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

Complex networks: Structure and dynamics

Thirty years of brain imaging research has converged to define the brain’s default network—a novel and only recently appreciated brain system that participates in internal modes of cognition Here we synthesize past observations to provide strong evidence that the default network is a specific, anatomically defined brain system preferentially active when individuals are not focused on the external environment Analysis of connectional anatomy in the monkey supports the presence of an interconnected brain system Providing insight into function, the default network is active when individuals are engaged in internally focused tasks including autobiographical memory retrieval, envisioning the future, and conceiving the perspectives of others Probing the functional anatomy of the network in detail reveals that it is best understood as multiple interacting subsystems The medial temporal lobe subsystem provides information from prior experiences in the form of memories and associations that are the building blocks of mental simulation The medial prefrontal subsystem facilitates the flexible use of this information during the construction of self-relevant mental simulations These two subsystems converge on important nodes of integration including the posterior cingulate cortex The implications of these functional and anatomical observations are discussed in relation to possible adaptive roles of the default network for using past experiences to plan for the future, navigate social interactions, and maximize the utility of moments when we are not otherwise engaged by the external world We conclude by discussing the relevance of the default network for understanding mental disorders including autism, schizophrenia, and Alzheimer’s disease

/pdf/the-brain-s-default-network-anatomy-function-and-relevance-3sxu9umxjl.pdf

The Brain's Default Network Anatomy, Function, and Relevance to Disease

In recent years, many new cortical areas have been identified in the macaque monkey. The number of identified connections between areas has increased even more dramatically. We report here on (1) a summary of the layout of cortical areas associated with vision and with other modalities, (2) a computerized database for storing and representing large amounts of information on connectivity patterns, and (3) the application of these data to the analysis of hierarchical organization of the cerebral cortex. Our analysis concentrates on the visual system, which includes 25 neocortical areas that are predominantly or exclusively visual in function, plus an additional 7 areas that we regard as visual-association areas on the basis of their extensive visual inputs. A total of 305 connections among these 32 visual and visual-association areas have been reported. This represents 31% of the possible number of pathways if each area were connected with all others. The actual degree of connectivity is likely to be closer to 40%. The great majority of pathways involve reciprocal connections between areas. There are also extensive connections with cortical areas outside the visual system proper, including the somatosensory cortex, as well as neocortical, transitional, and archicortical regions in the temporal and frontal lobes. In the somatosensory/motor system, there are 62 identified pathways linking 13 cortical areas, suggesting an overall connectivity of about 40%. Based on the laminar patterns of connections between areas, we propose a hierarchy of visual areas and of somatosensory/motor areas that is more comprehensive than those suggested in other recent studies. The current version of the visual hierarchy includes 10 levels of cortical processing. Altogether, it contains 14 levels if one includes the retina and lateral geniculate nucleus at the bottom as well as the entorhinal cortex and hippocampus at the top. Within this hierarchy, there are multiple, intertwined processing streams, which, at a low level, are related to the compartmental organization of areas V1 and V2 and, at a high level, are related to the distinction between processing centers in the temporal and parietal lobes. However, there are some pathways and relationships (about 10% of the total) whose descriptions do not fit cleanly into this hierarchical scheme for one reason or another. In most instances, though, it is unclear whether these represent genuine exceptions to a strict hierarchy rather than inaccuracies or uncertainities in the reported assignment.

/pdf/distributed-hierarchical-processing-in-the-primate-cerebral-1dvblm6d82.pdf

Distributed Hierarchical Processing in the Primate Cerebral Cortex

During performance of attention-demanding cognitive tasks, certain regions of the brain routinely increase activity, whereas others routinely decrease activity. In this study, we investigate the extent to which this task-related dichotomy is represented intrinsically in the resting human brain through examination of spontaneous fluctuations in the functional MRI blood oxygen level-dependent signal. We identify two diametrically opposed, widely distributed brain networks on the basis of both spontaneous correlations within each network and anticorrelations between networks. One network consists of regions routinely exhibiting task-related activations and the other of regions routinely exhibiting task-related deactivations. This intrinsic organization, featuring the presence of anticorrelated networks in the absence of overt task performance, provides a critical context in which to understand brain function. We suggest that both task-driven neuronal responses and behavior are reflections of this dynamic, ongoing, functional organization of the brain.

https://www.pnas.org/content/pnas/102/27/9673.full.pdf

The human brain is intrinsically organized into dynamic, anticorrelated functional networks

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

Evoked activity in the mammalian cortex and the resulting behavioral responses exhibit a large variability to repeated presentations of the same stimulus. This study examined whether the variability can be attributed to ongoing activity. Ongoing and evoked spatiotemporal activity patterns in the cat visual cortex were measured with real-time optical imaging; local field potentials and discharges of single neurons were recorded simultaneously, by electrophysiological techniques. The evoked activity appeared deterministic, and the variability resulted from the dynamics of ongoing activity, presumably reflecting the instantaneous state of cortical networks. In spite of the large variability, evoked responses in single trials could be predicted by linear summation of the deterministic response and the preceding ongoing activity. Ongoing activity must play an important role in cortical function and cannot be ignored in exploration of cognitive processes.

https://science.sciencemag.org/content/sci/273/5283/1868.full.pdf

Dynamics of Ongoing Activity: Explanation of the Large Variability in Evoked Cortical Responses

Optical imaging of cortical activity offers several advantages over conventional electrophysiological and anatomical techniques. One can map a relatively large region, obtain successive maps to different stimuli in the same cortical area and follow variations in response over time. In the intact mammalian brain this imaging has been accomplished with the aid of voltage sensitive dyes. However, it has been known for many years that some intrinsic changes in the optical properties of the tissue are dependent on electrical or metabolic activity. Here we show that these changes can be used to study the functional architecture of cortex. Optical maps of whisker barrels in the rat and the orientation columns in the cat visual cortex, obtained by reflection measurements of the intrinsic signal, were confirmed with voltage sensitive dyes or by electrophysiological recordings. In addition, we describe an intrinsic signal originating from small arteries which can be used to investigate the communication between local neuronal activity and the microvasculature. One advantage of the method is that it is non-invasive and does not require dyes, a clear benefit for clinical applications.

Functional architecture of cortex revealed by optical imaging of intrinsic signals

Modern neuroimaging techniques use signals originating from microcirculation to map brain function. In this study, activity-dependent changes in oxyhemoglobin, deoxyhemoglobin, and light scattering were characterized by an imaging spectroscopy approach that offers high spatial, temporal, and spectral resolution. Sensory stimulation of cortical columns initiates tissue hypoxia and vascular responses that occur within the first 3 seconds and are highly localized to individual cortical columns. However, the later phase of the vascular response is less localized, spreading over distances of 3 to 5 millimeters.

https://science.sciencemag.org/content/sci/272/5261/551.full.pdf

Interactions between electrical activity and cortical microcirculation revealed by imaging spectroscopy : Implications for functional brain mapping

We have shown previously the existence of small, activity-dependent changes in intrinsic optical properties of cortex that are useful for optical imaging of cortical functional architecture. In this study we introduce a higher resolution optical imaging system that offers spatial and temporal resolution exceeding that achieved by most alternative imaging techniques for imaging cortical functional architecture or for monitoring local changes in cerebral blood volume or oxygen saturation. In addition, we investigated the mechanisms responsible for the activity-dependent intrinsic signals evoked by sensory stimuli, and studied their origins and wavelength dependence. These studies enabled high-resolution visualization of cortical functional architecture at wavelengths ranging from 480 to 940 nm. With the use of near-infrared illumination it was possible to image cortical functional architecture through the intact dura or even through a thinned skull. In addition, the same imaging technique proved useful for imaging and discriminating sensory-evoked, activity-dependent changes in local blood volume and oxygen saturation (oxygen delivery). Illumination at 570 nm allowed imaging of activity-dependent blood volume increases, whereas at 600-630 nm, the predominant signal probably originated from activity-dependent oxygen delivery from capillaries. The onset of oxygen delivery started prior to the blood volume increase. Thus, optical imaging based on intrinsic signals is a minimally invasive procedure for monitoring short- and long-term changes in cerebral activity.

https://www.pnas.org/content/pnas/87/16/6082.full.pdf

Cortical functional architecture and local coupling between neuronal activity and the microcirculation revealed by in vivo high-resolution optical imaging of intrinsic signals.

Spontaneous cortical activity--ongoing activity in the absence of intentional sensory input--has been studied extensively, using methods ranging from EEG (electroencephalography), through voltage sensitive dye imaging, down to recordings from single neurons. Ongoing cortical activity has been shown to play a critical role in development, and must also be essential for processing sensory perception, because it modulates stimulus-evoked activity, and is correlated with behaviour. Yet its role in the processing of external information and its relationship to internal representations of sensory attributes remains unknown. Using voltage sensitive dye imaging, we previously established a close link between ongoing activity in the visual cortex of anaesthetized cats and the spontaneous firing of a single neuron. Here we report that such activity encompasses a set of dynamically switching cortical states, many of which correspond closely to orientation maps. When such an orientation state emerged spontaneously, it spanned several hypercolumns and was often followed by a state corresponding to a proximal orientation. We suggest that dynamically switching cortical states could represent the brain's internal context, and therefore reflect or influence memory, perception and behaviour.

/pdf/spontaneously-emerging-cortical-representations-of-visual-1x1lai59ee.pdf

Amiram Grinvald

Papers

Dynamics of Ongoing Activity: Explanation of the Large Variability in Evoked Cortical Responses

Functional architecture of cortex revealed by optical imaging of intrinsic signals

Interactions between electrical activity and cortical microcirculation revealed by imaging spectroscopy : Implications for functional brain mapping

Cortical functional architecture and local coupling between neuronal activity and the microcirculation revealed by in vivo high-resolution optical imaging of intrinsic signals.

Spontaneously emerging cortical representations of visual attributes