Left Posterior Parietal Cortex Research Articles

Recognition Memory Performance is Modulated by Transcranial Direct Current Stimulation Over the Left Posterior Parietal Cortex

Recognition Memory Performance is Modulated by Transcranial Direct Current Stimulation Over the Left Posterior Parietal Cortex

Hunting for right and left parietal hot spots using single-pulse TMS: modulation of visuospatial perception during line bisection judgment in the healthy brain.

A series of studies have consistently reproduced left neglect-like bias on line length estimation tasks in healthy participants by applying transcranial magnetic stimulation (TMS) over the right posterior parietal cortex (PPC), while no significant changes have been reported when stimulating the left PPC. However, a notable inter-individual variability in the right parietal site where TMS modulates visuospatial perception can be observed, and no general agreement exists on how to identify the optimal parietal site of stimulation. In the present study, we propose a new site-finding TMS protocol to easily identify the optimum parietal location, or “hot spot,” where TMS may modulate visuospatial perception on a line length estimation task (the Landmark task). Single-pulse TMS at 115% of motor threshold was applied 150 ms after the visual stimulus onset over nine different sites of a 3 cm × 3 cm grid, centred over right or left PPC (P4 and P3 according to the 10–20 EEG system, respectively) in eight healthy participants. Stimulation of right PPC induced a significant left neglect-like bias, when the coil was applied over the most posterior and dorso-posterior sites. Unexpectedly, TMS over left PPC also produced left neglect-like bias. However, in this case significant effects were found when targeting the most anterior and dorso-anterior portions of the grid. These results are discussed in relation to recent findings on neural networks underlying spatial cognition. The hunting protocol we propose might offer an economical and easy-to-use tool to functionally identify the optimal parietal site where TMS can modulate visuospatial perception, in healthy subjects and possibly in post-stroke patients undergoing repetitive transcranial magnetic stimulation treatment.

Delayed and disorganised brain activation detected with magnetoencephalography after mild traumatic brain injury

BackgroundAwareness to neurocognitive issues after mild traumatic brain injury (mTBI) is increasing, but currently no imaging markers are available for mTBI. Advanced structural imaging recently showed microstructural tissue changes and...

Enhanced long-term memory encoding after parietal neurostimulation.

Neurostimulation, e.g., transcranial direct current stimulation (tDCS), shows promise as an effective cognitive intervention. In spite of low spatial resolution, limited penetration, and temporary influence, evidence highlights tDCS-linked cognitive benefits in a range of cognitive domains. The left posterior parietal cortex (PPC) is an accessible node in frontoparietal networks engaged during long-term memory (LTM). Here, we tested the hypothesis that tDCS can facilitate LTM by pairing LTM encoding and retrieval with PPC stimulation. Healthy young adults performed a verbal LTM task (California Verbal Learning Task) with four different stimulation parameters. In Experiment 1, we applied tDCS to left PPC during LTM encoding. In Experiment 2, we applied tDCS just prior to retrieval to test the temporal specificity of tDCS during a LTM task. In later experiments, we tested hemispheric specificity by replicating Experiment 1 while stimulating the right PPC. Experiment 1 showed that tDCS applied during LTM encoding improved the pace of list learning and enhanced retrieval after a short delay. Experiment 2 indicated anodal left PPC tDCS only improved LTM when applied during encoding, and not during maintenance. Experiments 3 and 4 confirmed that tDCS effects were hemisphere specific and that no effects were found after right PPC stimulation during encoding. These findings indicate that anodal tDCS to the PPC helps verbal LTM in healthy young adults under certain conditions. First, when it is applied to the left, not the right, PPC and second, when it is applied during encoding.

Transcranial direct current stimulation over posterior parietal cortex modulates visuospatial localization.

Visual localization is based on the complex interplay of bottom-up and top-down processing. Based on previous work, the posterior parietal cortex (PPC) is assumed to play an essential role in this interplay. In this study, we investigated the causal role of the PPC in visual localization. Specifically, our goal was to determine whether modulation of the PPC via transcranial direct current stimulation (tDCS) could induce visual mislocalization similar to that induced by an exogenous attentional cue (Wright, Morris, & Krekelberg, 2011). We placed one stimulation electrode over the right PPC and the other over the left PPC (dual tDCS) and varied the polarity of the stimulation. We found that this manipulation altered visual localization; this supports the causal involvement of the PPC in visual localization. Notably, mislocalization was more rightward when the cathode was placed over the right PPC than when the anode was placed over the right PPC. This mislocalization was found within a few minutes of stimulation onset, it dissipated during stimulation, but then resurfaced after stimulation offset and lasted for another 10-15 min. On the assumption that excitability is reduced beneath the cathode and increased beneath the anode, these findings support the view that each hemisphere biases processing to the contralateral hemifield and that the balance of activation between the hemispheres contributes to position perception (Kinsbourne, 1977; Szczepanski, Konen, & Kastner, 2010).

Neural reinstatement and the amount of information recollected

Recent functional neuroimaging studies have attempted to understand the cognitive and neural bases of episodic memory retrieval, as well as the extent to which different retrieval judgments reflect qualitative as opposed to continuous changes in neural signals. The present study addressed this issue by investigating the reinstatement of episodic content according to the amount of information available at retrieval. Subjects encoded a series of words in the context of three distinct tasks, while a manipulation of presentation duration (4 or 8s) was also employed. A later recognition memory test was used to segregate trials according to whether or not they were accompanied by the recollection of details from encoding. Functional MRI data acquired during both the encoding and retrieval phases were used in conjunction with multi-voxel pattern-analysis (MVPA) to provide a measure of the degree to which encoding-related patterns of brain activity were later reactivated (reinstated) at the time of retrieval. Critically, the magnitude of reinstatement differed with respect to the encoding manipulation, such that reinstatement was stronger for items associated with the longer presentation duration. Together with duration-related differences in retrieval activity in left posterior parietal cortex, the results provide neural evidence for the reinstatement of different amounts of episodic information, consistent with the idea that recollection is based on a continuous neural signal.

ALTERED FUNCTIONAL CONNECTIVITY AND GLOBAL NETWORK PROPERTIES IN AMNESTIC MILD COGNITIVE IMPAIRMENT

Background: Amnestic mild cognitive impairment (aMCI) patients demonstrate lower global network interactions. However, graph theory (GT) analysis has not been used to examine networks compromised by strong amyloid (A β) deposition, such as the default mode network (DMN). In addition, no study directly compared GT and functional connectivity (FC) analysis between healthy controls (HCS) and aMCI with respect to Aβ and APOE genotype. Based on the known structural and functional impairments in MCI, we hypothesize that FC and global network interactions are reduced in MCI and in subjects with Aβ load and e4 allele presence. Methods: We compared 24 HCS to 15 matched aMCI by resting-state fMRI. FC (within the DMN) and GT (whole-brain) analyses were performed with the toolbox CONN at p<0.05 (corrected). Three group comparisons were examined: HCS versus aMCI, PiB- versus PiB+ (i.e., significant global PiB load), and APOE+ (e4 carrier) versus APOE-. The DMN regions were: superior prefrontal cortex (sPFC), medial PFC (mPFC), ventral PFC (vPFC), anterior (aPFC), posterior cingulate cortex (PCC), lateral posterior parietal cortex (PPC), retrosplenial cortex (RSC), parahippocampal gyrus (PHG), cerebellar tonsils (CT), and inferior temporal cortex (ITC). For GT we looked at global- and local efficiency (LE), clustering coefficient (CC), and cost. Results: HCS demonstrated stronger FC between sPFC - RSC, left PPC - right sPFC, and mPFC - left sPFC. PiB+ subjects showed lower FC for: mPFC - CT, vPFC - aPFC, vPFC - left sPFC, aPFC - left sPFC, and left PPC - dACC. PiB+ showed stronger FC between left and right PHG, left PPC - RSC, and left PHG - aPFC. APOE- demonstrated higher FC between left PPC - dACC, vPFC - aPFC, left sPFC - left PPC, mPFC - right ITC, vPFC - RSC, and aPFC - ITC. HCS showed higher cost in the sPFC and PCC, and LE in the dorsolateral PFC. PiB- yield stronger LE and cost in temporal, visual, and frontal regions. APOE- exhibits higher LE and CC in temporal and visual areas. Conclusions: The spatial extent of FC reductions (especially in prefrontal-related connections) depends on the presence of Aβ and APOE. GT results suggest lower local network interactions and cost in MCI, as well as higher efficiency in PiB- and APOE-individuals.

Dissociable causal roles for left and right parietal cortex in controlling attentional biases from the contents of working memory

The contents of working memory (WM) steer visual attention, but the extent of this guidance can be strategically enhanced or inhibited when WM content is reliably helpful or harmful to a visual task. Current understanding of the neural substrates mediating the cognitive control over WM biases is limited, however, by the correlational nature of functional MRI approaches. A recent fMRI study provided suggestive evidence for a functional lateralization of these control processes in posterior parietal cortex (PPC): activity in left PPC correlated with the presentation of WM cues that ought to be strategically enhanced to optimize performance, while activity in the right PPC correlated with the presentation of cues that ought to be inhibited to prevent detrimental attentional biases in a visual search. Here, we aimed to directly assess whether the left and right PPC are causally involved in the cognitive control of WM biases, and to clarify their precise functional contributions. We therefore applied 1Hz repetitive transcranial magnetic stimulation (rTMS) to left and right PPC (and a vertex control site) prior to administering a behavioral task assessing WM biasing control functions. We observed that the perturbation of left PPC eliminated the strategic benefit of predictably helpful WM cueing, while the perturbation of right PPC amplified the cost of unpredictable detrimental WM cueing. The left and right PPC thus play distinct causal roles in WM–attention interactions: the left PPC to maximize benefits, and the right PPC to minimize costs, of internally maintained content on visual attention.

The posterior parietal cortex: Comparing remember/know and source memory tests of recollection and familiarity

Numerous neuroimaging studies have shown a dissociation within the left posterior parietal cortex (PPC) between recollection and familiarity, with dorsal regions routinely active during familiarity and ventral regions active during recollection. The two most common methods for separating the neural correlates of these retrieval states are the remember/know paradigm and tests probing source memory. While relatively converging results have been found using these methods, the literature is lacking an adequate and direct comparison of the two procedures. We directly compared these two methodologies and found differences in both the magnitude and extent of activation within the left PPC. During familiarity, dorsal PPC regions were more strongly activated by the source test, while the remember/know test led to stronger recollection-related activations within the ventral regions of the PPC. This modulation of PPC activity is particularly important because it suggests that the neural correlates of familiarity and recollection depend on how they are operationalized. Previous assumptions that remember/know and source memory tests are functionally equivalent should therefore be re-evaluated. Additionally, any theories attempting to explain the functional role of the PPC during memory retrieval must take these differences into account.

TDCS increases cortical excitability: Direct evidence from TMS–EEG

Despite transcranial direct current stimulation (tDCS) is increasingly used in experimental and clinical settings, its precise mechanisms of action remain largely unknown. At a neuronal level, tDCS modulates the resting membrane potential in a polarity-dependent fashion: anodal stimulation increases cortical excitability in the stimulated region, while cathodal decreases it. So far, the neurophysiological underpinnings of the immediate and delayed effects of tDCS, and to what extent the stimulation of a given cerebral region may affect the activity of anatomically connected regions, remain unclear. In the present study, we used a combination of Transcranial Magnetic Stimulation (TMS) and Electroencephalography (EEG) in order to explore local and global cortical excitability modulation during and after active and sham tDCS. Single pulse TMS was delivered over the left posterior parietal cortex (PPC), before, during, and after 15 min of tDCS over the right PPC, while EEG was recorded from 60 channels.For each session, indexes of global and local cerebral excitability were obtained, computed as global and local mean field power (Global Mean Field Power, GMFP and Local Mean Field Power, LMFP) on mean TMS-evoked potentials (TEPs) for three temporal windows: 0–50, 50–100, and 100–150 msec. The global index was computed on all 60 channels. The local indexes were computed in six clusters of electrodes: left and right in frontal, parietal and temporal regions.GMFP increased, compared to baseline, both during and after active tDCS in the 0–100 msec temporal window. LMFP increased after the end of stimulation in parietal and frontal clusters bilaterally, while no difference was found in the temporal clusters. In sum, a diffuse rise of cortical excitability occurred, both during and after active tDCS. This evidence highlights the spreading of the effects of anodal tDCS over remote cortical regions of stimulated and contralateral hemispheres.

P645: Case report: seizure induction by single-pulse transcranial magnetic stimulation after anodal transcranial direct current stimulation in a stroke patient

s of Poster Presentations / Clinical Neurophysiology 125, Supplement 1 (2014) S1–S339 S225 Abstract P643 – Figure 1. Excitability modulation induced by paired pulses for the hand APB as a function of ISI for 4 capacitor combinations.P643 – Figure 1. Excitability modulation induced by paired pulses for the hand APB as a function of ISI for 4 capacitor combinations. Abstract P643 – Figure 2. Excitability modulation induced by paired pulses at ISIs of 200, 400 and 600 μs.P643 – Figure 2. Excitability modulation induced by paired pulses at ISIs of 200, 400 and 600 μs. different stimulation patterns in various channels. Multiple pulses operation with coils with various depth profiles can increase the focality of TMS effect in deep brain regions. P644 Anticipatory motor control is mediated via the left posterior parietal cortex (PPC) V. Krause, J. Weber, B. Pollok Heinrich-Heine-University, Institute of Clinical Neuroscience and Medical Psychology, Duesseldorf, Germany Question: Within the motor control network the posterior parietal cortex (PPC) has been related to anticipatory motor control while the primary motor cortex (M1) may be rather dedicated to movement execution. Assuming that cortical nodes within this network provide different contributions to motor control, the present study elucidates the specific relevance of left PPC for movement timing. Methods: Anodal vs. cathodal transcranial direct current stimulation (tDCS) was applied for 10 min over left PPC in right-handed subjects in separate sessions. Left M1 tDCS served as control condition. Anticipatory motor control was assessed by means of a synchronization task requiring subjects to synchronize their own finger taps with a regular auditory metronome. In addition, subjects performed a continuation task investigating interval reproduction and a simple auditory reaction task. Tasks were performed with the right and the left hand, respectively, prior to and immediately after tDCS. Results: Anodal left PPC tDCS decreased synchronization accuracy of either hand during the synchronization task. During continuation, cathodal left PPC tDCS increased the reproduced interval of the left hand but decreased the interval of the right hand. Reaction times were not significantly modulated by PPC stimulation. Left M1 tDCS was not efficient to significantly modulate performance in any of the tasks. Conclusions: The present data underline the superordinate role of left PPC for anticipatory motor control of either hand revealing a piece of evidence for left hemispheric PPC dominance. The observed effects cannot be explained by alterations of motor cortical excitability. P645 Case report: seizure induction by single-pulse transcranial magnetic stimulation after anodal transcranial direct current stimulation in a stroke patient S. Koganemaru1,2, A. Ikeda3, R. Takahashi3, H. Fukuyama1, T. Mima1 1Kyoto universtiy, Human Brain Research Center, Kyoto, Japan; 2Kyoto universtiy, Integrative Brain Science, Kyoto, Japan; 3Kyoto university, Neurology, Kyoto, Japan Transcranial direct current stimulation (tDCS) has been used as a safe therapeutic tool in neurological disorders, especially in stroke, because anodal stimulation given over the affected M1 area produces LTP-like effects on corticospinal excitability. Single-pulse transcranial magnetic stimulation (TMS) has been used as an evaluation tool of corticospinal excitability even in stroke, with a minimal risk of de novo seizure induction. Here, we report a 35 year-old male with postoperative subcortical hemorrhage in the left hemisphere, who developed a generalized convulsion starting from the affected upper limb after a single-pulse TMS subsequent to the anodal tDCS given over the ipsilesional M1 hand area for 10 minutes. He had no history of seizure or electroencephalogram (EEG) abnormality, and received several TMS tests without any side effects before tDCS intervention. EEG testing just after and 3 month after the event revealed no epileptiform discharges. He has been seizure-free at least for 3 months. The present case report raises the possibility that a single-pulse TMS after tDCS may be associated with a seizure risk in patients with brain lesion due to their accumulative excitatory effects. P646 Transcranial direct current stimulation (tDCS) in the treatment of adolescents with attention deficit hyperactivity disorder (ADHD) A. Sotnikova1, C. Soff1, K. Becker1, M. Siniatchkin2 1University of Marburg, Department of Child and Adolescents Psychiatry, Marburg, Germany; 2Goethe-University of Frankfurt, Department of Child and Adolescents Psychiatry, Frankfurt, Germany Question: The stimulation of the brain with weak direct current through the scalp (tDCS) is a safe, well tolerable and effective procedure which

Increased functional connectivity between dorsal posterior parietal and ventral occipitotemporal cortex during uncertain memory decisions

Retrieval of episodic memories is a multi-component act that relies on numerous operations ranging from processing the retrieval cue, evaluating retrieved information, and selecting the appropriate response given the demands of the task. Motivated by a rich functional neuroimaging literature, recent theorizing about various computations at retrieval has focused on the role of posterior parietal cortex (PPC). In a potentially promising line of research, recent neuroimaging findings suggest that different subregions of dorsal PPC respond distinctly to different aspects of retrieval decisions, suggesting that better understanding of their contributions might shed light on the component processes of retrieval. In an attempt to understand the basic operations performed by dorsal PPC, we used functional MRI and functional connectivity analyses to examine how activation in, and connectivity between, dorsal PPC and ventral temporal regions representing retrieval cues varies as a function of retrieval decision uncertainty. Specifically, participants made a five-point recognition confidence judgment for a series of old and new visually presented words. Consistent with prior studies, memory-related activity patterns dissociated across left dorsal PPC subregions, with activity in the lateral IPS tracking the degree to which participants perceived an item to be old, whereas activity in the SPL increased as a function of decision uncertainty. Importantly, whole-brain functional connectivity analyses further revealed that SPL activity was more strongly correlated with that in the visual word-form area during uncertain relative to certain decisions. These data suggest that the involvement of SPL during episodic retrieval reflects, at least in part, the processing of the retrieval cue, perhaps in service of attempts to increase the mnemonic evidence elicited by the cue.

Investigating the neural correlates of the Stroop effect with magnetoencephalography.

Reporting the ink color of a written word when it is itself a color name incongruent with the ink color (e.g. "red" printed in blue) induces a robust interference known as the Stroop effect. Although this effect has been the subject of numerous functional neuroimaging studies, its neuronal substrate is still a matter of debate. Here, we investigated the spatiotemporal dynamics of interference-related neural events using magnetoencephalography (MEG) and voxel-based analyses (SPM8). Evoked magnetic fields (EMFs) were acquired in 12 right-handed healthy subjects performing a color-word Stroop task. Behavioral results disclosed a classic interference effect with longer mean reaction times for incongruent than congruent stimuli. At the group level, EMFs' differences between incongruent and congruent trials spanned from 380 to 700ms post-stimulus onset. Underlying neural sources were identified in the left pre-supplementary motor area (pre-SMA) and in the left posterior parietal cortex (PPC) confirming the role of these regions in conflict processing.

Neuromodulation of parietal and motor activity affects motor planning and execution

Transcranial direct current stimulation (tDCS) is a non-invasive tool, which effectively modulates behavior, and related brain activity. When applied to the primary motor cortex (M1), tDCS affects motor function, enhancing or decreasing performance of both healthy participants and brain-damaged patients. Beyond M1, the posterior parietal cortex (PPC) is also crucially involved in controlling and guiding movement. Therefore, we explored whether the modulation of cortical excitability within PPC can also affect hand motor function in healthy right-handed participants. In Experiment 1, anodal tDCS (2 mA, 10 min) was applied to PPC and to M1 of both hemispheres. Skilled motor function of the non-dominant left hand, measured using the Jebsen–Taylor Hand Function Test (JTT), improved after anodal tDCS of the right, contralateral M1, as well as after the anodal stimulation of the left, ipsilateral PPC. Conversely, in Experiment 2, cathodal tDCS of the left PPC, or of the right M1, reduced motor performance of the left hand. Finally, Experiment 3 shows that the anodal tDCS of the left PPC selectively facilitated action planning, while the anodal tDCS of the right M1 modulated action execution only.This evidence shows that motor improvement induced by left parietal and right motor stimulations relies on substantial different mechanisms, opening up novel perspectives in the neurorehabilitation of stroke patients with motor and apraxic disorders.

Dissecting hemisphere-specific contributions to visual spatial imagery using parametric brain mapping

In the current study we aimed to empirically test previously proposed accounts of a division of labour between the left and right posterior parietal cortices during visuospatial mental imagery. The representation of mental images in the brain has been a topic of debate for several decades. Although the posterior parietal cortex is involved bilaterally, previous studies have postulated that hemispheric specialisation might result in a division of labour between the left and right parietal cortices. In the current fMRI study, we used an elaborated version of a behaviourally-controlled spatial imagery paradigm, the mental clock task, which involves mental image generation and a subsequent spatial comparison between two angles. By systematically varying the difference between the two angles that are mentally compared, we induced a symbolic distance effect: smaller differences between the two angles result in higher task difficulty. We employed parametrically weighed brain imaging to reveal brain areas showing a graded activation pattern in accordance with the induced distance effect. The parametric difficulty manipulation influenced behavioural data and brain activation patterns in a similar matter. Moreover, since this difficulty manipulation only starts to play a role from the angle comparison phase onwards, it allows for a top-down dissociation between the initial mental image formation, and the subsequent angle comparison phase of the spatial imagery task. Employing parametrically weighed fMRI analysis enabled us to top-down disentangle brain activation related to mental image formation, and activation reflecting spatial angle comparison. The results provide first empirical evidence for the repeatedly proposed division of labour between the left and right posterior parietal cortices during spatial imagery.

The enigma of Bálint's syndrome: neural substrates and cognitive deficits

The enigma of Bálint's syndrome: neural substrates and cognitive deficits

Distinct parietal sites mediate the influences of mood, arousal, and their interaction on human recognition memory

The two dimensions of emotion, mood valence and arousal, have independent effects on recognition memory. At present, however, it is not clear how those effects are reflected in the human brain. Previous research in this area has generally dealt with memory for emotionally valenced or arousing stimuli, but the manner in which interacting mood and arousal states modulate responses in memory substrates remains poorly understood. We investigated memory for emotionally neutral items while independently manipulating mood valence and arousal state by means of music exposure. Four emotional conditions were created: positive mood/high arousal, positive mood/low arousal, negative mood/high arousal, and negative mood/low arousal. We observed distinct effects of mood valence and arousal in parietal substrates of recognition memory. Positive mood increased activity in ventral posterior parietal cortex (PPC) and orbitofrontal cortex, whereas arousal condition modulated activity in dorsal PPC and the posterior cingulate. An interaction between valence and arousal was observed in left ventral PPC, notably in a parietal area distinct from the those identified for the main effects, with a stronger effect of mood on recognition memory responses here under conditions of relative high versus low arousal. We interpreted the PPC activations in terms of the attention-to-memory hypothesis: Increased arousal may lead to increased top-down control of memory, and hence dorsal PPC activation, whereas positive mood valence may result in increased activity in ventral PPC regions associated with bottom-up attention to memory. These findings indicate that distinct parietal sites mediate the influences of mood, arousal, and their interplay during recognition memory.

Cortical Hubs and Subcortical Cholinergic Pathways as Neural Substrates of Poststroke Dementia

A role of neural networks in the development of poststroke dementia has not been clearly established. We hypothesized that stroke-mediated disruption of subcortical cholinergic pathway or large-scale neural networks contributes to poststroke dementia. A matched case-control study was conducted in a predetermined cohort with acute ischemic stroke. Cases were defined as newly developed dementia diagnosed >3 months after stroke using the Korean Vascular Cognitive Impairment Harmonization Standards. Each case was matched to 2 controls for age, education, and initial stroke severity. The Cholinergic Pathways HyperIntensities Scale was applied with some modifications to characterize disruption of cholinergic pathways by acute stroke lesions. Involvement of major cortical hub locations of the default mode network, central executive network, and salience network was also investigated. After matching, 38 cases and 66 matched controls were included. Cholinergic Pathways HyperIntensities Scale scores were significantly higher in cases than in controls (2.2±2.9 versus 0.9±1.4). Acute ischemic lesions affecting the default mode and central executive networks were more frequently observed in cases compared with controls (36.8% versus 7.6% and 26.3% versus 6.1%, respectively). These findings remained significant in the multiple logistic regression models adjusted for various sets of potential confounders. Lesion location analysis revealed that cases were more likely to have acute lesions in the left corona radiata, hippocampal formation, and posterior parietal cortex. Disruption of cholinergic pathways and major hubs of large-scale neural networks might contribute to newly developed dementia after acute ischemic stroke.

Hand position-dependent modulation of errors in vibrotactile temporal order judgments: the effects of transcranial magnetic stimulation to the human posterior parietal cortex

The ability to decide which of the two stimuli is presented first can be probed using a temporal order judgment (TOJ) task. When the stimuli are delivered to the fingers, TOJ decisions can be confounded by the fact that the hands can be moved to different locations in space. How and where this confounded information is processed in the brain is poorly understood. In the present set of experiments, we addressed this knowledge gap by using single-pulse transcranial magnetic stimulation (TMS) to disrupt processing in the right or left posterior parietal cortex (PPC) during a vibrotactile TOJ task with stimuli applied to the right and left index fingers. In the first experiment, participants held their hands in an uncrossed configuration, and we found that when the index finger contralateral to the site of TMS was stimulated first, there was a significant increase in TOJ errors. This increase did not occur when stimuli were delivered to the ipsilateral finger first. In the second experiment, participants held their hands in a crossed configuration and the pattern of errors was reversed relative to the first experiment. In both the first two experiments, significant increases in TOJ error were present with TMS over either hemisphere, regardless of arm configuration; however, they were larger overall following TMS over the right PPC. Control experiments using sham TMS indicated the systematic modulation in error was not due to nonspecific effects of the stimulation. Additionally, we showed that these TMS-induced changes in TOJ errors were not due to a reduced ability to detect the timing of the vibrotactile stimuli. Taken together, these results demonstrate that both the right and left PPC contribute to the processing underlying vibrotactile TOJs by integrating vibrotactile information and proprioceptive information related to arm position in space.

Preliminary evidence for performance enhancement following parietal lobe stimulation in Developmental Dyscalculia

Nearly 7% of the population exhibit difficulties in dealing with numbers and performing arithmetic, a condition named Developmental Dyscalculia (DD), which significantly affects the educational and professional outcomes of these individuals, as it often persists into adulthood. Research has mainly focused on behavioral rehabilitation, while little is known about performance changes and neuroplasticity induced by the concurrent application of brain-behavioral approaches. It has been shown that numerical proficiency can be enhanced by applying a small—yet constant—current through the brain, a non-invasive technique named transcranial electrical stimulation (tES). Here we combined a numerical learning paradigm with transcranial direct current stimulation (tDCS) in two adults with DD to assess the potential benefits of this methodology to remediate their numerical difficulties. Subjects learned to associate artificial symbols to numerical quantities within the context of a trial and error paradigm, while tDCS was applied to the posterior parietal cortex (PPC). The first subject (DD1) received anodal stimulation to the right PPC and cathodal stimulation to the left PPC, which has been associated with numerical performance's improvements in healthy subjects. The second subject (DD2) received anodal stimulation to the left PPC and cathodal stimulation to the right PPC, which has been shown to impair numerical performance in healthy subjects. We examined two indices of numerical proficiency: (i) automaticity of number processing; and (ii) mapping of numbers onto space. Our results are opposite to previous findings with non-dyscalculic subjects. Only anodal stimulation to the left PPC improved both indices of numerical proficiency. These initial results represent an important step to inform the rehabilitation of developmental learning disabilities, and have relevant applications for basic and applied research in cognitive neuroscience, rehabilitation, and education.