Abstract
Cortical activity exhibits persistent metastable dynamics. Assemblies of neurons transiently couple (integrate) and decouple (segregate) at multiple spatiotemporal scales; both integration and segregation are required to support metastability. Integration of distant brain regions can be achieved through long range excitatory projections, but the mechanism supporting long range segregation is not clear. We argue that the thalamocortical matrix connections, which project diffusely from the thalamus to the cortex and have long been thought to support cortical gain control, play an equally-important role in cortical segregation. We present a computational model of the diffuse thalamocortical loop, called the competitive cross-coupling (CXC) spiking network. Simulations of the model show how different levels of tonic input from the brainstem to the thalamus could control dynamical complexity in the cortex, directing transitions between sleep, wakefulness and high attention or vigilance. The model also explains how mutually-exclusive activity could arise across large portions of the cortex, such as between the default-mode and task-positive networks. It is robust to noise but does not require noise to autonomously generate metastability. We conclude that the long range segregation observed in brain activity and required for global metastable dynamics could be provided by the thalamocortical matrix, and is strongly modulated by brainstem input to the thalamus.
Highlights
In wakefulness and in rapid eye movement (REM) sleep, cortical activity exhibits persistent ongoing complex dynamics (Breakspear et al, 2003; Honey et al, 2007)
Autonomous (Self-sustained) Metastable Dynamics To establish the baseline complex dynamics supported by the CXC network, constant input from the AAS was first set to the baseline level of 1 and synaptic conductances were set to standard values for the network
Metastable network dynamics were robust to moderate network parameter changes, significant modification of these parameters resulted in either collapse of network dynamics into a limit cycle resembling seizure or failure of activity to propagate causing the total network activity to fall to zero
Summary
In wakefulness and in rapid eye movement (REM) sleep, cortical activity exhibits persistent ongoing complex dynamics (Breakspear et al, 2003; Honey et al, 2007). During times when the brain is not actively processing sensory stimuli or task-related events, and as such is in a state known as the “resting” or “default-mode” state, cortical activity is Global segregation of cortical activity concentrated in a well-defined sub-network including regions of frontal and association cortices ( the ventromedial prefrontal cortex, posterior cingulate cortex, ventral precuneus, and parts of the medial temporal and medial, lateral and inferior parietal cortices) (Greicius et al, 2003; Uddin et al, 2008). Activity of this default-mode network (DMN) is anticorrelated with activity in much of the rest of the cortex—that is, activation of the DMN and of those cortical centers used for sensory and task-related processing is largely mutually exclusive (Greicius et al, 2003; Uddin et al, 2008; Tomasi and Volkow, 2011)
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