Sequential structure of neocortical spontaneous activity in vivo

A default mode of brain function: A brief history of an evolving idea

The concept of a default mode of brain function arose out of a focused need to explain the appearance of activity decreases in functional neuroimaging data when the control state was passive visual fixation or eyes closed resting. The problem was particularly compelling because these activity decreases were remarkably consistent across a wide variety of task conditions. Using PET, we determined that these activity decreases did not arise from activations in the resting state. Hence, their presence implied the existence of a default mode. While the unique constellation of brain areas provoking this analysis has come to be known as the default system, all areas of the brain have a high level of organized default functional activity. Most critically, this work has called attention to the importance of intrinsic functional activity in assessing brain behavior relationships.

Precisely timed spatiotemporal patterns of neural activity in dissociated cortical cultures

Recurring patterns of neural activity, a potential substrate of both information transfer and transformation in cortical networks, have been observed in the intact brain and in brain slices. Do these patterns require the inherent cortical microcircuitry of such preparations or are they a general property of self-organizing neuronal networks? In networks of dissociated cortical neurons from rats--which lack evidence of the intact brain's intrinsic cortical architecture--we have observed a robust set of spontaneously repeating spatiotemporal patterns of neural activity, using a template-matching algorithm that has been successful both in vivo and in brain slices. The observed patterns in cultured monolayer networks are stable over minutes of extracellular recording, occur throughout the culture's development, and are temporally precise within milliseconds. The identification of these patterns in dissociated cultures opens a powerful methodological avenue for the study of such patterns, and their persistence despite the topological and morphological rearrangements of cellular dissociation is further evidence that precisely timed patterns are a universal emergent feature of self-organizing neuronal networks.

Development and plasticity of spontaneous activity and Up states in cortical organotypic slices

Cortical computations are an emergent property of neural dynamics. To understand how neural dynamics emerges within local cortical networks, we characterized the development and underlying mechanisms of spontaneous dynamics in cortical organotypic slices. We observed not only a quantitative increase in the levels of spontaneous dynamics, but a qualitative transition from brief bursts of activity to well defined Up states during the first 4 weeks in vitro. Analysis of cellular and synaptic properties indicates that these changes are driven by increasing excitatory drive accompanied by changes in the balance of excitation and inhibition. Examination of the structure of spontaneous dynamics revealed no evidence of precisely repeating patterns. Slices exposed to chronic patterned stimulation exhibited decreased levels of spontaneous activity, suggesting homeostatic control of the levels of network activity. Together, these results suggest that Up states reflect a fundamental mode of network dynamics that emerges through the orchestrated regulation of multiple cellular and synaptic properties in parallel.

Cortical songs revisited: a lesson in statistics

Recordings from single neurons in the cortex have revealed precisely repeating patterns of synaptic events. These repeats are known as cortical "motifs" and have been suggested to reflect the precise replay of spatiotemporal firing sequences ("synfire" chains). In this issue of Neuron, Mokeichev et al. use compelling statistical analysis to show that, rather than being evidence of deterministic synfire chains, such cortical motifs are bound to appear by chance due to the natural dynamics of voltage fluctuations in neurons.

Interaction between neocortical and hippocampal networks via slow oscillations

Both the thalamocortical and limbic systems generate a variety of brain state-dependent rhythms but the relationship between the oscillatory families is not well understood. Transfer of information across structures can be controlled by the offset oscillations. We suggest that slow oscillation of the neocortex, which was discovered by Mircea Steriade, temporally coordinates the self-organized oscillations in the neocortex, entorhinal cortex, subiculum and hippocampus. Transient coupling between rhythms can guide bidirectional information transfer among these structures and might serve to consolidate memory traces.