Abstract
Cortical sensory systems are active with rich patterns of activity during sleep and under light anesthesia. Remarkably, this activity shares many characteristics with those present when the awake brain responds to sensory stimuli. We review two specific forms of such activity: slow-wave activity (SWA) in the adult brain and spindle bursts in developing brain. SWA is composed of 0.5–4 Hz resting potential fluctuations. Although these fluctuations synchronize wide regions of cortex, recent large-scale imaging has shown spatial details of their distribution that reflect underlying cortical structural projections and networks. These networks are regulated, as prior awake experiences alter both the spatial and temporal features of SWA in subsequent sleep. Activity patterns of the immature brain, however, are very different from those of the adult. SWA is absent, and the dominant pattern is spindle bursts, intermittent high frequency oscillations superimposed on slower depolarizations within sensory cortices. These bursts are driven by intrinsic brain activity, which act to generate peripheral inputs, for example via limb twitches. They are present within developing sensory cortex before they are mature enough to exhibit directed movements and respond to external stimuli. Like in the adult, these patterns resemble those evoked by sensory stimulation when awake. It is suggested that spindle-burst activity is generated purposefully by the developing nervous system as a proxy for true external stimuli. While the sleep-related functions of both slow-wave and spindle-burst activity may not be entirely clear, they reflect robust regulated phenomena which can engage select wide-spread cortical circuits. These circuits are similar to those activated during sensory processing and volitional events. We highlight these two patterns of brain activity because both are prominent and well-studied forms of spontaneous activity that will yield valuable insights into brain function in the coming years.
Highlights
A key feature noted by early investigators of brain activity was that its pattern depends on behavioral state
Sleep would suggest a brain at rest, it remains active with a structured set of ongoing spontaneous activity patterns that do not rely on external sensory inputs
Using voltage sensitive dye (VSD) imaging in cortex of rat pups, we have shown that highly specific patterns of activation across the cortex follow hindlimb or tail twitches (McVea et al, 2012; Figures 3B,C)
Summary
A key feature noted by early investigators of brain activity was that its pattern depends on behavioral state. Unique forms of spontaneous activity are present during sleep in the immature brain These activity patterns, like the adult brain activity discussed above, are not reflective of current sensory or motor processes involving the external world. We discuss spindle bursts, a pattern of intermittent fast oscillations at 5–25 Hz in the developing brain which play an important role in maturation of sensory systems (Khazipov and Luhmann, 2006; Blankenship and Feller, 2009; Blumberg et al, 2013; Cirelli and Tononi, 2015) In discussing how both patterns are generated and shaped by neural circuits, we hope to reinforce the concept that sleep-related activity within sensory systems and other regions of the brain are neither noise nor idle ‘‘holdingpatterns’’, but rather are actively formed and deployed for specific purposes
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