estimate internal noise and integration efficiency. First, we utilized a contour integration task modeled as a cortical-integration process involving multiple adjacent cortical columns in early visual areas. Second, we estimated internal noise and integration efficiency using the linear amplifier model (LAM). Fifty-seven mTBI patients and 24 normal controls performed a 4AFC task where they had to identify a valid contour amongst three invalid contours. Thresholds for contour amplitude were measured adaptively across three levels of added external orientation noise. Using the LAM, we found that mTBI increased internal noise without affecting integration efficiency. mTBI also caused hemifield bias differences, and efficiency was related to a change of visual habits. Using a controlled task reflecting cortical integration within the equivalent noise framework empowered us to detect increased computational noise that may be at the heart of mTBI deficits. Our approach is highly sensitive and translatable to rehabilitative efforts for the mTBI population, while also implicating a novel hypothesis of mTBI effects on basic visual processing—namely that cortical integration is maintained at the cost of increased internal noise.

Probing the Neural Mechanisms for Distractor Filtering and Their History-Contingent Modulation by Means of TMS

In visual search, the presence of a salient, yet task-irrelevant, distractor in the stimulus array interferes with target selection and slows down performance. Neuroimaging data point to a key role of the frontoparietal dorsal attention network in dealing with visual distractors; however, the respective roles of different nodes within the network and their hemispheric specialization are still unresolved. Here, we used transcranial magnetic stimulation (TMS) to evaluate the causal role of two key regions of the dorsal attention network in resisting attentional capture by a salient singleton distractor: the frontal eye field (FEF) and the cortex within the intraparietal sulcus (IPS). The task of the participants (male/female human volunteers) was to discriminate the pointing direction of a target arrow while ignoring a task-irrelevant salient distractor. Immediately after stimulus onset, triple-pulse 10 Hz TMS was delivered either to IPS or FEF on either side of the brain. Results indicated that TMS over the right FEF significantly reduced the behavioral cost engendered by the salient distractor relative to left FEF stimulation. No such effect was obtained with stimulation of IPS on either side of brain. Interestingly, this FEF-dependent reduction in distractor interference interacted with the contingent trial history, being maximal when no distractor was present on the previous trial relative to when there was one. Our results provide direct causal evidence that the right FEF houses key mechanisms for distractor filtering, pointing to a pivotal role of the frontal cortex of the right hemisphere in limiting interference from an irrelevant but attention-grabbing stimulus.SIGNIFICANCE STATEMENT Visually conspicuous stimuli attract our attention automatically and interfere with performance by diverting resources away from the main task. Here, we applied transcranial magnetic stimulation over four frontoparietal cortex locations (frontal eye field and intraparietal sulcus in each hemisphere) to identify regions of the dorsal attention network that help limit interference from task-irrelevant, salient distractors. Results indicate that the right FEF participates in distractor-filtering mechanisms that are recruited when a distracting stimulus is encountered. Moreover, right FEF implements adjustments in distraction-filtering mechanisms following recent encounters with distractors. Together, these findings indicate a different hemispheric contribution of the left versus right dorsal frontal cortex to distraction filtering. This study expands our understanding of how our brains select relevant targets in the face of task-irrelevant, salient distractors.

Unraveling Causal Mechanisms of Top-Down and Bottom-Up Visuospatial Attention with Non-invasive Brain Stimulation

Attention is a process of selection that allows us to intelligently navigate the abundance of information in our world. Attention can be either directed voluntarily based on internal goals-"top-down" or goal-directed attention-or captured automatically, by salient stimuli-"bottom-up" or stimulus-driven attention. Do these two modes of attention control arise from same or different brain circuits? Do they share similar or distinct neural mechanisms? In this review, we explore this dichotomy between the neural bases of top-down and bottom-up attention control, with a special emphasis on insights gained from non-invasive neurostimulation techniques, specifically, transcranial magnetic stimulation (TMS). TMS enables spatially focal and temporally precise manipulation of brain activity. We explore a significant literature devoted to investigating the role of fronto-parietal brain regions in top-down and bottom-up attention with TMS, and highlight key areas of convergence and debate. We also discuss recent advances in combinatorial paradigms that combine TMS with other imaging modalities, such as functional magnetic resonance imaging or electroencephalography. These paradigms are beginning to bridge essential gaps in our understanding of the neural pathways by which TMS affects behavior, and will prove invaluable for unraveling mechanisms of attention control, both in health and in disease.

Common and distinct neural correlates of dual-tasking and task-switching: a meta-analytic review and a neuro-cognitive processing model of human multitasking

Although there are well-known limitations of the human cognitive system in performing two tasks simultaneously (dual-tasking) or alternatingly (task-switching), the question for a common vs. distinct neural basis of these multitasking limitations is still open. We performed two Activation Likelihood Estimation meta-analyses of neuroimaging studies on dual-tasking or task-switching and tested for commonalities and differences in the brain regions associated with either domain. We found a common core network related to multitasking comprising bilateral intraparietal sulcus (IPS), left dorsal premotor cortex (dPMC), and right anterior insula. Meta-analytic contrasts revealed eight fronto-parietal clusters more consistently activated in dual-tasking (bilateral frontal operculum, dPMC, and anterior IPS, left inferior frontal sulcus and left inferior frontal gyrus) and, conversely, four clusters (left inferior frontal junction, posterior IPS, and precuneus as well as frontomedial cortex) more consistently activated in task-switching. Together with sub-analyses of preparation effects in task-switching, our results argue against purely passive structural processing limitations in multitasking. Based on these findings and drawing on current theorizing, we present a neuro-cognitive processing model of multitasking.

FEF Excitability in Attentional Bias: A TMS-EEG Study

The role of distinct cortical regions in guiding social orienting needs further investigation. Our aim was to explore the contribution of the frontal eye field (FEF) in early orienting of attention towards stimuli with social value. We used a TMS-EEG approach to investigate event related potentials (ERPs; no-TMS block) and TMS evoked potentials (TEPs; TMS block) during the cueing phase of a modified version of the dot-probe task, comparing competing (face vs. house) and not competing (house vs. house) conditions. Our results revealed an increased amplitude of ERP components in the competing condition, showing greater posterior N170 and fronto-central vertex positive potential (VPP) and an enhanced frontal negative component at 250-270 ms from cue onset. TMS pulses over the FEF induced similar N170 and VPP amplified components. In addition, in the ERPs, a reduced positivity at 400 ms was shown when the face appeared on the left side vs. the right side of space. In contrast, in the TMS blocks, we found lateralized effects on N170 depending on the side of face presentation. The enhanced cortical excitability induced by TMS over the right FEF significantly correlated with the performance on the behavioral task, suggesting a link between the FEF activity during the cueing phase of the dot-probe task and the subsequent behavioral response times to the targets.

Transcranial magnetic stimulation for diplopia in a patient with spinocerebellar ataxia type 6: a case report

Background

In Patients with spinocerebellar ataxia type 6 (SCA6) are often treated by transcranial magnetic stimulation (TMS) over the motor cortex and cerebellum. However, few reports have examined effective therapeutic modalities for diplopia in SCA6 patients. In the current case, we applied single-pulse TMS over the motor cortex and cerebellum to improve ataxia, and observed an unexpected improvement of diplopia.

Case presentation

A 62-year-old Japanese male with spinocerebellar ataxia type 6 (SCA6) was admitted to our hospital for exacerbation of ataxia. We administered single-pulse transcranial magnetic stimulation (TMS) over the hand motor area and the cerebellum with a circular coil to reduce ataxia. After the initiation of TMS, since diplopia unexpectedly improved, we started a quantitative assessment of diplopia by counting the number of fixation spots that he observed in his visual field. This assessment suggested that TMS had an immediate and cumulative effect on diplopia. We also delivered more localized stimulation only over the motor cortex with a Figure-8 coil, and diplopia improved immediately. Additionally, we administered a sham stimulation before the real stimulation over the motor cortex and the cerebellum. The sham stimulation improved diplopia, and greater improvement was observed with subsequent real stimulation. We also used a Hess chart examination and video recordings of binocular gross appearance to elucidate the changes in ocular movement objectively. However, these examinations did not reveal any obvious oculomotor changes.

Conclusions

We applied single-pulse TMS to a SCA6 patient with diplopia, which improved without any adverse effects. TMS may have potential for the treatment of diplopia in SCA6 patients.

TMS-EEG reveals hemispheric asymmetries in top-down influences of posterior intraparietal cortex on behavior and visual event-related potentials

Clinical data and behavioral studies using transcranial magnetic stimulation (TMS) suggest right-hemisphere dominance for top-down modulation of visual processing in humans. We used concurrent TMS-EEG to directly test for hemispheric differences in causal influences of the right and left intraparietal cortex on visual event-related potentials (ERPs). We stimulated the left and right posterior part of intraparietal sulcus (IPS1) while the participants were viewing and rating the visibility of bilaterally presented Gabor patches. Subjective visibility ratings showed that TMS of right IPS shifted the visibility toward the right hemifield, while TMS of left IPS did not have any behavioral effect. TMS of right IPS, but not left one, reduced the amplitude of posterior N1 potential, 180-220ms after stimulus-onset. The attenuation of N1 occurred bilaterally over the posterior areas of both hemispheres. Consistent with previous TMS-fMRI studies, this finding suggests that the right IPS has top-down control on the neural processing in visual cortex. As N1 most probably reflects reactivation of early visual areas, the current findings support the view that the posterior parietal cortex in the right hemisphere amplifies recurrent interactions in ventral visual areas during the time-window that is critical for conscious perception.

Effector-based attention systems

Visual processing is known to be enhanced at the end point of eye movements. Feedback within the oculomotor system has been shown to drive these alterations in visual processing. However, we do not simply view the world; we also reach out and interact using our hands. Consequently, it is not surprising that visual processing has also been shown to be altered in near-hand space. A growing body of work documents a myriad of alterations in near-hand visual processing, with little consensus on the neural underpinnings of the effect of the hand. Since movement of the eyes and hands is governed by parallel frontoparietal networks and since within the oculomotor system feedback from these motor control regions has been shown to drive enhanced visual processing at saccade end points, it is plausible that a similar feedback mechanism is at play in near-hand improvements in visual processing. Here, we compare and contrast oculomotor-driven and hand-driven changes in visual processing and provide support for the hypothesis that feedback within the reaching and grasping systems enhances visual processing near the hand in a novel way.

FEF-Controlled Alpha Delay Activity Precedes Stimulus-Induced Gamma-Band Activity in Visual Cortex

Recent findings in the visual system of nonhuman primates have demonstrated an important role of gamma-band activity (40-100 Hz) in the feedforward flow of sensory information, whereas feedback control appears to be established dynamically by oscillations in the alpha (8-13 Hz) and beta (13-18 Hz) bands (van Kerkoerle et al., 2014; Bastos et al., 2015). It is not clear, however, how alpha oscillations are controlled and how they interact with the flow of visual information mediated by gamma-band activity. Using noninvasive human MEG recordings in subjects performing a visuospatial attention task, we show that fluctuations in alpha power during a delay period in a spatial attention task preceded subsequent stimulus-driven gamma-band activity. Importantly, these interactions correlated with behavioral performance. Using Granger analysis, we further show that the right frontal-eye field (rFEF) exerted feedback control of the visual alpha oscillations. Our findings suggest that alpha oscillations controlled by the FEF route cortical information flow by modulating gamma-band activity.SIGNIFICANCE STATEMENT Visual perception relies on a feedforward flow of information from sensory regions, which is modulated by a feedback drive. We have identified the neuronal dynamics supporting integration of the feedforward and feedback information. Alpha oscillations in early visual regions reflect feedback control when spatial attention is allocated and this control is exercised by the right frontal eye field. Importantly, the alpha-band activity predicted both performance and activity in the gamma band. In particular, gamma activity was modulated by the phase of the alpha oscillations. These findings provide novel insight into how the brain operates as a network and suggest that the integration of feedforward and feedback information is implemented by cross-frequency interactions between slow and fast neuronal oscillations.

Functional connectivity between prefrontal and parietal cortex drives visuo-spatial attention shifts

It is well established that the frontal eye-fields (FEF) in the dorsal attention network (DAN) guide top-down selective attention. In addition, converging evidence implies a causal role for the FEF in attention shifting, which is also known to recruit the ventral attention network (VAN) and fronto-striatal regions. To investigate the causal influence of the FEF as (part of) a central hub between these networks, we applied thetaburst transcranial magnetic stimulation (TBS) off-line, combined with functional magnetic resonance (fMRI) during a cued visuo-spatial attention shifting paradigm.

We found that TBS over the right FEF impaired performance on a visual discrimination task in both hemifields following attention shifts, while only left hemifield performance was affected when participants were cued to maintain the focus of attention. These effects recovered ca. 20 min post stimulation. Furthermore, particularly following attention shifts, TBS suppressed the neural signal in bilateral FEF, right inferior and superior parietal lobule (IPL/SPL) and bilateral supramarginal gyri (SMG). Immediately post stimulation, functional connectivity was impaired between right FEF and right SMG as well as right putamen. Importantly, the extent of decreased connectivity between right FEF and right SMG correlated with behavioural impairment following attention shifts.

The main finding of this study demonstrates that influences from right FEF on SMG in the ventral attention network causally underly attention shifts, presumably by enabling disengagement from the current focus of attention.

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Thetaburst stimulation to the right FEF temporarily impairs bilateral attention shifts.

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Lateralised behavioural deficits in the contralateral hemifield are observed when cued to maintain attention.

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These effects recover ca. 20 min post stimulation.

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During shifts, neural activity is suppressed following right FEF TBS in the dorsal attention network and supramarginal gyri.

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Influences from right FEF to SMG causally underlie attention shifts, presumably by enabling disengagement from current focus.

Effects of inhibitory theta burst TMS to different brain sites involved in visuospatial attention - a combined neuronavigated cTBS and behavioural study

PURPOSE AND METHODS

The study sought to alter visual spatial attention in young healthy subjects by a neuronavigated inhibitory rTMS protocol (cTBS-600) to right brain areas thought to be involved in visual attentional processes, i.e. the temporoparietal junction (TPJ) and the posterior middle frontal gyrus (pMFG), and to test the reversibility of effects by an additional consecutive cTBS to the homologue left brain cortical areas.

RESULTS

Healthy subjects showed a leftward bias of the egocentric perspective for both visual-perceptive and visual-exploratory tasks specifically for items presented in the left hemifield. cTBS to the right TPJ, and less systematically to the right pMFG reduced this bias for visuo-spatial and exploratory visuo-motor behaviour. Further, a consecutive cTBS to the left TPJ changed the bias again towards the left for a visual-perceptive task.

CONCLUSIONS

The evidence supports the notion of an involvement of the right TPJ (and pMFG) in spatial visual attention. The observations further indicate that inhibitory non-invasive brain stimulation (cTBS) to the left TPJ has a potential for reversing a rightward bias of spatial attention when the right TPJ is dysfunctional. Accordingly, the findings could have implications for therapeutic rTMS development for right brain damaged patients with visual neglect.

Exploring the contributions of the supplementary eye field to subliminal inhibition using double-pulse transcranial magnetic stimulation

It is widely accepted that the supplementary eye fields (SEF) are involved in the control of voluntary eye movements. However, recent evidence suggests that SEF may also be important for unconscious and involuntary motor processes. Indeed, Sumner et al. ([2007]: Neuron 54:697-711) showed that patients with micro-lesions of the SEF demonstrated an absence of subliminal inhibition as evoked by masked-prime stimuli. Here, we used double-pulse transcranial magnetic stimulation (TMS) in healthy volunteers to investigate the role of SEF in subliminal priming. We applied double-pulse TMS at two time windows in a masked-prime task: the first during an early phase, 20-70 ms after the onset of the mask but before target presentation, during which subliminal inhibition is present; and the second during a late phase, 20-70 ms after target onset, during which the saccade is being prepared. We found no effect of TMS with the early time window of stimulation, whereas a reduction in the benefit of an incompatible subliminal prime stimulus was found when SEF TMS was applied at the late time window. These findings suggest that there is a role for SEF related to the effects of subliminal primes on eye movements, but the results do not support a role in inhibiting the primed tendency. Hum Brain Mapp 38:339-351, 2017. © 2016 Wiley Periodicals, Inc.

An Eye in the Palm of Your Hand: Alterations in Visual Processing Near the Hand, a Mini-Review

Feedback within the oculomotor system improves visual processing at eye movement end points, also termed a visual grasp. We do not just view the world around us however, we also reach out and grab things with our hands. A growing body of literature suggests that visual processing in near-hand space is altered. The control systems for moving either the eyes or the hands rely on parallel networks of fronto-parietal regions, which have feedback connections to visual areas. Since the oculomotor system effects on visual processing occur through feedback, both through the motor plan and the motor efference copy, a parallel system where reaching and/or grasping motor-related activity also affects visual processing is likely. Areas in the posterior parietal cortex, for example, receive proprioceptive and visual information used to guide actions, as well as motor efference signals. This trio of information channels is all that would be necessary to produce spatial allocation of reach-related visual attention. We review evidence from behavioral and neurophysiological studies that support the hypothesis that feedback from the reaching and/or grasping motor control networks affects visual processing while noting ways in which it differs from that seen within the oculomotor system. We also suggest that object affordances may represent the neural mechanism through which certain object features are selected for preferential processing when stimuli are near the hand. Finally, we summarize the two effector-based feedback systems and discuss how having separate but parallel effector systems allows for efficient decoupling of eye and hand movements.

Dissociable Roles of Dorsolateral Prefrontal Cortex and Frontal Eye Fields During Saccadic Eye Movements

The dorsolateral prefrontal cortex (DLPFC) and the frontal eye fields (FEF) have both been implicated in the executive control of saccades, yet possible dissociable roles of each region have not been established. Specifically, both establishing a “task set” as well as suppressing an inappropriate response have been linked to DLPFC and FEF activity, with behavioral outcome measures of these mechanisms mainly being the percentage of pro-saccade errors made on anti-saccade trials. We used continuous theta-burst stimulation (cTBS) to disrupt FEF or DLPFC function in humans during an anti-saccade task to assess the causal role of these regions in these executive control processes, and in programming saccades towards (pro-saccade) or away (anti-saccade) from visual targets. After right FEF cTBS, as compared to control cTBS to the right primary somatosensory cortex (rS1), anti-saccade amplitude of the first saccade decreased and the number of anti-saccades to acquire final position increased; however direction errors to the visual target were not different. In contrast, after left DLPFC cTBS, as compared to left S1 cTBS, subjects displayed greater direction errors for contralateral anti-saccades; however, there were no impairments on the number of saccades or the saccade amplitude. These results are consistent with the notion that DLPFC is necessary for executive control of saccades, whereas FEF is necessary for visuo-motor aspects of anti-saccade programming.

Arrhythmic activity in the left frontal eye field facilitates conscious visual perception in humans

The frontal eye field (FEF) is a brain region involved in several processes relevant for visual performance, including visuo-spatial attention, conscious access and decision-making. Prior research has causally demonstrated that high-beta FEF activity in the right hemisphere enhances conscious visual perception, an outcome that is in agreement with evidence of neural synchronization along a right dorsal fronto-parietal network during attentional orienting and a right-hemisphere dominance for visuospatial processing. Nonetheless, frontal regions in the left hemisphere have also been shown to modulate perceptual performance. To causally explore the neural basis of these modulations, we delivered high-beta frequency-specific bursts of transcranial magnetic stimulation (TMS) to the left FEF and report that, in this region, these patterns failed to modulate conscious perception. In contrast, non-frequency-specific TMS patterns yielded visual performance improvements similar to those formerly causally associated to the induction of high-beta activity on its right-hemisphere homotopic area. This noise-induced facilitation of conscious vision suggests a relevant role of the left frontal cortex in visual perception. Furthermore, taken together with prior causal right-FEF evidence, our study indicates that frontal regions of each hemisphere employ different coding strategies to modulate conscious perception.

Visual Contrast Sensitivity Improvement by Right Frontal High-Beta Activity Is Mediated by Contrast Gain Mechanisms and Influenced by Fronto-Parietal White Matter Microstructure

Behavioral and electrophysiological studies in humans and non-human primates have correlated frontal high-beta activity with the orienting of endogenous attention and shown the ability of the latter function to modulate visual performance. We here combined rhythmic transcranial magnetic stimulation (TMS) and diffusion imaging to study the relation between frontal oscillatory activity and visual performance, and we associated these phenomena to a specific set of white matter pathways that in humans subtend attentional processes. High-beta rhythmic activity on the right frontal eye field (FEF) was induced with TMS and its causal effects on a contrast sensitivity function were recorded to explore its ability to improve visual detection performance across different stimulus contrast levels. Our results show that frequency-specific activity patterns engaged in the right FEF have the ability to induce a leftward shift of the psychometric function. This increase in visual performance across different levels of stimulus contrast is likely mediated by a contrast gain mechanism. Interestingly, microstructural measures of white matter connectivity suggest a strong implication of right fronto-parietal connectivity linking the FEF and the intraparietal sulcus in propagating high-beta rhythmic signals across brain networks and subtending top-down frontal influences on visual performance.

Frontal eye fields control attentional modulation of alpha and gamma oscillations in contralateral occipitoparietal cortex

Covertly directing visuospatial attention produces a frequency-specific modulation of neuronal oscillations in occipital and parietal cortices: anticipatory alpha (8-12 Hz) power decreases contralateral and increases ipsilateral to attention, whereas stimulus-induced gamma (>40 Hz) power is boosted contralaterally and attenuated ipsilaterally. These modulations must be under top-down control; however, the control mechanisms are not yet fully understood. Here we investigated the causal contribution of the human frontal eye field (FEF) by combining repetitive transcranial magnetic stimulation (TMS) with subsequent magnetoencephalography. Following inhibitory theta burst stimulation to the left FEF, right FEF, or vertex, participants performed a visual discrimination task requiring covert attention to either visual hemifield. Both left and right FEF TMS caused marked attenuation of alpha modulation in the occipitoparietal cortex. Notably, alpha modulation was consistently reduced in the hemisphere contralateral to stimulation, leaving the ipsilateral hemisphere relatively unaffected. Additionally, right FEF TMS enhanced gamma modulation in left visual cortex. Behaviorally, TMS caused a relative slowing of response times to targets contralateral to stimulation during the early task period. Our results suggest that left and right FEF are causally involved in the attentional top-down control of anticipatory alpha power in the contralateral visual system, whereas a right-hemispheric dominance seems to exist for control of stimulus-induced gamma power. These findings contrast the assumption of primarily intrahemispheric connectivity between FEF and parietal cortex, emphasizing the relevance of interhemispheric interactions. The contralaterality of effects may result from a transient functional reorganization of the dorsal attention network after inhibition of either FEF.

Frontal eye field involvement in sustaining visual attention: evidence from transcranial magnetic stimulation

The frontal eye field (FEF), particularly the right FEF, is broadly implicated in top-down control of transient acts of attention, but less is known about its involvement in sustained attention. Although neuroimaging studies of sustained attention tasks commonly find FEF activation, it is unclear how this region contributes to moment-to-moment fluctuations in sustained performance. We sought to determine if the FEF plays a critical role in sustained attention, and if that role differs between periods of worse performance (out-of-the-zone) and periods of better performance (in-the-zone). We used offline 1 Hz repetitive transcranial magnetic stimulation (TMS) to temporarily attenuate either right or left FEF excitability while participants performed a go/no-go sustained attention task (the gradual onset continuous performance task). The results demonstrate that following TMS to the right FEF, sustained attention during in-the-zone periods significantly worsened both in terms of lower accuracy and increased reaction time variability. In contrast, applying TMS to the left FEF did not significantly affect accuracy or variability. These results demonstrate that the right FEF plays a crucial role in supporting optimal sustained attention.

The hybrid model of attentional control: New insights into hemispheric asymmetries inferred from TMS research

Several competing theories on the mechanisms underlying attentional control have emerged over the years that, despite their substantial differences, all emphasize the importance of hemispheric asymmetries. Transcranial magnetic stimulation (TMS) has proven particularly successful in teasing them apart by selective perturbation of the dorsal and ventral fronto-parietal network. We here critically review the TMS literature and show that hemispheric asymmetries within the dorsal attention network differ between parietal and frontal cortex. Specifically, posterior parietal cortex seems to be characterized by a contralateral bias of each hemisphere and competition between them. In contrast, the right frontal eye field seems to be involved in shifting attention toward both hemifields, whereas left frontal eye field is only involved on shifting attention toward the contralateral hemifield. In the light of presented evidence, we propose to revise the functional-anatomical model originally proposed by Corbetta and Shulman (2011, 2002) and introduce a hybrid model of hemispheric asymmetries in attentional control.

Occipital alpha activity during stimulus processing gates the information flow to object-selective cortex

A simultaneous EEG-fMRI study demonstrates that alpha-band activity in early visual cortex is associated with gating visual information to downstream regions, boosting attended information and suppressing distraction.

Given the limited processing capabilities of the sensory system, it is essential that attended information is gated to downstream areas, whereas unattended information is blocked. While it has been proposed that alpha band (8–13 Hz) activity serves to route information to downstream regions by inhibiting neuronal processing in task-irrelevant regions, this hypothesis remains untested. Here we investigate how neuronal oscillations detected by electroencephalography in visual areas during working memory encoding serve to gate information reflected in the simultaneously recorded blood-oxygenation-level-dependent (BOLD) signals recorded by functional magnetic resonance imaging in downstream ventral regions. We used a paradigm in which 16 participants were presented with faces and landscapes in the right and left hemifields; one hemifield was attended and the other unattended. We observed that decreased alpha power contralateral to the attended object predicted the BOLD signal representing the attended object in ventral object-selective regions. Furthermore, increased alpha power ipsilateral to the attended object predicted a decrease in the BOLD signal representing the unattended object. We also found that the BOLD signal in the dorsal attention network inversely correlated with visual alpha power. This is the first demonstration, to our knowledge, that oscillations in the alpha band are implicated in the gating of information from the visual cortex to the ventral stream, as reflected in the representationally specific BOLD signal. This link of sensory alpha to downstream activity provides a neurophysiological substrate for the mechanism of selective attention during stimulus processing, which not only boosts the attended information but also suppresses distraction. Although previous studies have shown a relation between the BOLD signal from the dorsal attention network and the alpha band at rest, we demonstrate such a relation during a visuospatial task, indicating that the dorsal attention network exercises top-down control of visual alpha activity.

In complex environments, our sensory systems are bombarded with information. Only a fraction of this information is processed, whereas most is ignored. As such, our brain must rely on powerful mechanisms to filter the relevant information. It has been proposed that alpha band oscillations (8–13 Hz) gate task-relevant visual information to downstream areas and supress irrelevant visual information. We tested this hypothesis in a study that combined electroencephalography (EEG) and functional MRI (fMRI) recordings. From the EEG, we directly measured alpha band oscillations in early visual regions. Using fMRI, we quantified neuronal activity in downstream regions. The participants performed a spatial working memory task that required them to encode pictures of objects presented in the left field of view while ignoring objects in the right field (or vice versa). We found that suppression of alpha band activity in visual areas opened the gate for relevant visual information to be routed to downstream regions. Conversely, an increase in alpha oscillations suppressed visual information that was irrelevant to the task. These findings suggest that alpha band oscillations are directly involved in boosting attended information and suppressing distraction in the ventral visual stream.

A practical application of text mining to literature on cognitive rehabilitation and enhancement through neurostimulation

The exponential growth in publications represents a major challenge for researchers. Many scientific domains, including neuroscience, are not yet fully engaged in exploiting large bodies of publications. In this paper, we promote the idea to partially automate the processing of scientific documents, specifically using text mining (TM), to efficiently review big corpora of publications. The “cognitive advantage” given by TM is mainly related to the automatic extraction of relevant trends from corpora of literature, otherwise impossible to analyze in short periods of time. Specifically, the benefits of TM are increased speed, quality and reproducibility of text processing, boosted by rapid updates of the results. First, we selected a set of TM-tools that allow user-friendly approaches of the scientific literature, and which could serve as a guide for researchers willing to incorporate TM in their work. Second, we used these TM-tools to obtain basic insights into the relevant literature on cognitive rehabilitation (CR) and cognitive enhancement (CE) using transcranial magnetic stimulation (TMS). TM readily extracted the diversity of TMS applications in CR and CE from vast corpora of publications, automatically retrieving trends already described in published reviews. TMS emerged as one of the important non-invasive tools that can both improve cognitive and motor functions in numerous neurological diseases and induce modulations/enhancements of many fundamental brain functions. TM also revealed trends in big corpora of publications by extracting occurrence frequency and relationships of particular subtopics. Moreover, we showed that CR and CE share research topics, both aiming to increase the brain's capacity to process information, thus supporting their integration in a larger perspective. Methodologically, despite limitations of a simple user-friendly approach, TM served well the reviewing process.

Using brain stimulation to disentangle neural correlates of conscious vision

Research into the neural correlates of consciousness (NCCs) has blossomed, due to the advent of new and increasingly sophisticated brain research tools. Neuroimaging has uncovered a variety of brain processes that relate to conscious perception, obtained in a range of experimental paradigms. But methods such as functional magnetic resonance imaging or electroencephalography do not always afford inference on the functional role these brain processes play in conscious vision. Such empirical NCCs could reflect neural prerequisites, neural consequences, or neural substrates of a conscious experience. Here, we take a closer look at the use of non-invasive brain stimulation (NIBS) techniques in this context. We discuss and review how NIBS methodology can enlighten our understanding of brain mechanisms underlying conscious vision by disentangling the empirical NCCs.

The effects of TMS over dorsolateral prefrontal cortex on trans-saccadic memory of multiple objects

Humans typically make several rapid eye movements (saccades) per second. It is thought that visual working memory can retain and spatially integrate three to four objects or features across each saccade but little is known about this neural mechanism. Previously we showed that transcranial magnetic stimulation (TMS) to the posterior parietal cortex and frontal eye fields degrade trans-saccadic memory of multiple object features (Prime, Vesia, & Crawford, 2008, Journal of Neuroscience, 28(27), 6938-6949; Prime, Vesia, & Crawford, 2010, Cerebral Cortex, 20(4), 759-772.). Here, we used a similar protocol to investigate whether dorsolateral prefrontal cortex (DLPFC), an area involved in spatial working memory, is also involved in trans-saccadic memory. Subjects were required to report changes in stimulus orientation with (saccade task) or without (fixation task) an eye movement in the intervening memory interval. We applied single-pulse TMS to left and right DLPFC during the memory delay, timed at three intervals to arrive approximately 100 ms before, 100 ms after, or at saccade onset. In the fixation task, left DLPFC TMS produced inconsistent results, whereas right DLPFC TMS disrupted performance at all three intervals (significantly for presaccadic TMS). In contrast, in the saccade task, TMS consistently facilitated performance (significantly for left DLPFC/perisaccadic TMS and right DLPFC/postsaccadic TMS) suggesting a dis-inhibition of trans-saccadic processing. These results are consistent with a neural circuit of trans-saccadic memory that overlaps and interacts with, but is partially separate from the circuit for visual working memory during sustained fixation.

Frontal eye field, where art thou? Anatomy, function, and non-invasive manipulation of frontal regions involved in eye movements and associated cognitive operations

The planning, control and execution of eye movements in 3D space relies on a distributed system of cortical and subcortical brain regions. Within this network, the Eye Fields have been described in animals as cortical regions in which electrical stimulation is able to trigger eye movements and influence their latency or accuracy. This review focuses on the Frontal Eye Field (FEF) a “hub” region located in Humans in the vicinity of the pre-central sulcus and the dorsal-most portion of the superior frontal sulcus. The straightforward localization of the FEF through electrical stimulation in animals is difficult to translate to the healthy human brain, particularly with non-invasive neuroimaging techniques. Hence, in the first part of this review, we describe attempts made to characterize the anatomical localization of this area in the human brain. The outcome of functional Magnetic Resonance Imaging (fMRI), Magneto-encephalography (MEG) and particularly, non-invasive mapping methods such a Transcranial Magnetic Stimulation (TMS) are described and the variability of FEF localization across individuals and mapping techniques are discussed. In the second part of this review, we will address the role of the FEF. We explore its involvement both in the physiology of fixation, saccade, pursuit, and vergence movements and in associated cognitive processes such as attentional orienting, visual awareness and perceptual modulation. Finally in the third part, we review recent evidence suggesting the high level of malleability and plasticity of these regions and associated networks to non-invasive stimulation. The exploratory, diagnostic, and therapeutic interest of such interventions for the modulation and improvement of perception in 3D space are discussed.

Translating working memory into action: behavioral and neural evidence for using motor representations in encoding visuo-spatial sequences

The neurobiological organization of action-oriented working memory is not well understood. To elucidate the neural correlates of translating visuo-spatial stimulus sequences into delayed (memory-guided) sequential actions, we measured brain activity using functional magnetic resonance imaging while participants encoded sequences of four to seven dots appearing on fingers of a left or right schematic hand. After variable delays, sequences were to be reproduced with the corresponding fingers. Recall became less accurate with longer sequences and was initiated faster after long delays. Across both hands, encoding and recall activated bilateral prefrontal, premotor, superior and inferior parietal regions as well as the basal ganglia, whereas hand-specific activity was found (albeit to a lesser degree during encoding) in contralateral premotor, sensorimotor, and superior parietal cortex. Activation differences after long versus short delays were restricted to motor-related regions, indicating that rehearsal during long delays might have facilitated the conversion of the memorandum into concrete motor programs at recall. Furthermore, basal ganglia activity during encoding selectively predicted correct recall. Taken together, the results suggest that to-be-reproduced visuo-spatial sequences are encoded as prospective action representations (motor intentions), possibly in addition to retrospective sensory codes. Overall, our study supports and extends multi-component models of working memory, highlighting the notion that sensory input can be coded in multiple ways depending on what the memorandum is to be used for.

Working memory improvement with non-invasive brain stimulation of the dorsolateral prefrontal cortex: a systematic review and meta-analysis

Recent studies have used non-invasive brain stimulation (NIBS) techniques, such as repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS), to increase dorsolateral prefrontal cortex (DLPFC) activity and, consequently, working memory (WM) performance. However, such experiments have yielded mixed results, possibly due to small sample sizes and heterogeneity of outcomes. Therefore, our aim was to perform a systematic review and meta-analyses on NIBS studies assessing the n-back task, which is a reliable index for WM. From the first data available to February 2013, we looked for sham-controlled, randomized studies that used NIBS over the DLPFC using the n-back task in PubMed/MEDLINE and other databases. Twelve studies (describing 33 experiments) matched our eligibility criteria. Active vs. sham NIBS was significantly associated with faster response times (RTs), higher percentage of correct responses and lower percentage of error responses. However, meta-regressions showed that tDCS (vs. rTMS) presented only an improvement in RT, and not in accuracy. This could have occurred in part because almost all tDCS studies employed a crossover design, possibly due to the reliable tDCS blinding. Study design was also associated with no improvement in correct responses in the active vs. sham groups. To conclude, rTMS of the DLPFC significantly improved all measures of WM performance whereas tDCS significantly improved RT, but not the percentage of correct and error responses. Mechanistic insights on the role of DLPFC in WM are further discussed, as well as how NIBS techniques could be used in neuropsychiatric samples presenting WM deficits, such as major depression, dementia and schizophrenia.

Consequences of cathodal stimulation for behavior: when does it help and when does it hurt performance?

Cathodal Transcranial Direct Current Stimulation (C-tDCS) has been reported, across different studies, to facilitate or hinder performance, or simply to have no tangible effect on behavior. This discrepancy is most prominent when C-tDCS is used to alter a cognitive function, questioning the assumption that cathodal stimulation always compromises performance. In this study, we aimed to study the effect of two variables on performance in a simple cognitive task (letter Flanker), when C-tDCS was applied to the left prefrontal cortex (PFC): (1) the time of testing relative to stimulation (during or after), and (2) the nature of the cognitive activity during stimulation in case of post-tDCS testing. In three experiments, we had participants either perform the Flanker task during C-tDCS (Experiment 1), or after C-tDCS. When the Flanker task was administered after C-tDCS, we varied whether during stimulation subjects were engaged in activities that posed low (Experiment 2) or high (Experiment 3) demands on the PFC. Our findings show that the nature of the task during C-tDCS has a systematic influence on the outcome, while timing per se does not.

Single pulse TMS differentially modulates reward behavior

Greater knowledge of cortical brain regions in reward processing may set the stage for using transcranial magnetic stimulation (TMS) as a treatment in patients with avolition, apathy or other drive-related symptoms. This study examined the effects of single pulse (sp) TMS to two reward circuit targets on drive in healthy subjects. Fifteen healthy subjects performed the monetary incentive delay task (MID) while receiving fMRI-guided spTMS to either inferior parietal lobe (IPL) or supplemental motor area (SMA). The study demonstrated decreasing reaction times (RT) for increasing reward. It also showed significant differences in RT modulation for TMS pulses to the IPL versus the SMA. TMS pulses during the delay period produced significantly more RT slowing when targeting the IPL than those to the SMA. This RT slowing carried over into subsequent trials without TMS stimulation, with significantly slower RTs in sessions that had targeted the IPL compared to those targeting SMA. The results of this study suggest that both SMA and IPL are involved in reward processing, with opposite effects on RT in response to TMS stimulation. TMS to these target cortical regions may be useful in modulating reward circuit deficits in psychiatric populations.

TMS of the anterior intraparietal area selectively modulates orientation change detection during action preparation

Perception of relevant visual object features can be modulated by the preparation of an action toward it (“action-modulated perception”). For instance, the perception of the orientation of a book can be enhanced when preparing to grasp it (but not when pointing to it). However, the underlying neuronal mechanisms are poorly understood. We argue that brain areas controlling arm movements are involved in establishing this effect through top-down feedback to early visual areas, similar to the neuronal mechanisms linking visual attention and eye movements. To investigate this involvement, we applied transcranial magnetic stimulation to a grasping motor area, the left anterior intraparietal sulcus (aIPS), during grasping or pointing preparation. Concurrently, an orientation change detection task was performed. As a control area, the vertex was stimulated. We found that stimulation of aIPS selectively modulates orientation sensitivity during action preparation compared with control stimulation (vertex), negating the increased orientation sensitivity with grasping preparation over pointing preparation. We argue that aIPS is a critical part of the mechanism underlying perceptual modulations during action preparation. The present results and recent literature suggest that this action-modulated perception for hand movements is implemented through a cortical feedback connection between aIPS and early visual areas.

Enhancement of human cognitive performance using transcranial magnetic stimulation (TMS)

Here we review the usefulness of transcranial magnetic stimulation (TMS) in modulating cortical networks in ways that might produce performance enhancements in healthy human subjects. To date over sixty studies have reported significant improvements in speed and accuracy in a variety of tasks involving perceptual, motor, and executive processing. Two basic categories of enhancement mechanisms are suggested by this literature: direct modulation of a cortical region or network that leads to more efficient processing, and addition-by-subtraction, which is disruption of processing which competes or distracts from task performance. Potential applications of TMS cognitive enhancement, including research into cortical function, rehabilitation therapy in neurological and psychiatric illness, and accelerated skill acquisition in healthy individuals are discussed, as are methods of optimizing the magnitude and duration of TMS-induced performance enhancement, such as improvement of targeting through further integration of brain imaging with TMS. One technique, combining multiple sessions of TMS with concurrent TMS/task performance to induce Hebbian-like learning, appears to be promising for prolonging enhancement effects. While further refinements in the application of TMS to cognitive enhancement can still be made, and questions remain regarding the mechanisms underlying the observed effects, this appears to be a fruitful area of investigation that may shed light on the basic mechanisms of cognitive function and their therapeutic modulation.

Fronto-tectal white matter connectivity mediates facilitatory effects of non-invasive neurostimulation on visual detection

The causal ability of pre-target FEF activity to modulate visual detection for perithreshold stimuli has been recently demonstrated in humans by means of non-invasive neurostimulation. Yet in spite of the network-distributed effects of these type of techniques, the white matter (WM) tracts and distant visual nodes contributing to such behavioral impact remain unknown. We hereby used individual data from a group of healthy human subjects, who received time-locked pulses of active or sham Transcranial Magnetic Stimulation (TMS) to the right Frontal Eye Field (FEF) region, and experienced increases in visual detection sensitivity. We then studied the extent to which interindividual differences in visual modulation might be dependent on the WM patterns linking the targeted area to other regions relevant for visuo-attentional behaviors. We report a statistically significant correlation between the probability of connection in a right fronto-tectal pathway (FEF-Superior Colliculus) and the modulation of visual sensitivity during a detection task. Our findings support the potential contribution of this pathway and the superior colliculus in the mediation of visual performance from frontal regions in humans. Furthermore, we also show the ability of a TMS/DTI correlational approach to contribute to the disambiguation of the specific long-range pathways driving network-wide neurostimulatory effects on behavior, anticipating their future role in guiding a more efficient use of focal neurostimulation.

Frontoparietal mechanisms supporting attention to location and intensity of painful stimuli

Attention can profoundly shape the experience of pain. However, little is known about the neural mechanisms that support directed attention to nociceptive information. In the present study, subjects were cued to attend to either the spatial location or the intensity of sequentially presented pairs of painful heat stimuli during a delayed match-to-sample discrimination task. We hypothesized that attention-related brain activation would be initiated after the presentation of the attentional cue and would be sustained through the discrimination task. Conjunction analysis confirmed that bilateral portions of the posterior parietal cortex (intraparietal sulcus [IPS] and superior parietal lobule) exhibited this sustained activity during attention to spatial but not intensity features of pain. Analyses contrasting activation during spatial and intensity attention tasks revealed that the right IPS region of the posterior parietal cortex was consistently more activated across multiple phases of the spatial task. However, attention to either feature of the noxious stimulus was associated with activation of frontoparietal areas (IPS and frontal eye fields) as well as priming of the primary somatosensory cortex. Taken together, these results delineate the neural substrates that support selective amplification of different features of noxious stimuli for utilization in discriminative processes.

Top-down modulation from inferior frontal junction to FEFs and intraparietal sulcus during short-term memory for visual features

Visual STM of simple features is achieved through interactions between retinotopic visual cortex and a set of frontal and parietal regions. In the present fMRI study, we investigated effective connectivity between central nodes in this network during the different task epochs of a modified delayed orientation discrimination task. Our univariate analyses demonstrate that the inferior frontal junction (IFJ) is preferentially involved in memory encoding, whereas activity in the putative FEFs and anterior intraparietal sulcus (aIPS) remains elevated throughout periods of memory maintenance. We have earlier reported, using the same task, that areas in visual cortex sustain information about task-relevant stimulus properties during delay intervals [Sneve, M. H., Alnæs, D., Endestad, T., Greenlee, M. W., & Magnussen, S. Visual short-term memory: Activity supporting encoding and maintenance in retinotopic visual cortex. Neuroimage, 63, 166-178, 2012]. To elucidate the temporal dynamics of the IFJ-FEF-aIPS-visual cortex network during memory operations, we estimated Granger causality effects between these regions with fMRI data representing memory encoding/maintenance as well as during memory retrieval. We also investigated a set of control conditions involving active processing of stimuli not associated with a memory task and passive viewing. In line with the developing understanding of IFJ as a region critical for control processes with a possible initiating role in visual STM operations, we observed influence from IFJ to FEF and aIPS during memory encoding. Furthermore, FEF predicted activity in a set of higher-order visual areas during memory retrieval, a finding consistent with its suggested role in top-down biasing of sensory cortex.

Causal frequency-specific contributions of frontal spatiotemporal patterns induced by non-invasive neurostimulation to human visual performance

Neural oscillatory activity is known to play a crucial role in brain function. In the particular domain of visual perception, specific frequency bands in different brain regions and networks, from sensory areas to large-scale frontoparietal systems, have been associated with distinct aspects of visual behavior. Nonetheless, their contributions to human visual cognition remain to be causally demonstrated. We hereby used non-uniform (and thus non-frequency-specific) and uniform (frequency-specific) high-beta and gamma patterns of noninvasive neurostimulation over the right frontal eye field (FEF) to isolate the behavioral effects of oscillation frequency and provide causal evidence that distinct visual behavioral outcomes could be modulated by frequency-specific activity emerging from a single cortical region. In a visual detection task using near-threshold targets, high-beta frequency enhanced perceptual sensitivity (d') without changing response criterion (beta), whereas gamma frequency shifted response criterion but showed no effects on perceptual sensitivity. The lack of behavioral modulations by non-frequency-specific patterns demonstrates that these behavioral effects were specifically driven by burst frequency. We hypothesize that such frequency-coded behavioral impact of oscillatory activity may reflect a general brain mechanism to multiplex functions within the same neural substrate. Furthermore, pathological conditions involving impaired cerebral oscillations could potentially benefit in the near future from the use of neurostimulation to restore the characteristic oscillatory patterns of healthy systems.