Functional circuitry underlying natural and interventional cancellation of visual neglect

Brain Network Dysfunction in Poststroke Delirium and Spatial Neglect: An fMRI Study

BACKGROUND AND PURPOSE

Delirium, an acute reduction in cognitive functioning, hinders stroke recovery and contributes to cognitive decline. Right-hemisphere stroke is linked with higher delirium incidence, likely, due to the prevalence of spatial neglect (SN), a right-brain disorder of spatial processing. This study tested if symptoms of delirium and SN after right-hemisphere stroke are associated with abnormal function of the right-dominant neural networks specialized for maintaining attention, orientation, and arousal.

METHODS

Twenty-nine participants with right-hemisphere ischemic stroke undergoing acute rehabilitation completed delirium and SN assessments and functional neuroimaging scans. Whole-brain functional connectivity of 4 right-hemisphere seed regions in the cortical-subcortical arousal and attention networks was assessed for its relationship to validated SN and delirium severity measures.

RESULTS

Of 29 patients, 6 (21%) met the diagnostic criteria for delirium and 16 (55%) for SN. Decreased connectivity of the right basal forebrain to brain stem and basal ganglia predicted more severe SN. Increased connectivity of the arousal and attention network regions with the parietal, frontal, and temporal structures in the unaffected hemisphere was also found in more severe delirium and SN.

CONCLUSIONS

Delirium and SN are associated with decreased arousal network activity and an imbalance of cortico-subcortical hemispheric connectivity. Better understanding of neural correlates of poststroke delirium and SN will lead to improved neuroscience-based treatment development for these disorders. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT03349411.

Neural Mechanisms of Prism Adaptation in Healthy Adults and Individuals with Spatial Neglect after Unilateral Stroke: A Review of fMRI Studies

Functional disability due to spatial neglect hinders recovery in up to 30% of stroke survivors. Prism adaptation treatment (PAT) may alleviate the disabling consequences of spatial neglect, but we do not yet know why some individuals show much better outcomes following PAT than others. The goal of this scoping review and meta-analysis was to investigate the neural mechanisms underlying prism adaptation (PA). We conducted both quantitative and qualitative analyses across fMRI studies investigating brain activity before, during, and after PA, in healthy individuals and patients with right or left brain damage (RBD or LBD) due to stroke. In healthy adults, PA was linked with activity in posterior parietal and cerebellar clusters, reduced bilateral parieto-frontal connectivity, and increased fronto-limbic and sensorimotor network connectivity. In contrast, RBD individuals with spatial neglect relied on different circuits, including an activity cluster in the intact left occipital cortex. This finding is consistent with a shift in hemispheric dominance in spatial processing to the left hemisphere. However, more studies are needed to clarify the contribution of lesion location and load on the circuits involved in PA after unilateral brain damage. Future studies are also needed to clarify the relationship of decreasing resting state functional connectivity (rsFC) to visuomotor function.

Revisiting 'brain modes' in a new computational era: approaches for the characterization of brain-behavioural associations

The study of brain-function relationships is undergoing a conceptual and methodological transformation due to the emergence of network neuroscience and the development of multivariate methods for lesion-deficit inferences. Anticipating this process, in 1998 Godefroy and co-workers conceptualized the potential of four elementary typologies of brain-behaviour relationships named 'brain modes' (unicity, equivalence, association, summation) as building blocks able to describe the association between intact or lesioned brain regions and cognitive processes or neurological deficits. In the light of new multivariate lesion inference and network approaches, we critically revisit and update the original theoretical notion of brain modes, and provide real-life clinical examples that support their existence. To improve the characterization of elementary units of brain-behavioural relationships further, we extend such conceptualization with a fifth brain mode (mutual inhibition/masking summation). We critically assess the ability of these five brain modes to account for any type of brain-function relationship, and discuss past versus future contributions in redefining the anatomical basis of human cognition. We also address the potential of brain modes for predicting the behavioural consequences of lesions and their future role in the design of cognitive neurorehabilitation therapies.

Low frequency transcranial magnetic stimulation of right posterior parietal cortex reduces reaction time to perithreshold low spatial frequency visual stimuli

Research in humans and animal models suggests that visual responses in early visual cortical areas may be modulated by top-down influences from distant cortical areas, particularly in the frontal and parietal regions. The right posterior parietal cortex is part of a broad cortical network involved in aspects of visual search and attention, but its role in modulating activity in early visual cortical areas is less well understood. This study evaluated the influence of right posterior parietal cortex (PPC) on a direct measure of visual processing in humans. Contrast sensitivity (CS) and detection response times were recorded using a visual detection paradigm to two types of centrally-presented stimuli. Participants were tested on the detection task before, after, and 1 hour after low-frequency repetitive transcranial magnetic stimulation (rTMS) to the right PPC or to the scalp vertex. Low-frequency rTMS to the right PPC did not significantly change measures of contrast sensitivity, but increased the speed at which participants responded to visual stimuli of low spatial frequency. Response times returned to baseline 1-hour after rTMS. These data indicate that low frequency rTMS to the right PPC speeds up aspects of early visual processing, likely due to a disinhibition of the homotopic left posterior parietal cortex.

Perturbation-driven paradoxical facilitation of visuo-spatial function: Revisiting the 'Sprague effect'

The 'Sprague Effect' described in the seminal paper of James Sprague (Science 153:1544-1547, 1966a) is an unexpected paradoxical effect in which a second brain lesion reversed functional deficits induced by an earlier lesion. It was observed initially in the cat where severe and permanent contralateral visually guided attentional deficits generated by the ablation of large areas of the visual cortex were reversed by the subsequent removal of the superior colliculus (SC) opposite to the cortical lesion or by the splitting of the collicular commissure. Physiologically, this effect has been explained in several ways-most notably by the reduction of the functional inhibition of the ipsilateral SC by the contralateral SC, and the restoration of normal interactions between cortical and midbrain structures after ablation. In the present review, we aim at reappraising the 'Sprague Effect' by critically analyzing studies that have been conducted in the feline and human brain. Moreover, we assess applications of the 'Sprague Effect' in the rehabilitation of visually guided attentional impairments by using non-invasive therapeutic approaches such as transcranial magnetic stimulation (TMS) and transcranial direct-current stimulation (tDCS). We also review theoretical models of the effect that emphasize the inhibition and balancing between the two hemispheres and show implications for lesion inference approaches. Last, we critically review whether the resulting inter-hemispheric rivalry theories lead toward an efficient rehabilitation of stroke in humans. We conclude by emphasizing key challenges in the field of 'Sprague Effect' applications in order to design better therapies for brain-damaged patients.

Recovery from Spatial Neglect with Intra- and Transhemispheric Functional Connectivity Changes in Vestibular and Visual Cortex Areas-A Case Study

Objective

Vestibular signals are involved in higher cortical functions like spatial orientation and its disorders. Vestibular dysfunction contributes, for example, to spatial neglect which can be transiently improved by caloric stimulation. The exact roles and mechanisms of the vestibular and visual systems for the recovery of neglect are not yet known.

Methods

Resting-state functional connectivity (fc) magnetic resonance imaging was recorded in a patient with hemispatial neglect during the acute phase and after recovery 6 months later following a right middle cerebral artery infarction before and after caloric vestibular stimulation. Seeds in the vestibular [parietal operculum (OP2)], the parietal [posterior parietal cortex (PPC); 7A, hIP3], and the visual cortex (VC) were used for the analysis.

Results

During the acute stage after caloric stimulation the fc of the right OP2 to the left OP2, the anterior cingulum, and the para/hippocampus was increased bilaterally (i.e., the vestibular network), while the interhemispheric fc was reduced between homologous regions in the VC. After 6 months, similar fc increases in the vestibular network were found without stimulation. In addition, fc increases of the OP2 to the PPC and the VC were seen; interhemispherically this was true for both PPCs and for the right PPC to both VCs.

Conclusion

Improvement of neglect after caloric stimulation in the acute phase was associated with increased fc of vestibular cortex areas in both hemispheres to the para-hippocampus and the dorsal anterior cingulum, but simultaneously with reduced interhemispheric VC connectivity. This disclosed a, to some extent, similar but also distinct short-term mechanism (vestibular stimulation) of an improvement of spatial orientation compared to the long-term recovery of neglect.

Patching for Diplopia Contraindicated in Patients with Brain Injury?

PURPOSE

Patching for double vision is a common palliative treatment for head-trauma patients with acquired strabismus when prisms are not feasible.

METHODS

We review literature on spatial neglect and discuss possible effects of monocular occlusion on spatial attention.

RESULTS

Patching the left eye has been shown to worsen spatial judgments in some brain-injured patients with left neglect by inhibiting the right superior colliculus further impairing contralateral leftward orienting (the Sprague Effect).

CONCLUSIONS

Because more peripheral parts of the visual field increasingly project to the contralateral superior colliculus with the temporal crescent being entirely contralateral, avoiding patching of the temporal crescent was advised, and in most cases can be achieved by taping off the spectacle lens and avoiding an elastic eye patch.

A critical role of temporoparietal junction in the integration of top-down and bottom-up attentional control

Information processing can be biased toward behaviorally relevant and salient stimuli by top-down (goal-directed) and bottom-up (stimulus-driven) attentional control processes respectively. However, the neural basis underlying the integration of these processes is not well understood. We employed functional magnetic resonance imaging (fMRI) and transcranial direct-current stimulation (tDCS) in humans to examine the brain mechanisms underlying the interaction between these two processes. We manipulated the cognitive load involved in top-down processing and stimulus surprise involved in bottom-up processing in a factorial design by combining a majority function task and an oddball paradigm. We found that high cognitive load and high surprise level were associated with prolonged reaction time compared to low cognitive load and low surprise level, with a synergistic interaction effect, which was accompanied by a greater deactivation of bilateral temporoparietal junction (TPJ). In addition, the TPJ displayed negative functional connectivity with right middle occipital gyrus, which is involved in bottom-up processing (modulated by the interaction effect), and the right frontal eye field (FEF), which is involved in top-down control. The enhanced negative functional connectivity between the TPJ and right FEF was accompanied by a larger behavioral interaction effect across subjects. Application of cathodal tDCS over the right TPJ eliminated the interaction effect. These results suggest that the TPJ plays a critical role in processing bottom-up information for top-down control of attention.

The relevance of aging-related changes in brain function to rehabilitation in aging-related disease

The effects of aging on rehabilitation of aging-related diseases are rarely a design consideration in rehabilitation research. In this brief review we present strong coincidental evidence from these two fields suggesting that deficits in aging-related disease or injury are compounded by the interaction between aging-related brain changes and disease-related brain changes. Specifically, we hypothesize that some aphasia, motor, and neglect treatments using repetitive transcranial magnetic stimulation (rTMS) or transcranial direct current stimulation (tDCS) in stroke patients may address the aging side of this interaction. The importance of testing this hypothesis and addressing the larger aging by aging-related disease interaction is discussed. Underlying mechanisms in aging that most likely are relevant to rehabilitation of aging-related diseases also are covered.

Application of Parallel Factor Analysis (PARAFAC) to electrophysiological data

The identification of important features in multi-electrode recordings requires the decomposition of data in order to disclose relevant features and to offer a clear graphical representation. This can be a demanding task. Parallel Factor Analysis (PARAFAC; Hitchcock, 1927; Carrol and Chang, 1970; Harshman, 1970) is a method to decompose multi-dimensional arrays in order to focus on the features of interest, and provides a distinct illustration of the results. We applied PARAFAC to analyse spatio-temporal patterns in the functional connectivity between neurons, as revealed in their spike trains recorded in cat primary visual cortex (area 18). During these recordings we reversibly deactivated feedback connections from higher visual areas in the pMS (posterior middle suprasylvian) cortex in order to study the impact of these top-down signals. Cross correlation was computed for every possible pair of the 16 electrodes in the electrode array. PARAFAC was then used to reveal the effects of time, stimulus, and deactivation condition on the correlation patterns. Our results show that PARAFAC is able to reliably extract changes in correlation strength for different experimental conditions and display the relevant features. Thus, PARAFAC proves to be well-suited for the use in the context of electrophysiological (action potential) recordings.

Rehabilitation of poststroke cognition

Given the increasing rates of stroke and our aging population, it is critical that we continue to foster innovation in stroke rehabilitation. Although there is evidence supporting cognitive rehabilitation in stroke, the set of cognitive domains effectively addressed to date represents only a small subset of the problems experienced by stroke survivors. Further, a gap remains between investigational treatments and our evolving theories of brain function. These limitations present opportunities for improving the functional impact of stroke rehabilitation. The authors use a case example to encourage the reader to consider the evidence base for cognitive rehabilitation in stroke, focusing on four domains critical to daily life function: (1) speech and language, (2) functional memory, (3) executive function and skilled learned purposive movements, and (4) spatial-motor systems. Ultimately, they attempt to draw neuroscience and practice closer together by using translational reasoning to suggest possible new avenues for treating these disorders.

Is neuroenhancement by noninvasive brain stimulation a net zero-sum proposition?

In the past several years, the number of studies investigating enhancement of cognitive functions through noninvasive brain stimulation (NBS) has increased considerably. NBS techniques, such as transcranial magnetic stimulation and transcranial current stimulation, seem capable of enhancing cognitive functions in patients and in healthy humans, particularly when combined with other interventions, including pharmacologic, behavioral and cognitive therapies. The "net zero-sum model", based on the assumption that brain resources are subjected to the physical principle of conservation of energy, is one of the theoretical frameworks proposed to account for such enhancement of function and its potential cost. We argue that to guide future neuroenhancement studies, the net-zero sum concept is helpful, but only if its limits are tightly defined.

Multiple sessions of transcranial direct current stimulation to the intact hemisphere improves visual function after unilateral ablation of visual cortex

Damage to cerebral systems is frequently followed by the emergence of compensatory mechanisms, which serve to reduce the effects of brain damage and allow recovery of function. Intrinsic recovery, however, is rarely complete. Non-invasive brain stimulation technologies have the potential to actively shape neural circuits and enhance recovery from brain damage. In this study, a stable deficit for detecting and orienting to visual stimuli presented in the contralesional visual hemifield was generated by producing unilateral brain damage of the right posterior parietal and contiguous visual cortical areas. A long regimen of inhibitory non-invasive transcranial direct-current stimulation (cathodal tDCS, 2 mA, 20 min) was applied to the contralateral (intact) posterior parietal cortex over 14 weeks (total of 70 sessions, one per day, 5 days per week) and behavioral outcomes were periodically assessed. In three out of four stimulated cats, lasting recovery of visuospatial function was observed. Recovery started after 2-3 weeks of stimulation, and recovered targets were located first in the periphery, and moved to more central visual field locations with the accrual of stimulation sessions. Recovery for moving tasks followed a biphasic pattern before reaching plateau levels. Recovery did not occur for more difficult visual tasks. These findings highlight the ability of multiple sessions of transcranial direct-current stimulation to produce recovery of visuospatial function after unilateral brain damage.

Non-invasive brain stimulation in neglect rehabilitation: an update

Here, we review the effects of non-invasive brain stimulation such as transcranial magnetic stimulation (TMS) or transcranial direct current stimulation (tDCS) in the rehabilitation of neglect. We found 12 studies including 172 patients (10 TMS studies and 2 tDCS studies) fulfilling our search criteria. Activity of daily living measures such as the Barthel Index or, more specifically for neglect, the Catherine Bergego Scale were the outcome measure in three studies. Five studies were randomized controlled trials with a follow-up time after intervention of up to 6 weeks. One TMS study fulfilled criteria for Class I and one for Class III evidence. The studies are heterogeneous concerning their methodology, outcome measures, and stimulation parameters making firm comparisons and conclusions difficult. Overall, there are however promising results for theta-burst stimulation, suggesting that TMS is a powerful add-on therapy in the rehabilitation of neglect patients.

Rehabilitation of spatial neglect

Spatial neglect is a frequent cause of disability associated with high costs and duration of hospital stay, increased family burden, and requirements for skilled chronic care. This condition is disproportionately more frequent with right than left hemispheric injury and it is characterized by perceptual, representational, and behavioral deficits involving or directed towards the left hemispace or the left hemibody. Spatial dysfunction is conceptualized into two major components: the perceptual/representational "where" component that results mainly from injury to posterior brain regions and the premotor/intentional "aiming" component that results mostly from damage to anterior brain regions. Additionally, deficits in arousal, vigilance, affective symptoms, and disorders of emotional communication may compound the clinical manifestations of spatial neglect. Evidence-based sources that evaluate the effectiveness of rehabilitation treatments for neglect are, unfortunately, unable to provide a unified consensus for the efficacy of a given treatment approach. The reasons for this failure are related to internal inconsistencies defining appropriate criteria for treatment success and lack of characterization of neglect mechanisms and considerations of patient characteristics related to treatment failure. In this chapter we advocate the use of visual scanning, limb activation therapy, and "general treatment" because we believe that they are appropriately supported by different sources and they may be useful for experimental trials and standardized clinical care. We advocate an integrative approach that takes advantage of the same rehabilitation strategy or task to treat different perceptual, representational, and premotor components of neglect. A variety of therapies that may be familiar to the rehabilitation team may be useful as long as they are applied in a systematized program and are based on good clinical judgment. Information regarding adjuvant pharmacological therapy is sparse but different agents with aminergic and cholinergic activity may be useful. Medication with sedative, antidopaminergic or anticholinergic properties may interfere with the rehabilitation process and should be avoided.

Relevance of animal models of subarachnoid hemorrhage for examining neurobehavioral changes

BACKGROUND AND PURPOSE

For many years survival and neurological functionality of patients were the main outcome measures after treatment of intracranial aneurysms. But, the variable outcomes of patients operated on in a delayed fashion or before the aneurysm rupture indicate that more precise measures are needed for assessment of not only the neurological but also the neuropsychological outcome. However, development and testing of such new tools requires better understanding of pathomechanisms of neurobehavioral changes evoked by aneurysmal subarachnoid hemorrhage (aSAH), which can be achieved using animal models.

METHODS

We reviewed and selected (1) animal models developed to investigate delayed cerebral vasospasm that could be useful for examining effects of brain injury evoked by aSAH and (2) a battery of neurobehavioral animal testing that can be used for assessment of patients after aSAH.

RESULTS

For every species used as an aSAH model, a battery of neurobehavioral test exists.

CONCLUSION

Albeit some limitations must be recognized, research using animal models of SAH should continue to play a critical role in assessment of cognitive and behavioral functions after aSAH.

Model approach to neurological variants of visuo-spatial neglect

Neglect is a neurological disorder of spatial attention with reduced awareness of visual stimuli in the hemifield contralateral to an acute temporo-parietal lesion mainly of the right hemisphere. There is a close association of multisensory orientation centers (MSO) and vestibular tonus imbalance. A lesion of the dominant right MSO causes a left-sided neglect due to a lack of ipsilateral activation of the visual cortex, which is further enhanced by increasing inhibition from the contralateral visual cortex. The nondominant MSO in the left hemisphere might be involved in the manifestation of the less frequent and more transient right-sided neglect and in the plastic mechanisms of gradual recovery from left-sided neglect or extinction. There is evidence that a vestibular tonus inbalance due to peripheral or central vestibular pathway lesions may also induce a neglect. In a first model approach using an attractor network and assuming that there is only one MSO in the right hemisphere, it is possible to simulate attentional shifts into a visual hemifield and to induce a neglect. The neural network model consists of four layers of neurons: retina, MSO, visual cortex V1, and superior colliculus. The superior colliculus layer is modeled as a recurrent attractor network with one inhibitory interneuron and synaptic weights chosen to implement a winner-take-all network that centers the hill of activity on the strongest input. We are well aware of the simplifications used in the conceptual drawings and the computational model, but nevertheless hope that they will serve as an inspiration for further modeling and clinical studies.

Prism adaptation for spatial neglect after stroke: translational practice gaps

Spatial neglect increases hospital morbidity and costs in around 50% of the 795,000 people per year in the USA who survive stroke, and an urgent need exists to reduce the care burden of this condition. However, effective acute treatment for neglect has been elusive. In this article, we review 48 studies of a treatment of intense neuroscience interest: prism adaptation training. Due to its effects on spatial motor 'aiming', prism adaptation training may act to reduce neglect-related disability. However, research failed, first, to suggest methods to identify the 50-75% of patients who respond to treatment; second, to measure short-term and long-term outcomes in both mechanism-specific and functionally valid ways; third, to confirm treatment utility during the critical first 8 weeks poststroke; and last, to base treatment protocols on systematic dose-response data. Thus, considerable investment in prism adaptation research has not yet touched the fundamentals needed for clinical implementation. We suggest improved standards and better spatial motor models for further research, so as to clarify when, how and for whom prism adaptation should be applied.

Spatial neglect: hypothetical mechanisms of disturbed interhemispheric crosstalk for orientation

Schematic drawings are presented of the major anatomical structures involved, along with their functional excitatory and inhibitory connections as the basis of a hypothetical model of visuospatial neglect. It is assumed that multisensory integration centers for attention and orientation (MSO) are represented in the temporo-parietal cortex of both hemispheres, with the dominant MSO being in the right hemisphere (nondominant hemisphere in right-handedness). A lesion of the dominant MSO will cause a left-sided neglect due to a lack of ipsilateral activation of the visual cortex, which is further enhanced by increased inhibition from the contralateral visual cortex. The nondominant MSO in the left hemisphere might be involved in the manifestation of the less frequent and more transient right-sided neglect and in the plastic mechanisms of gradual recovery from a left-sided neglect or extinction.

Monocular patching may induce ipsilateral "where" spatial bias

Spatial bias is an asymmetry of perception and/or representation of spatial information - "where" bias -, or of spatially directed actions - "aiming" bias. A monocular patch may induce contralateral "where" spatial bias (the Sprague effect [Sprague, J. M. (1966). Interaction of cortex and superior colliculus in mediation of visually guided behavior in cat. Science, 153(3743), 1544-1547]). However, an ipsilateral patch-induced spatial bias may be observed if visual occlusion results in top-down, compensatory re-allocation of spatial perceptual or representational resources toward the region of visual deprivation. Tactile distraction from a monocular patch may also contribute to an ipsilateral bias. To examine these hypotheses, neurologically normal adults bisected horizontal lines at baseline without a patch, while wearing a monocular patch, and while wearing tactile-only and visual-only monocular occlusion. We fractionated "where" and "aiming" spatial bias components using a video apparatus to reverse visual feedback for half of the test trials. The results support monocular patch-induced ipsilateral "where" spatial errors, which are not consistent with the Sprague effect. Further, the present findings suggested that the induced ipsilateral bias may be primarily induced by visual deprivation, consistent with compensatory "where" resource re-allocation.

Spatial neglect: clinical and neuroscience review: a wealth of information on the poverty of spatial attention

Hemispatial neglect (HSN) is a frequent, conspicuous neurobehavioral accompaniment of brain injury. Patients with HSN share several superficial similarities, leading earlier clinical neuroscientists to view neglect as a unitary condition associated with brain structures that mediate relatively discrete spatial cognitive mechanisms. Over the last two decades, research largely deconstructed the neglect syndrome, revealing a remarkable heterogeneity of behaviors and providing insight into multiple component processes, both spatial and nonspatial, that contribute to hemispatial neglect. This review surveys visual HSN, presenting first the means for detection and diagnosis in its manifold variations. We summarize cognitive operations relevant to spatial attention and evidence for their role in neglect behaviors and then briefly consider neural systems that may subserve the component processes. Finally, we propose several methods for rehabilitating HSN, including the challenges facing remediation of such a heterogeneous cognitive disorder.

Oculomotor functions of the parietal lobe: Effects of chronic lesions in humans

This review summarises research in patients with chronic lesions of parietal oculomotor cortex and compares their oculomotor performance to patients with lesions of the frontal eye field (FEF). The observations identify the oculomotor functions for which these regions are indispensable, and explore dynamic interactions within cortical and subcortical networks for oculomotor control. The experiments examined endogenously generated saccades, saccades to visual targets, antisaccades, saccade choice and saccade remapping for inhibitory spatial tagging. The findings suggest that the key function of parietal oculomotor cortex is the computation of sensorimotor transformations, rather than the initiation of either voluntary or reflexive saccades. They also reveal the re-organisation of cortico-subcortical networks after brain injury, and provide insight into their dynamic interactions: FEF lesions result in disinhibition of reflexive saccades toward the contralesional field and an impairment of reflexive saccades toward the ipsilesional field; whereas parietal lesion result in the opposite pattern.

Opposite impact on 14C-2-deoxyglucose brain metabolism following patterns of high and low frequency repetitive transcranial magnetic stimulation in the posterior parietal cortex

Repetitive transcranial magnetic stimulation (rTMS) appears capable of modulating human cortical excitability beyond the duration of the stimulation train. However, the basis and extent of this "off-line" modulation remains unknown. In a group of anesthetized cats, we applied patterns of real or sham focal rTMS to the visuo-parietal cortex (VP) at high (HF) or low (LF) frequency and recorded brain glucose uptake during (on-line), immediately after (off-line), or 1 h after (late) stimulation. During the on-line period LF and HF rTMS induced a significant relative reduction of (14)C-2DG uptake in the stimulated VP cortex and tightly linked cortical and subcortical structures (e.g. the superficial superior colliculus, the pulvinar, and the LPl nucleus) with respect to homologue areas in the unstimulated hemisphere. During the off-line period HF rTMS induced a significant relative increase in (14)C-2DG uptake in the targeted VP cortex, whereas LF rTMS generated the opposite effect, with only mild network impact. Moderate distributed effects were only recorded after LF rTMS in the posterior thalamic structures. No long lasting cortical or subcortical effects were detected during the late period. Our findings demonstrate opposite modulation of rTMS on local and distant effects along a specific network, depending on the pattern of stimulation. Such effects are demonstrated in the anesthetized animal, ruling out behavioral and non-specific reasons for the differential impact of the stimulation. The findings are consistent with previous differential electrophysiological and behavioral effects of low and high frequency rTMS patterns and provide support to uses of rTMS in neuromodulation.

Low frequency transcranial magnetic stimulation on the posterior parietal cortex induces visuotopically specific neglect-like syndrome

The visuo-parietal (VP) region of the cerebral cortex is critically involved in the generation of orienting responses towards visual stimuli. In this study we use repetitive transcranial magnetic stimulation (rTMS) to unilaterally and non-invasively deactivate the VP cortex during a simple spatial visual detection task tested in real space. Adult cats were intensively trained over 4 months on a task requiring them to detect and orient to a peripheral punctuate static LED presented at a peripheral location between 0 degrees and 90 degrees , to the right or left of a 0 degrees fixation target. In 16 different interleaved sessions, real or sham low frequency (1 Hz) rTMS was unilaterally applied during 20 min (1,200 pulses) to the VP cortex. The percentage of mistakes detecting and orienting to contralateral visual targets increased significantly during the 15-20 min immediately following real but not sham rTMS. Behavioral deficits were most marked in peripheral eccentricities, whereas more central locations were largely unaffected. Performance returned to baseline (pre-TMS) levels when animals were tested 45 min later and remained in pre-TMS levels 24 h after the end of the stimulation. Our results confirm that the VP cortex of the cat is critical for successful detection and orienting to visual stimuli presented in the corresponding contralateral visual field. In addition, we show that rTMS disrupts a robust behavioral task known to depend on VP cortex and does so for the far periphery of the visual field, but not for more central targets.