Anterior and posterior association cortex contributions to the somatosensory P300

A P300 (P3)-evoked response is generated in a variety of mammalian species upon detection of significant environmental events. The P3 component has been proposed to index a neural system involved in attention and memory capacity. We investigated the contribution of anterior and posterior association cortex to somatosensory P3 generation. Somatosensory event-related potentials (ERPs) were recorded in controls (n = 10) and patients with unilateral lesions in temporal-parietal junction (n = 8), lateral parietal cortex (n = 8), or dorsolateral frontal cortex (n = 10). Subjects pressed a button to mechanical taps of the fifth finger (targets; p = 0.12), randomly interposed in sequences of taps to the second (standards; p = 0.76) and the third or fourth finger (tactile novels; p = 0.06). Occasional shock stimuli were delivered to the wrist (shock novels; p = 0.06). The scalp-recorded P3 was differentially affected by anterior and posterior association cortex lesions. Subjects with temporal-parietal lesions showed markedly reduced P3s to all types of stimuli at all scalp locations. The reductions were largest at the parietal electrode site over the lesioned hemisphere. Parietal patients had normal P3s for all stimulus types except for contralateral shock novels, which generated reduced P3s. Frontal lesions had reductions of the novelty P3 over frontal sites with minimal changes in the target P3. The data support the existence of multiple intracranial P3 sources. The data further indicate that association cortex in the temporal-parietal junction is critical for generating the scalp-recorded target and novelty P3s, whereas dorsolateral frontal cortex contributes preferentially to novelty P3 generation. The N2 component was reduced by parietal and frontal lesions in patients who had intact target P3s, suggesting that different neural systems underlie N2 and P3 generation.     

P300 generation by novel somatosensory stimuli.

Event-related potentials (ERPs) to task-relevant target and task-irrelevant novel stimuli were recorded in a somatosensory discrimination task. Subjects pressed a button to mechanical taps of the fifth finger (targets, P = 0.12), randomly interposed in sequences of taps to the second finger (standards, P = 0.76). Two types of infrequent novel stimuli were delivered; one was a mechanical tap to the third or fourth finger (tactile novels, P = 0.06), another was an electric shock at the wrist (shock novels, P = 0.06). Correctly detected targets generated a parietal maximal P300 (P3b, latency 335 msec). Shock novels generated a central maximal P300 with a shorter peak latency (298 msec) than the P3b. Tactile novels generated a P300 with a scalp distribution comparable to the shock novels. Unlike the P3b, P300 amplitude to both the shock and tactile novel stimuli habituated by 20-30% across the first several stimulus presentations. These results indicate that, similar to the auditory and visual modality, task-irrelevant novel somatosensory stimuli generate a novelty P300 ERP. Differences in scalp distribution, latency and habituation characteristics suggest that the novelty P300 may have contributions from intracranial generators independent from target P300 sources.     

Event-related brain potentials in response to novel sounds in dementia.

OBJECTIVE: Non-target, deviant stimuli generate an earlier latency, front-central novelty P3, whereas correctly detected task-relevant stimuli generate a parietal maximal target P3. We examined whether the P3 component to novel stimuli is affected by dementing processes, and is therefore useful for distinguishing Alzheimer's type dementia (AD) from vascular dementia (VD). METHODS: We recorded ERPs to task-relevant stimuli (target P3) and novel task-irrelevant stimuli (novelty P3) in an auditory oddball task in AD (n = 16), VD (n = 16), and age-matched controls (n = 18). The amplitude, latency, and scalp topography of target and novelty P3 were compared among 3 groups using ANOVA. The relationship between P3 measures and intelligence scores were evaluated by correlation analysis. RESULTS: The amplitude, latency and scalp topography of the target P3 were comparably affected by both AD and VD. However, the amplitude of the novelty P3 was markedly reduced in VD, but not in AD, and the scalp topographics were different in the 3 groups. The amplitude was maximal at frontal sites in controls, at central sites in AD, and at parietal sites in VD. The target P3 latency was prolonged in both AD and VD, whereas the novelty P3 latency was only prolonged in VD. AD was discriminated satisfactorily from VD by using the novelty amplitude at Cz and the ratio of the amplitudes at Fz and Pz as independent variables. CONCLUSIONS: These results suggest that the response to novel stimuli is differentially affected by dementia with degenerative and vascular etiology.     

Hemispheric asymmetry of event-related potentials in a patient with callosal disconnection syndrome: a comparison of auditory,visual and somatosensory modalities

To investigate whether callosal lesions affect the distribution of event-related potentials (ERP) between the two hemispheres and whether hemispheric ERP distribution differs among sensory modalities, a patient with interhemispheric disconnection syndrome and 47 controls were subjected to an oddball paradigm. High (target) and low tone bursts for auditory, red (target) and green lights for visual and electrical stimuli delivered to the index (target) or fifth finger for somatosensory ERPs were presented to the unilateral ear, visual field and hand, respectively. The subjects were instructed to press a button with the hand on the stimulated side. The results showed that the hemispheric asymmetry of the patient's auditory ERPs was not significantly different from that of the controls, regardless of which ear was stimulated. In contrast, the visual and somatosensory ERPs showed a delay of the P3 latency and an attenuation of the N1-P2 and N2-P3 amplitude over the hemisphere ipsilateral to the stimulus, regardless of the stimulated side. These findings suggest that the source of P3 generation is relatively lateralized to the hemisphere contralateral to the stimulus, and that the callosal transfer of visual and somatosensory information is involved in the P3 generation in the hemisphere ipsilateral to the stimulus.     

Contributions of temporal-parietal junction to the human auditory P3

The P3 component of the event-related potential (ERP) is generated in humans and other mammalian species when attention is drawn to infrequent stimuli. We assessed the role of subregions of human posterior association cortex in auditory P3 generation in groups of patients with focal cortical lesions. Auditory P3s were recorded to target (P3b) and unexpected novel stimuli (P3a) in monaural and dichotic signal detection experiments. Two groups of patients were studied with lesions of: (1) temporal-parietal junction including posterior superior temporal plane and adjacent caudal inferior parietal cortex; and (2) the lateral parietal lobe including the rostral inferior parietal lobe and portions of superior parietal lobe. Extensive lateral parietal cortex lesions had no effect on the P3. In contrast, discrete unilateral lesions centered in the posterior superior temporal plane eliminated both the auditory P3b and P3a at electrodes over the posterior scalp. The results indicate that auditory association cortex in the human temporal-parietal junction is critical for auditory P3 generation.     

接触性热痛诱发电位在多发性硬化患者中的应用

目的 建立接触性热痛诱发电位(CHEP)标准,并对多发性硬化(MS)患者进行痛温觉传导通路功能评价,评价CHEP在MS中的应用价值.方法 选取确诊的MS患者36例和健康人40名,应用接触性热痛诱发电位刺激器(Medoc,Israel),在2个强度水平(47、51℃)应用可调节脉冲,刺激部位选择右上肢前臂掌侧前1/3处、右下肢内踝上皮肤和腰部.受试者在每次刺激后,对刺激强度分级.以Keypoint.net仪器记录,记录点为Cz.测定刺激强度和疼痛分级的关系、诱发电位的主要成分的潜伏期和波幅.结果 健康对照组予以47℃和51℃分别刺激上下肢,CHEP的引出率为100%;MS组上肢4例,下肢5例未引出肯定CHEP波形.我们通过对下肢和腰部刺激计算得出A8纤维传导速度为(18.1±7.3)m/s.MS患者存在痛温觉减退症状的上肢21例,下肢29例,其反应性疼痛分级[视觉模拟评分(VAS),上肢6.1±0.9,下肢5.6±1.3]较对照组(上肢8.0±0.7,下肢7.9±0.7)低,差异有统计学意义(Z=-3.249、-5.272,P＜0.01).存在痛温觉障碍且能够诱发出CHEP波形(上肢17例,下肢24例)的上下肢N波潜伏期[上肢(387.3±34.2)ms,下肢(489.9±70.2)ms]较对照组[上肢(346.0±25.5)ms,下肢(400.8±24.4)ms]明显延长(t=4.790、4.798,P＜0.01)、N-P波幅[上肢(30.5±12.8)μV,下肢(28.2±16.2)μV]较对照组[上肢(49.3±16.0)μV,下肢(42.2±16.7)μV]明显减低(t=-4.612、-3.144,P＜0.01).MS患者下肢CHEP检测的异常率(26/36,72.2%)高于上肢(16/36,44.4%,P=0.031)和体感诱发电位(19/36,52.8%,χ~2=4.261,P=0.039).上肢未存在痛温觉障碍的15例MS患者中,有3例CHEP异常,下肢7例中有2例CHEP异常.结论 CHEP提供了一种临床实用的、非侵入性的客观检查方法,它与侧重于结构异常的MRI不同,重点检测伤害性通路的功能改变,所以结合MRI和其他诱发电位能够更有效的辅助诊断MS,评价伤害性通路的情况,揭示亚临床病灶的存在.     

Abnormalities of P300 cortical current density in unmedicated depressed patients revealed by LORETA analysis of event-related potentials

The purpose of the present study was to investigate the neural substrates underlying event-related potential (ERP) abnormalities, with respect to the generators of the ERP components in depressed patients. Using an oddball paradigm, ERP from auditory stimuli were recorded from 22 unmedicated patients with current depressive episodes and compared with those from 22 age- and gender-matched normal controls. Cortical current densities of the N100 and P300 components were analyzed using low-resolution electromagnetic tomography (LORETA). Group differences in cortical current density were mapped on a 3-D cortex model. The results revealed that N100 cortical current densities did not differ between the two groups, while P300 cortical current densities were significantly lower in depressed patients over the bilateral temporal lobes, the left frontal region, and the right temporal-parietal area. Furthermore, the cortical area in which the group difference in P300 current density had been identified was remarkably larger over the right than the left hemisphere, thus supporting the hypothesis of right hemisphere dysfunction in depression.     

Predictive value of novel stimuli modifies visual event-related potentials and behavior.

OBJECTIVE: We examined how behavioral context influences novelty processing by varying the degree that a novel event predicted the occurrence of a subsequent target stimulus. METHODS: Visual event-related potentials (ERPs) and reaction times (RTs) were recorded in 3 detection experiments (23 subjects). The predictive value of a novel stimulus on the occurrence of a subsequent target was varied as was novel-target pairing intervals (200-900 ms). In Experiment 1, novel stimuli always preceded a target, in Experiment 2, 40% of novel stimuli were followed by a target, and in Experiment 3, novel stimuli occurred randomly. RESULTS: In Experiment 1, RTs following 100% predictive novels were shortened for targets at all spatial locations and novel-target pairing intervals. Novel stimuli predicting a target generated a central negativity peaking at 300 ms and reduced P3a and P3b ERPs. In Experiments 2 and 3, target RTs were prolonged only when novel and target stimuli were presented in the same spatial location at short ISIs (200 ms). The central novel N2 was smaller in amplitude in comparison to Experiment 1, and novelty P3a and target extrastriate N2 and posterior scalp P3b ERPs were enhanced. CONCLUSIONS: The enhanced N2 for 100% predictive novel stimuli appears to index an alerting system facilitating behavioral detection. The same novel stimuli with no predictive value distract attention and generate a different ERP pattern characterized by increased novelty P3a and target P3b responses. The results indicate that behavioral context determines how novel stimuli are processed and influence behavior.     

Contribution of subregions of human frontal cortex to novelty processing

Novelty processing was studied in patients with lesions centered in either OFC or lateral pFC (LPFC). An auditory novelty oddball ERP paradigm was applied with environmental sounds serving as task irrelevant novel stimuli. Lesions to the LPFC as well as the OFC resulted in a reduction of the frontal Novelty P3 response, supporting a key role of both frontal subdivisions in novelty processing. The posterior P3b to target sounds was unaffected in patients with frontal lobe lesions in either location, indicating intact posterior cortical target detection mechanisms. LPFC patients displayed an enhanced sustained negative slow wave (NSW) to novel sounds not observed in OFC patients, indicating prolonged resource allocation to task-irrelevant stimuli after LPFC damage. Both patient groups displayed an enhanced NSW to targets relative to controls. However, there was no difference in behavior between patients and controls suggesting that the enhanced NSW to targets may index an increased resource allocation to response requirements enabling comparable performance in the frontal lesioned patients. The current findings indicate that the LPFC and OFC have partly shared and partly differential contributions to the cognitive subcomponents of novelty processing.     

High-frequency magnetic oscillations evoked by posterior tibial nerve stimulation

Magnetocephalographic recordings of the primary somatosensory response (P37m) and high-frequency oscillations (HFOs) evoked by posterior tibial nerve stimulation were obtained in normal subjects. Electrical stimuli were delivered to the posterior tibial nerve and magnetic recordings were taken over the superior aspect of the left hemisphere with a 37-channel biomagnetometer. In order to separate the high-frequency oscillations from the underlying P37m, the wide-band (0.1-1200 Hz) recorded responses were digitally filtered with a 500-800 Hz band-pass filter. The localization of the HFOs were estimated to be in somatosensory area 3b, very close to the P37m source. Our data suggest that the HFOs are somatotopically arranged in the primary somatosensory cortex, and are a ubiquitous phenomenon of the primary somatosensory cortex.     

What is novel in the novelty oddball paradigm? Functional significance of the novelty P3 event-related potential as revealed by independent component analysis

To better understand whether voluntary attention affects how the brain processes novel events, variants of the auditory novelty oddball paradigm were presented to two different groups of human volunteers. One group of subjects (n=16) silently counted rarely presented 'infrequent' tones (p=0.10), interspersed with 'novel' task-irrelevant unique environmental sounds (p=0.10) and frequently presented 'standard' tones (p=0.80). A second group of subjects (n=17) silently counted the 'novel' environmental sounds, the 'infrequent' tones now serving as the task-irrelevant deviant events. Analysis of event-related potentials (ERPs) recorded from 63 scalp channels suggested a spatiotemporal overlap of fronto-central novelty P3 and centro-parietal P3 (P3b) ERP features in both groups. Application of independent component analysis (ICA) to concatenated single trials revealed two independent component clusters that accounted for portions of the novelty P3 and P3b response features, respectively. The P3b-related ICA cluster contributed to the novelty P3 amplitude response to novel environmental sounds. In contrast to the scalp ERPs, the amplitude of the novelty P3 related cluster was not affected by voluntary attention, that is, by the target/nontarget distinction. This result demonstrates the usefulness of ICA for disentangling spatiotemporally overlapping ERP processes and provides evidence that task irrelevance is not a necessary feature of novelty processing.     

Pathophysiology underlying diminished attention to novel events in patients with early AD

BACKGROUND: Patients with mild to moderate AD often are apathetic and fail to attend to novel aspects of their environment. OBJECTIVE: To investigate the mechanisms underlying these changes by studying the novelty P3 response that measures shifts of attention toward novel events. METHODS: While event-related potentials were recorded, mildly impaired AD patients and matched normal controls (NC) viewed line drawings that included a repetitive background stimulus, an infrequent target stimulus, and infrequent novel stimuli. Subjects controlled how long they viewed each stimulus by pressing a button. This served as a measure of their allocation of attention. They also responded to targets by depressing a foot pedal. Patients did not differ from NC in age, education, estimated IQ, or mood but were judged by informants to be more apathetic. RESULTS: P3 amplitude to novel stimuli was significantly smaller for AD patients than NC. However, P3 amplitude to target stimuli did not differ between groups. For NC, P3 response to novel stimuli was much larger than to background stimuli. In contrast, for patients with AD, there was no difference in P3 response to novel vs background stimuli. Although NC spent more time looking at novel than background stimuli, patients with AD distributed their viewing time evenly. Remarkably, for patients with AD, the amplitude of the novelty P3 response powerfully predicted how long they would spend looking at novel stimuli (R2 = 0.52) and inversely correlated with apathy severity. CONCLUSIONS: The decreased attention to novel events exhibited by patients with AD cannot be explained by a nonspecific reduction in their attentional abilities. The novelty P3 response is markedly diminished in mild AD, at a time when the target P3 response is preserved. The disruption of the novelty P3 response predicts diminished attention to novel stimuli and is associated with the apathy exhibited by patients with AD.     

Impaired novelty detection and frontal lobe dysfunction in Parkinson's disease

Recent evidence suggests that the frontal lobe plays an important role in an orienting response to novel events, and that frontal lobe dysfunction is linked to attentional and cognitive deficits in Parkinson's disease (PD). We tested the hypothesis that the neural network involved in novelty detection may be impaired in PD patients by studying event-related brain potentials to target and novel stimuli and their correlation to performance in neuropsychological tests in non-demented PD patients. The PD patients showed prolonged P3 latency to novel stimuli compared with age-matched controls, whereas their P3 latency to target stimuli was not different from that in controls. The PD patients also manifested amplitude reduction and less habituation of the P3 to novel stimuli over frontal scalp sites compared with controls. The prolonged latency and frontal reduction of novelty P3 correlated with a poor performance in the Wisconsin Card Sorting Test. These results suggest that the orienting response of PD patients to novel events is impaired and that recording novelty P3 might provide a neurophysiological and quantitative measure of attentional and cognitive deficits linked to the frontal lobe in non-demented PD patients.     

Scalp topography of the short latency somatosensory evoked potentials following posterior tibial nerve stimulation in man

The scalp topography of the short latency somatosensory evoked potentials (SEPs) to unilateral posterior tibial nerve stimulation at the ankle was studied by using a non-cephalic reference in 22 normal young adults. At least 3 components (P28, N31 and N32) were identified preceding the major positive peak (P36). The first 2 components had similar peak latency at all scalp electrodes, and were considered to be generated in deep structures. However, N32 was localized to the hemisphere contralateral to the side of stimulation. P36 was maximal at the midline foot sensory area, or at the contralateral parasagittal area, and its amplitude decreased more steeply anteriorly than posteriorly. The peak latency of P36 progressively increased from ipsilateral to the side of stimulation in the coronal plane. P36 occurred earlier in the somatosensory area, and increased in peak latency anteriorly. Generator source of scalp-recorded far-field potentials (P28 and N31) remains to be elucidated. N32 might reflect activities of the thalamo-cortical pathway or an initial cortical response. P36 appeared to be generated in the somatosensory foot area.     

Frontal and parietal components of a cerebral network mediating voluntary attention to novel events

Despite the important role that attending to novel events plays in human behavior, there is limited information about the neuroanatomical underpinnings of this vital activity. This study investigated the relative contributions of the frontal and posterior parietal lobes to the differential processing of novel and target stimuli under an experimental condition in which subjects actively directed attention to novel events. Event-related potentials were recorded from well-matched frontal patients, parietal patients, and non-brain-injured subjects who controlled their viewing duration (by button press) of line drawings that included a frequent, repetitive background stimulus, an infrequent target stimulus, and infrequent, novel visual stimuli. Subjects also responded to target stimuli by pressing a foot pedal. Damage to the frontal cortex resulted in a much greater disruption of response to novel stimuli than to designated targets. Frontal patients exhibited a widely distributed, profound reduction of the novelty P3 response and a marked diminution of the viewing duration of novel events. In contrast, damage to posterior parietal lobes was associated with a substantial reduction of both target P3 and novelty P3 amplitude; however, there was less disruption of the processing of novel than of target stimuli. We conclude that two nodes of the neuroanatomical network for responding to and processing novelty are the prefrontal and posterior parietal regions, which participate in the voluntary allocation of attention to novel events. Injury to this network is indexed by reduced novelty P3 amplitude, which is tightly associated with diminished attention to novel stimuli. The prefrontal cortex may serve as the central node in determining the allocation of attentional resources to novel events, whereas the posterior parietal lobe may provide the neural substrate for the dynamic process of updating one's internal model of the environment to take into account a novel event.     

Somatosensory event-related potentials following different stimulus conditions.

We studied somatosensory event-related potentials (ERPs) in response to rare target, frequent nontarget and rare nontarget stimuli in 12 healthy subjects. Compared with the corresponding peaks following frequent stimuli, the responses elicited by rare target stimuli showed higher amplitudes for N70, P100 and N140 components and those evoked by rare nontarget stimuli showed higher amplitudes for N70 and N140 components. The P300 component following rare nontarget stimuli was shorter in latency and lower in amplitude than that elicited by target stimuli. ERP waveforms evoked by rare nontarget stimuli also showed obvious N240 and P300 components, which differed from those following frequent nontarget stimuli. These findings suggest that there are differences in signal processing in response to target, frequent nontarget and rare nontarget stimuli. The P300 component was distributed symmetrically and there was no significant hemispheric predominance with regard to any ERP component in response to either right or left side stimulation.     

Impaired novelty P3 potentials in multiple system atrophy--correlation with orthostatic hypotension

Although neuropsychological tests demonstrate frontal lobe dysfunction in multiple system atrophy (MSA), assessment of frontal function using event-related brain potentials (ERPs) has not been sufficiently performed in MSA. The correlation between frontal lobe dysfunction and orthostatic hypotension (OH), which is known to cause frontal hypoperfusion, remains unclear. Our objectives were to assess frontal lobe dysfunction in MSA patients using ERPs and to elucidate the relevance of OH to changes in ERPs. Nine consecutive patients with MSA and nine age- and gender-matched healthy controls were compared by performance in the Wisconsin Card Sorting Test (WCST) and somatosensory ERPs to target and novel stimuli, namely, parietal maximal P3 (target P3) and fronto-central P3 (novelty P3), respectively. The correlation between novelty P3 and OH was evaluated in the MSA group. The MSA group showed a poorer performance in categories achieved (CA), total errors (TE) and perseverative errors by Nelson's (PEN) method in the WCST compared with the control group (CA and PEN: p<0.01; TE: p<0.02). Novelty and target P3s in the MSA group showed significantly prolonged latency (novelty: p<0.05; target: p<0.01) and reduced amplitude (novelty: p<0.02; target: p<0.01) compared with the control group. There was a significant negative correlation between novelty P3 latency and a drop in systolic blood pressure (r=0.76; p<0.02). Abnormalities of novelty P3 in the MSA group might reflect frontal lobe dysfunction, namely failure of attentional set-shifting, that was identified by the WCST. OH may play a role in the development of frontal lobe dysfunction in MSA.     

Brain response to unexpected novel noises in children with low and high trait anxiety

The behavioral inhibition system [Gray, J. A. The neuropsychology of anxiety: An enquiry into the functions of the septo-hippocampal system. Oxford: Oxford University Press, 1982] proposes that anxiety is associated with the processing of novel stimuli. We aimed to explore this relationship by recording auditory event-related potentials associated with unexpected novel noises in typically developing children. Children aged 10-14 years with low (n = 12) and high (n = 11) self-report trait anxiety were assessed using a novelty oddball task. The N1 associated with novel stimuli, specifically the "N1c" component maximal at temporal lobe sites, was of significantly longer latency (p = .014) and greater amplitude (p = .004) in the high compared with the low anxious group. This group difference was supported by linear correlations between N1c amplitude and trait anxiety scores. There was no effect of anxiety on the later novelty P3. These data suggest a subtle moderating role of trait anxiety on brain response to novelty, and further research with clinically anxious children is indicated.     

Event related potentials (ERP) and behavioral measurements to verbal stimulation of visual fields

Event related potentials (ERP) to visually presented linguistic stimuli were examined using a lexical-decision task and an oddball paradigm. Stimuli were presented to the central, right or left visual fields (CVF, RVF and LVF) and generated ERP with very clear N100-P300 components. The question addressed was whether there is ERP evidence for left hemisphere (LH) superiority in linguistic discrimination as reported behaviorally. Nineteen young, right-handed male subjects participated. The main factor influencing the latency and amplitude of N100 was that of contralateral versus ipsilateral stimulation. Shorter N100 latency and larger amplitude were recorded over the hemisphere contralateral to the visual field stimulated. In contrast, the factors influencing the P300 parameters were the visual field stimulated and the hemisphere over which the ERP was recorded. P300 amplitude was significantly larger and P300 latency significantly shorter over the LH than over the RH. Significantly shorter P300 latency and larger peak amplitude were found for RVF than for LVF stimulation.     

Evidence for distinct beta resonance frequencies in human EEG related to specific sensorimotor cortical areas.

OBJECTIVE: We studied event-related synchronization (ERS) of beta rhythms related to voluntary movement vs. stimulation of upper and lower limbs. The aim of this study was to investigate whether the frequency of the beta response is related to specific regions within the sensorimotor strip. METHODS: Self-paced movement and electrical stimulation of the dominant hand and foot/leg was investigated in 10 right-handed volunteers. The electroencephalogram was recorded from closely spaced electrodes over central areas and processed time-locked to movement-offset or stimulation. In order to identify the dominant frequency of the induced beta oscillations, time-frequency maps were calculated using the continuous wavelet transformation. For the specific beta frequency bands, the band power time courses were analyzed by quantifying the event-related (de-)synchronization (ERD/ERS). RESULTS: Both limb movement and somatosensory stimulation induced bursts of beta oscillations appearing within 1 s after movement/stimulation with a clear focus close to the corresponding sensorimotor representation area. The peak frequency was significantly lower over the hand area (below approximately 20 Hz) than at mid-central sites overlying the foot representation area (above approximately 20 Hz). But no difference was found between movement and stimulation of the respective limb. CONCLUSIONS: Analyzing the frequency of induced beta activity revealed concomitant oscillations at slightly different frequencies over neighboring cortical areas. These oscillations might be indicative for a resonance-like behavior of connected sub-networks in sensorimotor areas.