Single-trial coupling of EEG and fMRI reveals the involvement of early anterior cingulate cortex activation in effortful decision making

While the precise role of the anterior cingulate cortex (ACC) is still being discussed, it has been suggested that ACC activity might reflect the amount of mental effort associated with cognitive processing. So far, not much is known about the temporal dynamics of ACC activity in effort-related decision making or auditory attention, because fMRI is limited concerning its temporal resolution and electroencephalography (EEG) is limited concerning its spatial resolution. Single-trial coupling of EEG and fMRI can be used to predict the BOLD signal specifically related to amplitude variations of electrophysiological components. The striking feature of single-trial coupling is its ability to separate different aspects of the BOLD signal according to their specific relationship to a distinct neural process. In the present study we investigated 10 healthy subjects with a forced choice reaction task under both low and high effort conditions and a control condition (passive listening) using simultaneous EEG and fMRI. We detected a significant effect of mental effort only for the N1 potential, but not for the P300 potential. In the fMRI analysis, ACC activation was present only in the high effort condition. We used single-trial coupling of EEG and fMRI in order to separate information specific to N1-amplitude variations from the unrelated BOLD response. Under high effort conditions we were able to detect circumscribed BOLD activations specific to the N1 potential in the ACC (t=4.7) and the auditory cortex (t=6.1). Comparing the N1-specific BOLD activity of the high effort condition versus the control condition we found only activation of the ACC (random effects analysis, corrected for multiple comparisons, t=4.4). These findings suggest a role of early ACC activation in effort-related decision making and provide a direct link between the N1 component and its corresponding BOLD signal.     

Single-trial P3 amplitude and latency informed event-related fMRI models yield different BOLD response patterns to a target detection task

Using single-trial parameters as a regressor in the General Linear Model (GLM) is becoming an increasingly popular method for informing fMRI analysis. However, the parameter used to characterise or to differentiate brain regions involved in the response to a particular task varies across studies (e.g. ERP amplitude, ERP latency, reaction time). Furthermore, the way in which the single-trial information is used in the fMRI analysis is also important. For example, the single-trial parameters can be used as regressors in the GLM or to modify the duration of the events modelled in the GLM. The aim of this study was to investigate the BOLD response to a target detection task when including P3 amplitude, P3 latency and reaction time parameters in the GLM. Simultaneous EEG-fMRI was recorded from fifteen subjects in response to a visual choice reaction time task. Including P3 amplitude as a regressor in the GLM yielded activation in left central opercular cortex, left postcentral gyrus, left insula, left middle frontal gyrus, left insula and left parietal operculum. Using P3 latency and reaction time as an additional regressor yielded no additional activation in comparison with the conventional fMRI analysis. However, when P3 latency or reaction time was used to determine the duration of events at a single-trial level, additional activation was observed in the left postcentral gyrus, left precentral gyrus, anterior cingulate cortex and supramarginal gyrus. Our findings suggest that ERP amplitudes and latencies can yield different activation patterns when used to modify relevant aspects of the GLM.     

Single-trial coupling of the gamma-band response and the corresponding BOLD signal

Oscillations in the gamma-band frequency range have been described to be more closely connected to hemodynamic changes as assessed with functional magnetic resonance imaging (fMRI) than other aspects of neuronal activity. In addition, gamma-band oscillations have attracted much interest during the last few years since they are thought to play a crucial role in many aspects of brain function related to perception and cognition. It was the aim of the present simultaneous EEG-fMRI study to identify brain regions specifically involved in the generation of the auditory gamma-band response (GBR) using single-trial coupling of EEG and fMRI. Ten healthy subjects participated in this study. Three different runs of an auditory choice reaction task with increasing difficulty were performed. Brain activity was recorded simultaneously with high density EEG (61 channels) and fMRI (1.5 T). BOLD correlates of the GBR have been predicted using the single-trial amplitude of the GBR.Reaction times (p < 0.001), error rates (p < 0.05) and self-ratings of task difficulty and effort demands (p < 0.001) were related to the level of difficulty in the task. In addition, we found a significant influence of task difficulty on the amplitude of the GBR at Cz (p < 0.05). Using single-trial coupling of EEG and fMRI GBR-specific activations were found only in the auditory cortex, the thalamus and the anterior cingulate cortex (ACC) in the most difficult run. Single-trial coupling might be a useful method in order to increase our knowledge about the functional neuroanatomy of “neural ensembles” coupled by 40 Hz oscillations.     

Hemodynamic nonlinearities affect BOLD fMRI response timing and amplitude

The interpretation of functional magnetic resonance imaging (fMRI) studies based on blood oxygen-level dependent (BOLD) contrast generally relies on the assumption of a linear relationship between evoked neuronal activity and fMRI response. While nonlinearities in this relationship have been suggested by a number of studies, it remains unclear to what extent they relate to the neurovascular response and are therefore inherent to BOLD fMRI. Full characterization of potential vascular nonlinearities is required for accurate inferences about the neuronal system under study. To investigate the extent of vascular nonlinearities, evoked activity was studied in humans with BOLD fMRI (n = 28) and magnetoencephalography (MEG) (n = 5). Brief (600–800 ms) rapidly repeated (1 Hz) visual stimuli were delivered using a stimulation paradigm that minimized neuronal nonlinearities. Nevertheless, BOLD fMRI experiments showed substantial remaining nonlinearities. The smallest stimulus separation (200–400 ms) resulted in significant response broadening (15–20% amplitude decrease; 10–12% latency increase; 6–14% duration increase) with respect to a linear prediction. The substantial slowing and widening of the response in the presence of preceding stimuli suggest a vascular rather than neuronal origin to the observed nonlinearity. This was confirmed by the MEG data, which showed no significant neuro-electric nonlinear interactions between stimuli as little as 200 ms apart. The presence of substantial vascular nonlinearities has important implications for rapid event-related studies by fMRI and other imaging modalities that infer neuronal activity from hemodynamic parameters.     

Cognitive neurophysiology: beyond averaging

Averaging of repeated responses to sensory stimuli is the standard approach in cognitive electrophysiology. This procedure can give rise to inappropriate interpretations in some situations, because two factors contribute to the average ERP responses: the amplitude of the responses during the individual experimental trials, and the concentration of the phases (phase-locking) across responses. Larger poststimulus single-trial amplitudes compared to prestimulus baseline are thought to correspond to a stimulus-related increase of postsynaptic potentials or/and activation of an increased amount of neural assemblies. But the functional interpretation of an enhanced inter-trial phase-locking is unclear. BOLD responses are probably related to single-trial EEG amplitudes, but not to the phase concentration across trials. Therefore, separation of amplitude and phase contributions is indispensable to avoid misinterpretations and to gain a deeper understanding of the relation between event-related EEG and fMRI.     

Enhanced EEG gamma-band activity reflects multisensory semantic matching in visual-to-auditory object priming

An important step in perceptual processing is the integration of information from different sensory modalities into a coherent percept. It has been suggested that such crossmodal binding might be achieved by transient synchronization of neurons from different modalities in the gamma-frequency range (> 30 Hz). Here we employed a crossmodal priming paradigm, modulating the semantic congruency between visual–auditory natural object stimulus pairs, during the recording of the high density electroencephalogram (EEG). Subjects performed a semantic categorization task. Analysis of the behavioral data showed a crossmodal priming effect (facilitated auditory object recognition) in response to semantically congruent stimuli. Differences in event-related potentials (ERP) were found between 250 and 350 ms, which were localized to left middle temporal gyrus (BA 21) using a distributed linear source model. Early gamma-band activity (40–50 Hz) was increased between 120 ms and 180 ms following auditory stimulus onset for semantically congruent stimulus pairs. Source reconstruction for this gamma-band response revealed a maximal increase in left middle temporal gyrus (BA 21), an area known to be related to the processing of both complex auditory stimuli and multisensory processing. The data support the hypothesis that oscillatory activity in the gamma-band reflects crossmodal semantic-matching processes in multisensory convergence sites.     

Face recognition and cortical responses: effect of stimulus duration

To clarify the relationship between face perception and cortical activation, we manipulated the face recognition performance of 9 subjects by varying the duration (DUR) of stimuli while cortical neuromagnetic responses were recorded. A face image replaced a continuous pixel-noise mask for 17–200 ms, and the subject reported which of the pre-learned faces had been presented. Two cortical responses were clearly stronger to intact than phase-scrambled faces: the temporo-occipital response peaking at 140–200 ms (M170) and a more widely distributed response peaking at 200–500 ms (M300). For the shortest DURs (17–33 ms), face recognition was at chance level and the cortical responses negligible. For DURs of 50–83 ms, the proportion of recognized faces as well as the strength of cortical responses increased steeply. Recognition performance saturated at DURs of around 100 ms, whereas cortical responses continued to increase until the longest DUR of 200 ms. Amplitudes of both M170 and M300 were thus tightly correlated with recognition performance (r = 0.98), but comparison of the increment rates as a function of DUR showed the recognition performance to have an even closer similarity to M170 than to M300. In single-trial analysis the variability of response strengths increased in a direct proportion to response amplitude, demonstrating the averaged responses to be composed of graded rather than of all-or-nothing-type single responses.     

The somatosensory representation of the human clitoris: An fMRI study

We studied the central representation of pudendal afferents arising from the clitoral nerves in 15 healthy adult female subjects using electrical dorsal clitoral nerve stimulation and fMRI. As a control body region, we electrically stimulated the right hallux in eight subjects. In a block design experiment, we applied bilateral clitoral stimulation and unilateral (right) hallux stimulation. Activation maps were calculated for the contrasts ‘electrical dorsal clitoral nerve stimulation versus rest’ and ‘electrical hallux stimulation versus rest’.A random-effect group analysis for the clitoral stimulation showed significant activations bilateral in the superior and inferior frontal gyri, insulae and putamen and in the postcentral, precentral and inferior parietal gyri (including the primary and secondary somatosensory cortices). No activation was found on the mesial surface of the postcentral gyrus. For the hallux, activations occurred in a similar neuronal network but the activation in the primary somatosensory cortex was localized in the inter-hemispheric fissure.The results of this study demonstrate that the central representation of pudendal afferents arising from the clitoral nerves and sensory inputs from the hallux can be studied and distinguished from each other by fMRI. From the somatotopic order described in the somatosensory homunculus one would expect for electrical clitoral nerve stimulation activation of the mesial wall of the postcentral gyrus. In contrast, we found activations on the lateral surface of the postcentral gyrus.     

fMRI activation during spike and wave discharges evoked by photic stimulation

Photoparoxysmal response (PPR) is an electroencephalographic (EEG) trait characterized by the occurrence of epileptiform discharges in response to visual stimulation. Studying this trait helps to learn about mechanisms of epileptogenicity. While simultaneous recordings of EEG and functional MRI (EEG–fMRI) in patients with spontaneous generalised spike-wave discharges (GSW) have revealed activation of the thalamus and deactivation in frontoparietal areas, EEG–fMRI studies on evoked GSW such as PPR are lacking. In this EEG–fMRI study, 30 subjects with reported generalised PPR underwent intermittent photic stimulation (IPS) in a 3 T MR scanner. PPR was elicited in 6 subjects, four diagnosed with idiopathic generalised epilepsy and two with tension-type headache. Because PPR is preceded by synchronization of cortical gamma oscillations, blood oxygenation level-dependent (BOLD) signal changes were analysed at the onset of the PPR (standard regressor) and 3 s before the onset of PPR (early regressor) in one model. In all subjects, IPS led to a significant activation of the visual cortex. Based on the early regressor, PPR associated activation was found in the parietal cortex adjacent to the intraparietal sulcus in five and in the premotor cortex in all 6 subjects. The standard regressor revealed deactivation in early activated areas in all subjects and thalamic activation in one subject. In contrast to spontaneous GSW, these results suggest that PPR is a cortical phenomenon with an involvement of the parietal and frontal cortices. Pronounced haemodynamic changes seen with the early regressor could mirror gamma activity that is known to precede PPR.     

Development, validation, and comparison of ICA-based gradient artifact reduction algorithms for simultaneous EEG-spiral in/out and echo-planar fMRI recordings

EEG data acquired in an MRI scanner are heavily contaminated by gradient artifacts that can significantly compromise signal quality. We developed two new methods based on independent component analysis (ICA) for reducing gradient artifacts from spiral in–out and echo-planar pulse sequences at 3 T, and compared our algorithms with four other commonly used methods: average artifact subtraction (Allen, P., Josephs, O., Turner, R., 2000. A method for removing imaging artifact from continuous EEG recorded during functional MRI. NeuroImage 12, 230–239.), principal component analysis (Niazy, R., Beckmann, C., Iannetti, G., Brady, J., Smith, S., 2005. Removal of FMRI environment artifacts from EEG data using optimal basis sets. NeuroImage 28, 720–737.), Taylor series ( Wan, X., Iwata, K., Riera, J., Kitamura, M., Kawashima, R., 2006. Artifact reduction for simultaneous EEG/fMRI recording: adaptive FIR reduction of imaging artifacts. Clin. Neurophysiol. 117, 681–692.) and a conventional temporal ICA algorithm. Models of gradient artifacts were derived from simulations as well as a water phantom and performance of each method was evaluated on datasets constructed using visual event-related potentials (ERPs) as well as resting EEG. Our new methods recovered ERPs and resting EEG below the beta band (< 12.5 Hz) with high signal-to-noise ratio (SNR > 4). Our algorithms outperformed all of these methods on resting EEG in the theta and alpha bands (SNR > 4); however, for all methods, signal recovery was modest (SNR  1) in the beta band and poor (SNR < 0.3) in the gamma band and above. We found that the conventional ICA algorithm performed poorly with uniformly low SNR (< 0.1). Taken together, our new ICA-based methods offer a more robust technique for gradient artifact reduction when scanning at 3 T using spiral in–out and echo-planar pulse sequences. We provide new insights into the strengths and weaknesses of each method using a unified subspace framework.     

Hemodynamic changes preceding the interictal EEG spike in patients with focal epilepsy investigated using simultaneous EEG-fMRI

EEG-fMRI is a non-invasive technique that allows the investigation of epileptogenic networks in patients with epilepsy. Lately, BOLD changes occurring before the spike were found in patients with generalized epilepsy. The study of metabolic changes preceding spikes might improve our knowledge of spike generation. We tested this hypothesis in patients with idiopathic and symptomatic focal epilepsy.Eleven consecutive patients were recorded at 3 T: five with idiopathic focal and 6 with symptomatic focal epilepsy. Thirteen spike types were analyzed separately. Statistical analysis was performed using the timing of spikes as events, modeled with HRFs peaking between − 9 s and + 9 s around the spike. HRFs were calculated the most focal BOLD response. Eleven of the thirteen studies showed prespike BOLD responses. Prespike responses were more focal than postspike responses. Three studies showed early positive followed by later negative BOLD responses in the spike field. Three had early positive BOLD responses in the spike field, which remained visible in the later maps. Three others had positive BOLD responses in the spike field, later propagating to surrounding areas. HRFs peaked between − 5 and + 6 s around the spike timing. No significant EEG changes could be identified prior to the spike.BOLD changes prior to the spike frequently occur in focal epilepsies. They are more focal than later BOLD changes and strongly related to the spike field. Early changes may result from increased neuronal activity in the spike field prior to the EEG spike and reflect an event more localized than the spike itself.     

An fMRI study of neural interaction in large-scale cortico-thalamic visual network

To date there is still no proper neuroimaging methods suitable for noninvasively providing both detailed spatial and temporal information of neural interaction across large-scale brain networks. This limitation has impeded the advance of neuroscience research. In an attempt to overcome this challenge, Ogawa et al. applied a paired-stimulus paradigm, which is composed of a pair of stimuli separated by a variable inter-stimulus interval (ISI), to decode temporal information of neural interaction from amplitude modulation of the blood-oxygenation-level-dependent (BOLD) responses elicited by the neural interaction pursued [Ogawa, S., Lee, T.-M., Stepnoski, R., Chen, W., Zhu, X.H., Ugurbil, K., 2000. An approach to probe neural systems interaction by functional MRI at neural time scale down to milliseconds. Proc. Natl. Acad Sci. U S A 97, 11026–11031.]. Although application of this paradigm has been demonstrated in a few publications, most of them only focused on investigating cortico-cortical interaction. Considering the vital roles that cortico-thalamic networks play in brain communication and function, extending the applicability of this method to studying cortico-thalamic neural interaction should be significant. In this study, we applied the paired-visual-stimulus paradigm to simultaneously measure the BOLD amplitude modulations as a function of ISI in the lateral geniculate nucleus (LGN) and primary visual cortex (V1) in the cat brain. The results reveal that both V1 and LGN BOLD responses were significantly suppressed when the visual system was within the refractory period at ISI ≤ 1 s and the suppression extent was gradually recovered when ISI became longer. Both BOLD and electrophysiological measurements show a facilitatory activity in V1 at ISI ≈ 1.5 s, but not in LGN. Furthermore, there was additional and consistent reduction in the LGN BOLD response compared to V1 within the range of ISI below 4 s, which is likely controlled by inhibitory effects through the cortico-geniculate feedback. These findings together suggest that the dynamic fMRI approach applied in this study is sensitive to neuronal inhibitory and facilitatory interactions and it should be useful for noninvasively investigating large-scale cortico-thalamic neural networks.     

Improved spatial localization of post-stimulus BOLD undershoot relative to positive BOLD

The negative blood oxygenation level-dependent (BOLD) signal following the cessation of stimulation (post-stimulus BOLD undershoot) is observed in functional magnetic resonance imaging (fMRI) studies. However, its spatial characteristics are unknown. To investigate this, gradient-echo BOLD fMRI in response to visual stimulus was obtained in isoflurane-anesthetized cats at 9.4 T. Since the middle cortical layer (layer 4) is known to have the highest metabolic and cerebral blood volume (CBV) responses, images were obtained to view the cortical cross-section. Robust post-stimulus BOLD undershoot was observed in all studies, and lasted longer than 30 s after the cessation of 40-60 s stimulation. The magnitude of post-stimulus BOLD undershoot was linearly dependent on echo time with little intercept when extrapolating to TE = 0, indicating that the T2* change is the major cause of the BOLD undershoot. The post-stimulus BOLD undershoot was observed within the cortex and near the surface of the cortex, while the prolonged CBV elevation was observed only at the middle of the cortex. Within the cortex, the largest post-stimulus undershoot was detected at the middle of the cortex, similar to the CBV increase during the stimulation period. Our findings demonstrate that, even though there is significant contribution from pial vessel signals, the post-stimulus undershoot BOLD signal is useful to improve the spatial localization of fMRI to active cortical sites.     

A simultaneous EEG-fMRI study of painful electric stimulation

Together with a detailed behavioral analysis, simultaneous measurement of functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) permits a better elucidation of cortical pain processing. We applied painful electrical stimulation to 6 healthy subjects and acquired fMRI simultaneously with an EEG measurement. The subjects rated various stimulus properties and the individual affective state. Stimulus-correlated BOLD effects were found in the primary and secondary somatosensory areas (SI and SII), the operculum, the insula, the supplementary motor area (SMA proper), the cerebellum, and posterior parts of the anterior cingulate gyrus (ACC). Perceived pain intensity was positively correlated with activation in these areas. Higher unpleasantness rating was associated with suppression of activity in areas known to be involved in stimulus categorization and representation (ventral premotor cortex, PCC, parietal operculum, insula) and enhanced activation in areas initiating, propagating, and executing motor reactions (ACC, SMA proper, cerebellum, primary motor cortex). Concordant dipole localizations in SI and ACC were modeled. Using the dipole strength in SI, the network was restricted to SI. The BOLD signal change in ACC was positively correlated to the individual dipole strength of the source in ACC thus revealing a close relationship of BOLD signal and possibly underlying neuronal electrical activity in SI and the ACC. The BOLD signal change decreased in SI over time. Dipole strength of the ACC source decreased over the experiment and increased during the stimulation block suggesting sensitization and habituation effects in these areas.     

Cortical source estimates of gamma band amplitude and phase are different in schizophrenia

Reductions in gamma band phase synchrony and evoked power have been reported in schizophrenic subjects in response to auditory stimuli. These results have been observed in the EEG at one or two electrode sites. We wished to extend these results using magnetic field data to estimate the responses at the neural generators themselves in each hemisphere. Whole head magnetoencephalographic (MEG) recordings were used to estimate the phase and amplitude behavior of sources in primary auditory cortex in both hemispheres of schizophrenic and comparison subjects. Both ipsi- and contralateral cases were evaluated using a driving (40 Hz modulated 1 kHz carrier) and a non-driving (1 kHz tone) stimulus. We used source space projection (SSP) to collapse the magnetic field data into estimates of the time course of source strengths in individual trials. Complex wavelet based time–frequency decomposition was used to compute inter-trial phase locking factor (PLF), and mean evoked and induced amplitude for each cortical generator. Schizophrenic subjects showed reduced SSP PLF and evoked source strength for contralateral generators responding to the driving stimulus in both hemispheres. For the pure tone stimulus, only the left hemisphere PLF's in the transient window were reduced. In contrast, subjects with schizophrenia exhibited higher induced 40 Hz power to both stimulus types, consistent with the reduced PLF findings. The method of SSP combined with wavelet based complex demodulation produces a significant improvement in signal-to-noise ratio, and directly estimates the activity of the cortical generators responsible for gamma band auditory MEG evoked fields. Schizophrenic subjects exhibit significant impairment of generation and phase locking of this activity in auditory cortex, suggesting an impairment of GABA-ergic inhibitory interneuronal modulation of pyramidal cell activity.     

Functional imaging of the human superior colliculus: An optimised approach

Effective functional imaging of the human Superior Colliculus (SC) has often been regarded as difficult because of the small size of the SC and its proximity to sources of pulsatile (cardiac) noise. An optimised approach to functional imaging of the SC with fMRI is presented, based upon the novel finding that visually-induced BOLD responses in the SC are qualitatively different from responses in both cortical (V1) and sub-cortical (LGN) comparison areas. An optimised model with a Haemodynamic Response Function (HRF) which peaks early (4–5 s) and then falls rapidly is shown to be best suited for revealing SC responses, while a model peaking at 6 s and falling more slowly was most sensitive in the two comparison areas. Additionally, a method of correcting for the noise characteristics of fMRI responses proposed recently by de Zwart et al. (de Zwart, J. A., van Gelderen, P., Fukunaga, M., & Duyn, J. H. (2008). Reducing correlated noise in fMRI data. Magn Reson Med, 59, 939–945) is modified for use in the SC, and shown to be highly effective at further improving the statistical detectability of responses by modelling out noise. Together these methods represent a significant advance over previous approaches to functional imaging of the human SC. They permit the routine detection of strong SC activity in single subjects at standard spatial resolutions.     

Event-related functional MRI of cortical activity evoked by microsaccades, small visually-guided saccades, and eyeblinks in human visual cortex

We used event-related functional magnetic resonance imaging (fMRI) to determine blood oxygen-level-dependent (BOLD) signal changes following microsaccades, visually-guided saccades, and eyeblinks in retinotopically mapped visual cortical areas V1–V3 and hMT+. A deconvolution analysis revealed a similar pattern of BOLD activation following a microsaccade, 0.16° voluntary saccade, and 0.16° displacement of the image under conditions of fixation. In all areas, an initial increase in BOLD signal peaking at approximately 4.5 s after the event was followed by a decline and decrease below baseline. This modulation appears most pronounced for microsaccades and small voluntary saccades in V1, diminishing in strength from V1 to V3. In contrast, 0.16° real motion under conditions of fixation yields the same level of BOLD signal increase in V1 through V3. BOLD signal modulates parametrically with the size of voluntary saccades (0.16°, 0.38°, 0.82°, 1.64°, and 3.28°) in V1–V3, but not in hMT+. Eyeblinks generate larger modulation that peaks by 6.5 s, and dips below baseline by 10 s post-event, and also exhibits diminishing modulation from V1 to V3. Our results are consistent with the occurrence of transient neural excitation driven by changes in input to retinal ganglion cell receptive fields that are induced by microsaccades, visually-guided saccades, or small image shifts. The pattern of results in area hMT+ exhibits no significant modulation by microsaccades, relatively small modulation by eyeblinks, and substantial responses to saccades and background jumps, suggesting that spurious image motion signal arising from microsaccades and eyeblinks is relatively diminished by hMT+.     

Integration of fMRI and simultaneous EEG: towards a comprehensive understanding of localization and time-course of brain activity in target detection

fMRI and EEG are complimentary methods for the analysis of brain activity since each method has its strength where the other one has limits: The spatial resolution is thus in the range of millimeters with fMRI and the time resolution is in the range of milliseconds with EEG. For a comprehensive understanding of brain activity in target detection, nine healthy subjects (age 24.2 +/- 2.9) were investigated with simultaneous EEG (27 electrodes) and fMRI using an auditory oddball paradigm. As a first step, event-related potentials, measured inside the scanner, have been compared with the potentials recorded in a directly preceding session in front of the scanner. Attenuated amplitudes were found inside the scanner for the earlier N1/P2 component but not for the late P300 component. Second, an independent analysis of the localizations of the fMRI activations and the current source density as revealed by low resolution electromagnetic tomography (LORETA) has been done. Concordant activations were found in most regions, including the temporoparietal junction (TPJ), the supplementary motor area (SMA)/anterior cingulate cortex (ACC), the insula, and the middle frontal gyrus, with a mean Euclidean distance of 16.0 +/- 6.6 mm between the BOLD centers of gravity and the LORETA-maxima. Finally, a time-course analysis based on the current source density maxima was done. It revealed different time-course patterns in the left and right hemisphere with earlier activations in frontal and parietal regions in the right hemisphere. The results suggest that the combination of EEG and fMRI permits an improved understanding of the spatiotemporal dynamics of brain activity.     

低频振幅结合功能连接对原发性甲亢患者感觉运动网络的功能MRI研究

目的采用低频振幅(amplitude of low frequency fluctuation,ALFF)与功能连接(functional connectivity,FC)相结合的方法,研究原发性甲状腺功能亢进患者静息状态下脑功能改变。材料与方法 12名未经治疗的甲亢患者为病例组,12名自愿接受扫描的健康人(年龄、性别无显著性差异)为对照组。采集所有被试静息态f MRI数据,采用REST及DPARSF软件分析原始数据,得出全脑ALFF,利用双样本t检验的方法比较病例组和对照组ALFF的变化,并以两组间ALFF值有显著性差异的脑区为感兴趣区(regions of interest,ROI)校正后行FC分析。结果与对照组相比,甲亢患者在双侧尾状核及双侧丘脑ALFF值降低(P0.001)。以上脑区的ALFF值均未发现与T3、T4有显著相关性(P0.05)。FC分析显示,左侧丘脑与双侧感觉运动区(包括中央前回、中央后回)FC增强(P0.001);右侧丘脑与右侧中央前回及中央后回连接增强(P0.001)。结论甲亢患者双侧丘脑的局部活动及其与大脑感觉运动网络的FC模式存在异常,增强的FC可能与代谢损伤所导致运动机能受损而使甲状腺功能异常患者有更强的功能需求有关。     

Age effects on the P300 potential and the corresponding fMRI BOLD-signal

Age has been reported to influence amplitude and latency of the P300 potential. Nevertheless, it is not yet fully understood which brain regions are responsible for these effects. The aim of this study was to investigate age-effects on the P300 potential and the simultaneously acquired BOLD signal of functional MRI. 32 healthy male subjects were investigated using an auditory oddball paradigm. The functional MRI data were acquired in temporal synchrony to the task. The evoked potential data were recorded during the intervals in between MR image acquisitions in order to reduce the influence of the scanner noise on the presentation of the tones and to reduce gradient artifacts. The age-effects were calculated by means of regression analyses. In addition, brain regions modulated by the task-induced amplitude variation of the P300 were identified (single trial analysis). The results indicated an age effect on the P300 amplitude. Younger subjects demonstrated increased parietal P300 amplitudes and increased BOLD responses in a network of brain regions including the anterior and posterior cingulate cortex, the insula, the temporo-parietal junction, the superior temporal gyrus, the caudate body, the amygdala and the parahippocampal gyrus. Single trial coupling of EEG and fMRI indicated that P300 amplitudes were predominantly associated with neural responses in the anterior cingulate cortex, the putamen and temporal brain areas. Taken together, the results indicate diminished neural responses in older compared to younger subjects especially in frontal, temporo-parietal and subcortical brain regions.