http://www.maths.qmul.ac.uk/%7Elatora/report_06.pdf

Complex networks: Structure and dynamics

The majority of functional neuroscience studies have focused on the brain's response to a task or stimulus. However, the brain is very active even in the absence of explicit input or output. In this Article we review recent studies examining spontaneous fluctuations in the blood oxygen level dependent (BOLD) signal of functional magnetic resonance imaging as a potentially important and revealing manifestation of spontaneous neuronal activity. Although several challenges remain, these studies have provided insight into the intrinsic functional architecture of the brain, variability in behaviour and potential physiological correlates of neurological and psychiatric disease.

/pdf/spontaneous-fluctuations-in-brain-activity-observed-with-32z1nyfc7n.pdf

Spontaneous fluctuations in brain activity observed with functional magnetic resonance imaging.

The emergence of a unified cognitive moment relies on the coordination of scattered mosaics of functionally specialized brain regions. Here we review the mechanisms of large-scale integration that counterbalance the distributed anatomical and functional organization of brain activity to enable the emergence of coherent behaviour and cognition. Although the mechanisms involved in large-scale integration are still largely unknown, we argue that the most plausible candidate is the formation of dynamic links mediated by synchrony over multiple frequency bands.

The brainweb: phase synchronization and large-scale integration.

https://www.cell.com/trends/cognitive-sciences/pdf/S1364-6613(05)00242-1.pdf

A mechanism for cognitive dynamics: neuronal communication through neuronal coherence

Neuronal activity in the brain gives rise to transmembrane currents that can be measured in the extracellular medium. Although the major contributor of the extracellular signal is the synaptic transmembrane current, other sources — including Na+ and Ca2+ spikes, ionic fluxes through voltage- and ligand-gated channels, and intrinsic membrane oscillations — can substantially shape the extracellular field. High-density recordings of field activity in animals and subdural grid recordings in humans, combined with recently developed data processing tools and computational modelling, can provide insight into the cooperative behaviour of neurons, their average synaptic input and their spiking output, and can increase our understanding of how these processes contribute to the extracellular signal.

/pdf/the-origin-of-extracellular-fields-and-currents-eeg-ecog-lfp-2e79zygxj0.pdf

The origin of extracellular fields and currents — EEG, ECoG, LFP and spikes

Functional magnetic resonance imaging (fMRI) is widely used to study the operational organization of the human brain, but the exact relationship between the measured fMRI signal and the underlying neural activity is unclear. Here we present simultaneous intracortical recordings of neural signals and fMRI responses. We compared local field potentials (LFPs), single- and multi-unit spiking activity with highly spatio-temporally resolved blood-oxygen-level-dependent (BOLD) fMRI responses from the visual cortex of monkeys. The largest magnitude changes were observed in LFPs, which at recording sites characterized by transient responses were the only signal that significantly correlated with the haemodynamic response. Linear systems analysis on a trialby-trial basis showed that the impulse response of the neurovascular system is both animal- and site-specific, and that LFPs yield a better estimate of BOLD responses than the multi-unit responses. These findings suggest that the BOLD contrast mechanism reflects the input and intracortical processing of a given area rather than its spiking output.

/pdf/neurophysiological-investigation-of-the-basis-of-the-fmri-1pwsdtcobg.pdf

Neurophysiological investigation of the basis of the fMRI signal

Most functional brain imaging studies use task-induced hemodynamic responses to infer underlying changes in neuronal activity. In addition to increases in cerebral blood flow and blood oxygenation level–dependent (BOLD) signals, sustained negative responses are pervasive in functional imaging. The origin of negative responses and their relationship to neural activity remain poorly understood. Through simultaneous functional magnetic resonance imaging and electrophysiological recording, we demonstrate a negative BOLD response (NBR) beyond the stimulated regions of visual cortex, associated with local decreases in neuronal activity below spontaneous activity, detected 7.15 ± 3.14 mm away from the closest positively responding region in V1. Trial-by-trial amplitude fluctuations revealed tight coupling between the NBR and neuronal activity decreases. The NBR was associated with comparable decreases in local field potentials and multiunit activity. Our findings indicate that a significant component of the NBR originates in neuronal activity decreases.

Negative functional MRI response correlates with decreases in neuronal activity in monkey visual area V1.

Increasing evidence has implicated the cerebellum in providing forward models of motor plants predicting the sensory consequences of actions Assuming that cerebellar input to the cerebral cortex contributes to the cerebro-cortical processing by adding forward model signals, we would expect to find projections emphasising motor and sensory cortical areas However, this expectation is only partially met by studies of cerebello–cerebral connections Here we show that by electrically stimulating the cerebellar output and imaging responses with functional magnetic resonance imaging, evoked blood oxygen level-dependant activity is observed not only in the classical cerebellar projection target, the primary motor cortex, but also in a number of additional areas in insular, parietal and occipital cortex, including sensory cortical representations Further probing of the responses reveals a projection system that has been optimized to mediate fast and temporarily precise information In conclusion, both the topography of the stimulation effects and its emphasis on temporal precision are in full accordance with the concept of cerebellar forward model information modulating cerebro-cortical processing The cerebellum has important roles in motor control and sensory perception Sultan and colleagues use electrical stimulation and functional magnetic resonance imaging to reveal that the cerebellum provides inputs to specialized cortical regions of the brain that modulate cerebro-cortical processing

/pdf/unravelling-cerebellar-pathways-with-high-temporal-precision-3gu7ac62ya.pdf

Unravelling cerebellar pathways with high temporal precision targeting motor and extensive sensory and parietal networks

In vivo measurement of cortical impedance spectrum in monkeys: implications for signal propagation.

Hippocampal ripples, episodic high-frequency field-potential oscillations primarily occurring during sleep and calmness, have been described in mice, rats, rabbits, monkeys and humans, and so far they have been associated with retention of previously acquired awake experience. Although hippocampal ripples have been studied in detail using neurophysiological methods, the global effects of ripples on the entire brain remain elusive, primarily owing to a lack of methodologies permitting concurrent hippocampal recordings and whole-brain activity mapping. By combining electrophysiological recordings in hippocampus with ripple-triggered functional magnetic resonance imaging, here we show that most of the cerebral cortex is selectively activated during the ripples, whereas most diencephalic, midbrain and brainstem regions are strongly and consistently inhibited. Analysis of regional temporal response patterns indicates that thalamic activity suppression precedes the hippocampal population burst, which itself is temporally bounded by massive activations of association and primary cortical areas. These findings suggest that during off-line memory consolidation, synergistic thalamocortical activity may be orchestrating a privileged interaction state between hippocampus and cortex by silencing the output of subcortical centres involved in sensory processing or potentially mediating procedural learning. Such a mechanism would cause minimal interference, enabling consolidation of hippocampus-dependent memory.

Axel Oeltermann

Papers

Neurophysiological investigation of the basis of the fMRI signal

Negative functional MRI response correlates with decreases in neuronal activity in monkey visual area V1.

Unravelling cerebellar pathways with high temporal precision targeting motor and extensive sensory and parietal networks

In vivo measurement of cortical impedance spectrum in monkeys: implications for signal propagation.

Hippocampal–cortical interaction during periods of subcortical silence