Stimulation Of Posterior Parietal Cortex Research Articles

The influence of posterior parietal cortex on extrastriate visual activity: A concurrent TMS and fast optical imaging study

The posterior parietal cortex (PPC) is a critical node in attentional and saccadic eye movement networks of the cerebral cortex, exerting top-down control over activity in visual cortex. Here, we sought to further elucidate the properties of PPC feedback by providing a time-resolved map of functional connectivity between parietal and occipital cortex using single-pulse TMS to stimulate the left PPC while concurrently recording fast optical imaging data from bilateral occipital cortex. Magnetic stimulation of the PPC induced transient ipsilateral occipital activations (BA 18) 24–48ms post-TMS. Concurrent TMS and fast optical imaging results demonstrate a clear influence of PPC stimulation on activity within human extrastriate visual cortex and further extend this time- and space-resolved method for examining functional connectivity.

The effect of 10 Hz repetitive transcranial magnetic stimulation of posterior parietal cortex on visual attention.

Repetitive transcranial magnetic stimulation (rTMS) of the posterior parietal cortex (PPC) at frequencies lower than 5 Hz transiently inhibits the stimulated area. In healthy participants, such a protocol can induce a transient attentional bias to the visual hemifield ipsilateral to the stimulated hemisphere. This bias might be due to a relatively less active stimulated hemisphere and a relatively more active unstimulated hemisphere. In a previous study, Jin and Hilgetag (2008) tried to switch the attention bias from the hemifield ipsilateral to the hemifield contralateral to the stimulated hemisphere by applying high frequency rTMS. High frequency rTMS has been shown to excite, rather than inhibit, the stimulated brain area. However, the bias to the ipsilateral hemifield was still present. The participants’ performance decreased when stimuli were presented in the hemifield contralateral to the stimulation site. In the present study we tested if this unexpected result was related to the fact that participants were passively resting during stimulation rather than performing a task. Using a fully crossed factorial design, we compared the effects of high frequency rTMS applied during a visual detection task and high frequency rTMS during passive rest on the subsequent offline performance in the same detection task. Our results were mixed. After sham stimulation, performance was better after rest than after task. After active 10 Hz rTMS, participants’ performance was overall better after task than after rest. However, this effect did not reach statistical significance. The comparison of performance after rTMS with task and performance after sham stimulation with task showed that 10 Hz stimulation significantly improved performance in the whole visual field. Thus, although we found a trend to better performance after rTMS with task than after rTMS during rest, we could not reject the hypothesis that high frequency rTMS with task and high frequency rTMS during rest equally affect performance.

Transcranial direct current stimulation modulates spatial memory in cognitively intact adults

Two forms of spatial processing are involved in object location memory. Coordinate processing uses a fine-grained code to provide exact knowledge for the location and is believed dependent on the right posterior parietal cortex (PPC). Categorical processing relies on spatial relationships between objects and is believed dependent on the left PPC. We used transcranial direct current stimulation (tDCS) to test these brain–behavior relationships during the encoding and subsequent recall of object location associations. Twelve right-handed, healthy young participants received 20min of tDCS (2mA) during three separate sessions. Stimulation delivery was counterbalanced across participants and sessions and included anodal (“excitatory”) stimulation of right PPC with concurrent left PPC cathodal (“inhibitory”) stimulation (R+L−), the reverse montage (R−L+), and sham stimulation. Participants completed different versions of the Object Location Touchscreen Task (OLTT) during each session, which assesses coordinate (recall of the location without the environment) and categorical processing (recall of the location with the environment). Encoding occurred during the last 5min of stimulation, while the delay phase occurred 15min after stimulation. Participants performed more accurately during the coordinate phase following R−L+ stimulation when compared to R+L− performance. Categorical performance was not significantly affected by stimulation. Findings suggest two possibilities that will be examined in future studies with larger sample sizes: (1) The R−L+ facilitates left-hemisphere dominant categorical processing, the benefits of which persists even when environmental details are absent, possibly due to increased mental imagery; (2) Cathodal stimulation decreased spurious neuronal noise thereby allowing for more efficient processing by the “critical” neuronal populations in the right PPC.

Activation and inhibition of posterior parietal cortex have bi-directional effects on spatial errors following interruptions.

Interruptions to ongoing mental activities are omnipresent in our modern digital world, but the brain networks involved in interrupted performance are not known, nor have the activation of those networks been modulated. Errors following interruptions reflect failures in spatial memory, whose maintenance is supported by a brain network including the right posterior parietal cortex (PPC). The present study therefore used bi-directional transcranial Direct Current Stimulation (tDCS) of right PPC to examine the neuromodulation of spatial errors following interruptions, as well as performance on another PPC-dependent task, mental rotation. Anodal stimulation significantly reduced the number of interruption-based errors and increased mental rotation accuracy whereas cathodal stimulation significantly increased errors and reduced mental rotation accuracy. The results provide evidence for a causal role of the PPC in the maintenance of spatial representations during interrupted task performance.

Improving ideomotor limb apraxia by electrical stimulation of the left posterior parietal cortex.

Limb apraxia, a deficit of planning voluntary gestures, is most frequently caused by damage to the left hemisphere, where, according to an influential neurofunctional model, gestures are planned, before being executed through the motor cortex of the hemisphere contralateral to the acting hand. We used anodal transcranial direct current stimulation delivered to the left posterior parietal cortex (PPC), the right motor cortex (M1), and a sham stimulation condition, to modulate the ability of six left-brain-damaged patients with ideomotor apraxia, and six healthy control subjects, to imitate hand gestures, and to perform skilled hand movements using the left hand. Transcranial direct current stimulation delivered to the left PPC reduced the time required to perform skilled movements, and planning, but not execution, times in imitating gestures, in both patients and controls. In patients, the amount of decrease of planning times brought about by left PPC transcranial direct current stimulation was influenced by the size of the parietal lobe damage, with a larger parietal damage being associated with a smaller improvement. Of interest from a clinical perspective, left PPC stimulation also ameliorated accuracy in imitating hand gestures in patients. Instead, transcranial direct current stimulation to the right M1 diminished execution, but not planning, times in both patients and healthy controls. In conclusion, by using a transcranial stimulation approach, we temporarily improved ideomotor apraxia in the left hand of left-brain-damaged patients, showing a role of the left PPC in planning gestures. This evidence opens up novel perspectives for the use of transcranial direct current stimulation in the rehabilitation of limb apraxia.

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.

Training Transfers the Limits on Perception from Parietal to Ventral Cortex

SummaryVisually guided behavior depends on (1) extracting and (2) discriminating signals from complex retinal inputs, and these perceptual skills improve with practice [1]. For instance, training on aerial reconnaissance facilitated World War II Allied military operations [2]; analysts pored over stereoscopic photographs, becoming expert at (1) segmenting pictures into meaningful items to break camouflage from (noisy) backgrounds, and (2) discriminating fine details to distinguish V-weapons from innocuous pylons. Training is understood to optimize neural circuits that process scene features (e.g., orientation) for particular purposes (e.g., judging position) [3, 4, 5, 6]. Yet learning is most beneficial when it generalizes to other settings [7, 8] and is critical in recovery after adversity [9], challenging understanding of the circuitry involved. Here we used repetitive transcranial magnetic stimulation (rTMS) to infer the functional organization supporting learning generalization in the human brain. First, we show dissociable contributions of the posterior parietal cortex (PPC) versus lateral occipital (LO) circuits: extracting targets from noise is disrupted by PPC stimulation, in contrast to judging feature differences, which is affected by LO rTMS. Then, we demonstrate that training causes striking changes in this circuit: after feature training, identifying a target in noise is not disrupted by PPC stimulation but instead by LO stimulation. This indicates that training shifts the limits on perception from parietal to ventral brain regions and identifies a critical neural circuit for visual learning. We suggest that generalization is implemented by supplanting dynamic processing conducted in the PPC with specific feature templates stored in the ventral cortex.

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.

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

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.

P 141. Global/local attention: Effects of tDCS on brain dynamics

Introduction Posterior parietal cortex (PPC) has been identified as a critical region for the control of spatial and non-spatial attention. Hierarchical letter stimuli (large letters composed of smaller letters) have been used to investigate the contribution of PPC to global/local processing of features in the visual field, and also to the switching of attention between the global and local level. Objectives We utilized transcranial direct current stimulation (tDCS) and high-density EEG to characterize brain dynamics during global/local attentional switching. Materials and methods 128-channel EEG data was recorded from 14 healthy, right-handed adults who made motor responses to hierarchical letter stimuli. A cue before each block of 4–8 hierarchical stimuli directed attention to either global or local features. tDCS was applied over left posterior parietal cortex (electrode P3, forearm reference; 2mA for 20min) during separate anodal, cathodal and sham stimulation sessions. The EEG data were analyzed using Brainstorm (Tadel et al., 2011) to extract brain-based activation waveforms for regions defined in the BrainVisa Tzourio-Mazoyer Atlas. Time–frequency plots were computed for left and right superior and inferior parietal and mid occipital corteces. Results Behavioral responses differed for the first stimuli following switching of attention between global/local features, but not for visual processing per se . Attentional shifts from local-to-global features were degraded for at least 20min following anodal stimulation (Stone and Tesche, 2009). Practice effects were observed in the theta-band between baseline data and data recorded immediately after sham stimulation for both G/L cues to switch attention and for the first compound letter stimuli after the G/L cues. In left PPC, anodal and cathodal stimulation increased theta for global-to-local switches for the first hierarchical letters after a cue to switch, whereas anodal increased and cathodal decreased theta for local-to-global switches. In right PPC, anodal tDCS increased and cathodal tDCS decreased theta for both global-to-local and local-to-global switches. Differential time–frequency activations were observed in left (but not right) mid occipital cortex following tDCS; anodal stimulation enhanced theta/alpha activity for local-to-global, but not global-to-local, switches. Conclusion The data suggest that tDCS of left PPC induces changes in theta-band activity in a distributed network which includes left and right PPC and occipital cortex, and that theta-band activity may be salient for PPC control of early(100–300ms) global/local processing.

1 Hz rTMS of the left posterior parietal cortex (PPC) modifies sensorimotor timing

In order to investigate the relevance of the left posterior parietal cortex (PPC) for precise sensorimotor timing we applied 1Hz repetitive transcranial magnetic stimulation (rTMS) over left PPC, right PPC and visual cortex of healthy participants for 10min, respectively. The impact on sensorimotor timing of the right hand was assessed using a synchronization task that required subjects to synchronize their right index finger taps with respect to constant auditory, visual or auditory–visual pacing. Our results reveal reduced negative tap-to-pacer asynchronies following rTMS of the left PPC in all pacing conditions. This effect lasted for about 5min after cessation of rTMS. Right PPC and visual cortex stimulation did not yield any significant behavioural effects. Since suppression of left PPC modified right-hand synchronization accuracy independent of the pacing signal's modality, the present data support the significance of left PPC for anticipatory motor control over a primary role in multisensory integration. The present data suggest that 1Hz rTMS might interrupt a matching process of anticipated and real sensorimotor feedback within PPC. Alternatively, downregulation of left PPC activity may affect M1 excitability via functional connections leading to a delay in motor output and, thus, smaller tap-to-pacer asynchronies.

Optical imaging in galagos reveals parietal–frontal circuits underlying motor behavior

The posterior parietal cortex (PPC) of monkeys and prosimian galagos contains a number of subregions where complex, behaviorally meaningful movements, such as reaching, grasping, and body defense, can be evoked by electrical stimulation with long trains of electrical pulses through microelectrodes. Shorter trains of pulses evoke no or simple movements. One possibility for the difference in effectiveness of intracortical microstimulation is that long trains activate much larger regions of the brain. Here, we show that long-train stimulation of PPC does not activate widespread regions of frontal motor and premotor cortex but instead, produces focal, somatotopically appropriate activations of frontal motor and premotor cortex. Shorter stimulation trains activate the same frontal foci but less strongly, showing that longer stimulus trains do not produce less specification. Because the activated sites in frontal cortex correspond to the locations of direct parietal-frontal anatomical connections from the stimulated PPC subregions, the results show the usefulness of optical imaging in conjunction with electrical stimulation in showing functional pathways between nodes in behavior-specific cortical networks. Thus, long-train stimulation is effective in evoking ethologically relevant sequences of movements by activating nodes in a cortical network for a behaviorally relevant period rather than spreading activation in a nonspecific manner.

Dissociating the contributions of human frontal eye fields and posterior parietal cortex to visual search

Imaging, lesion, and transcranial magnetic stimulation (TMS) studies have implicated a number of regions of the brain in searching for a target defined by a combination of attributes. The necessity of both frontal eye fields (FEF) and posterior parietal cortex (PPC) in task performance has been shown by the application of TMS over these regions. The effects of stimulation over these two areas have, thus far, proved to be remarkably similar and the only dissociation reported being in the timing of their involvement. We tested the hypotheses that 1) FEF contributes to performance in terms of visual target detection (possibly by modulation of activity in extrastriate areas with respect to the target), and 2) PPC is involved in translation of visual information for action. We used a task where the presence (and location) of the target was indicated by an eye movement. Task disruption was seen with FEF TMS (with reduced accuracy on the task) but not with PPC stimulation. When a search task requiring a manual response was presented, disruption with PPC TMS was seen. These results show dissociation of FEF and PPC contributions to visual search performance and that PPC involvement seems to be dependent on the response required by the task, whereas this is not the case for FEF. This supports the idea of FEF involvement in visual processes in a manner that might not depend on the required response, whereas PPC seems to be involved when a manual motor response to a stimulus is required.

Organization of the posterior parietal cortex in galagos: I. Functional zones identified by microstimulation

We used half-second trains of intracortical microstimulation to study the functional organization of the posterior parietal cortex (PPC) in prosimian galagos. These trains of current pulses evoked meaningful behaviors from the anterior, but not posterior, half of PPC. Stimulation of dorsal PPC caused contralateral forelimb movements, including defensive, hand-to-mouth, and reaching movements. Defensive and hand-to-mouth movement territories overlapped, although hand-to-mouth movements were usually evoked from more rostrolateral sites than defensive movements. Reaching movement sites were typically more caudal than defensive or hand-to-mouth movement sites. Stimulation of the most medial PPC sites evoked complex movements of forelimbs and hindlimbs. Ventral PPC commonly represented defensive face movements. Similar defensive movements, with the addition of widely opening the mouth to expose the teeth, were elicited from a small area in front of the PPC defensive face zone. Sometimes defensive face movements occurred with forelimb movements. Thus, subregions of PPC relate to different ethologically relevant categories of behavior. Most movements were initiated within 33-100 msec after stimulus onset. Face, eye blink, and ear movements were generally less delayed than forelimb movements. The present results in galagos, together with those obtained from macaque monkeys by Graziano and coworkers (Graziano et al. [2002a] Neuron 34:841-851; Cooke et al., [2003] Proc. Natl. Acad. Sci. U.S.A. 100:6163-6168), suggest that the functional involvement of the PPC in specific types of sensorimotor behavior evolved early in the course of primate evolution and that networks for complex movements involving motor and posterior parietal areas are characteristic of all primate brains.

Interhemispheric balance of overt attention: a theta burst stimulation study

Interhemispheric imbalance is discussed as a pathophysiological mechanism in visuospatial neglect. It is suggested that after a lesion of the right hemisphere the mutual transcallosal inhibition is impaired, resulting in an increased activity of the left hemisphere. We investigated the interhemispheric balance of attention in healthy subjects by using a free visual exploration task and by interfering with the neural activity of the posterior parietal cortex (PPC) of either hemisphere using an inhibitory transcranial magnetic stimulation routine with theta burst stimulation (TBS). Subjects explored colour photographs of real-life scenes presented on a computer screen under four conditions: (i) without TBS; (ii) after TBS over the right PPC; (iii) after TBS over the left PPC; and (iv) after TBS over the right PPC and, after the first half of the task, over the left PPC. Eye movements were measured, and distribution of mean cumulative fixation duration over screen halves was analyzed. TBS over the right PPC resulted in a significant rightward shift of mean cumulative fixation duration of approximately 30 min. The shift could be reversed when a subsequent train of TBS was applied over the left PPC. However, left PPC stimulation alone had no significant effect on visual exploration behaviour. The present study shows that the effect of TBS on the PPC depends on which hemisphere is stimulated and on the state of the contralateral homologue area. These findings are in accordance with the predictions of the interhemispheric rivalry model in neglect.

The timing of the involvement of the frontal eye fields and posterior parietal cortex in visual search

The frontal eye fields (FEFs) and posterior parietal cortex (PPC) are important for target detection in conjunction visual search but the relative timings of their contribution have not been compared directly. We addressed this using temporally specific double pulse transcranial magnetic stimulation delivered at different times over FEFs and PPC during performance of a visual search task. Disruption of performance was earlier (0/40 ms) with FEF stimulation than with PPC stimulation (120/160 ms), revealing a clear and substantial temporal dissociation of the involvement of these two areas in conjunction visual search. We discuss these timings with reference to the respective roles of FEF and PPC in the modulation of extrastriate visual areas and selection of responses.

TMS can reveal contrasting functions of the dorsal and ventral visual processing streams

In order to investigate the functional specificity of the dorsal and ventral visual processing steams we used transcranial magnetic stimulation (TMS) to briefly disrupt one or the other while subjects performed three tasks, involving discrimination of colour or shape or relative position. TMS was delivered over right posterior parietal cortex (PPC) or right lateral occipital (LO) cortex, regions known to have visuo-spatial and object processing properties respectively. LO but not PPC stimulation had a significant effect on reaction time when subjects were asked to make a discrimination of relative shape. PPC stimulation had a significant effect when subjects were asked to discriminate relative position of the same shapes. Stimulation of LO also lengthened reaction times on the position task. There were no effects of stimulation at either site on colour discrimination. Results are discussed within the framework of how the dorsal stream and ventral stream are dissociated following their damage in neurological patients and possible ways in which they may interact in the normal brain.

Differential Effects of Transcranial Magnetic Stimulation of Left and Right Posterior Parietal Cortex on Mental Rotation Tasks

A recently published study used the interference strategy of transcranial magnetic stimulation (TMS) to demonstrate the role of the right posterior parietal cortex (PPC) in the mental rotation of alphanumeric stimuli. We used similar stimulation parameters over the same left and right PPC regions, and examined the ability to rotate more complex 3D Shepard and Metzler (1971) images. There was reduced accuracy of performance with both right and left PPC stimulation for different angles of rotation of the visual stimuli. Right PPC stimulation led to reduced accuracy to rotate stimuli by 120°, whereas left PPC stimulation affected 180° rotation. We hypothesise that the two hemispheres make different contributions to the processing underlying visuospatial mental imagery: the right PPC is important for spatial rotations through smaller angles; the left hemisphere has a unique role when the stimuli to be compared are rotated through 180°, a task that engages verbal strategies due to the well-documented special nature of enantiomorphs.

Hemispheric lateralization of number comparison

In order to clarify the respective contribution of the right and left posterior parietal cortex (PPC) to number comparison, transcranial magnetic stimulation (TMS) was used to disrupt PPC processing in subjects instructed to determine whether a digit was smaller or larger than 5. Single pulse TMS was applied over the PPC, either unilaterally or bilaterally, 150, 200, or 250 ms after digit presentation. Sham TMS was used as a control condition to take into account the unspecific effects of TMS on reaction time (RT). The main finding of the present study is a significant increase in RTs when comparing digits close to 5 following a disruption either of the left PPC alone or of both PPC simultaneously. The comparison of digits far from 5 was unaltered by disrupting only one PPC but RTs were found increased after bilateral PPC stimulation. These disruptive effects were observed irrespective of the TMS delay. We concluded that coding precise numerical values requires the integrity of the left PPC, as suggested by the deficit in discriminating close digits consequent to its disruption. In contrast, approximate comparisons can be processed either by the left or right PPC, since simultaneous bilateral TMS was needed to alter the comparison of digits far from 5.