NeuroImage
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Stimulus repetition can produce neural response attenuation in stimulus-category selective networks within the occipito-temporal lobe. It is hypothesized that this neural suppression reflects the functional sharpening of local neuronal assemblies which boosts information processing efficiency. This neural suppression phenomenon has been mainly reported during conditions of conscious stimulus perception. ⋯ By measuring the temporal dynamics of high-frequency broadband gamma activity in VOTC and testing for main and interaction effects, we report that early processing of words in word-form selective networks exhibits a temporal cascade of modulations by stimulus repetition and masking: neuronal attenuation initially is observed in response to repeated words (irrespective of consciousness), that is followed by a second modulation contingent upon word reportability (irrespective of stimulus repetition). Later on (>300ms post-stimulus), a significant effect of conscious perception on the extent of repetition suppression was observed. The temporal dynamics of consciousness, the recognition memory processes and their interaction revealed in this study advance our understanding of their contributions to the neural mechanisms of word processing in VOTC.
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Studies employing functional connectivity-type analyses have established that spontaneous fluctuations in functional magnetic resonance imaging (fMRI) signals are organized within large-scale brain networks. Meanwhile, fMRI signals have been shown to exhibit 1/f-type power spectra - a hallmark of scale-free dynamics. We studied the interplay between functional connectivity and scale-free dynamics in fMRI signals, utilizing the fractal connectivity framework - a multivariate extension of the univariate fractional Gaussian noise model, which relies on a wavelet formulation for robust parameter estimation. ⋯ Third, in addition to a decrease of the Hurst exponent and inter-regional correlations, task performance modified cross-temporal dynamics, inducing a larger contribution of the highest frequencies within the scale-free range to global correlation. Lastly, we found that across individuals, a weaker task modulation of the frequency contribution to inter-regional connectivity was associated with better task performance manifesting as shorter and less variable reaction times. These findings bring together two related fields that have hitherto been studied separately - resting-state networks and scale-free dynamics, and show that scale-free dynamics of human brain activity manifest in cross-regional interactions as well.
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Hippocampal activity is characterized by the coordinated firing of a subset of neurons. Such neuronal ensembles can either be driven by external stimuli to form new memory traces or be reactivated by intrinsic mechanisms to reactivate and consolidate old memories. Hippocampal network oscillations orchestrate this coherent activity. ⋯ Interestingly, cells were more tightly clustered in large ensembles than in smaller groups. Together, our data show that spatiotemporal activity patterns of hippocampal neuronal ensembles can be reliably detected with deconvolution-based imaging techniques in mouse hippocampal slices. The here presented techniques are fully applicable to similar studies of distributed optical measurements of neuronal activity (in vivo), where signal-to-noise ratio is critical.
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Cerebral dysfunction occurring in mental disorders can show metabolic disturbances which are limited to circumscribed brain areas. Auditory hallucinations have been shown to be related to defined cortical areas linked to specific language functions. Here, we investigated if the study of metabolic changes in auditory hallucinations requires a functional rather than an anatomical definition of their location and size to allow a reliable investigation by magnetic resonance spectroscopy (MRS). ⋯ This study supports the neurodegenerative hypothesis of schizophrenia only in a frontal region whereas the results obtained from temporal regions are in contrast to the majority of previous studies. Future research should test the hypothesis raised by this study that a functional definition of language regions is needed if neurochemical imbalances are expected to be restricted to functional foci.
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Brain function critically relies on the supply with energy substrates (oxygen and glucose) via blood flow. Alterations in energy demand as during neuronal activation induce dynamic changes in substrate fluxes and blood flow. To study the complex system that regulates cerebral metabolism requires the combination of methods for the simultaneous assessment of multiple parameters. ⋯ In the vicinity of the ischemic territory, we observed PIDs that were characterized by reduced CMRO2 and increased oxygen extraction fraction (OEF), indicating a limitation of oxygen supply. Simultaneously measured PET showed an increased (18)F-FDG uptake in these regions. Our combined system proved to be a novel tool for the simultaneous study of dynamic spatiotemporal alterations of cortical blood flow, oxygen metabolism and glucose consumption under normal and pathologic conditions.