Scientific Understanding of Consciousness
Sleep in Individual Neurons
Nature, 472, 443–447, (28 April 2011)
Local sleep in awake rats
Vladyslav V. Vyazovskiy,1 Umberto Olcese,1, 2 Erin C. Hanlon,1 Yuval Nir,1
Chiara Cirelli1 & Giulio Tononi1
1Department of Psychiatry, University of Wisconsin-Madison, Madison, 6001 Research Park Boulevard, Wisconsin 53719, USA
2PERCRO Laboratory, Scuola Superiore Sant'Anna, 56217, Via Martiri, 11, Pisa, Italy
In an awake state, neurons in the cerebral cortex fire irregularly and electroencephalogram (EEG) recordings display low-amplitude, high-frequency fluctuations. During sleep, neurons oscillate between ‘on’ periods, when they fire as in an awake brain, and ‘off’ periods, when they stop firing altogether and the EEG displays high-amplitude slow waves. However, what happens to neuronal firing after a long period of being awake is not known. Here we show that in freely behaving rats after a long period in an awake state, cortical neurons can go briefly ‘offline’ as in sleep, accompanied by slow waves in the local EEG. Neurons often go offline in one cortical area but not in another, and during these periods of ‘local sleep’, the incidence of which increases with the duration of the awake state, rats are active and display an ‘awake’ EEG. However, they are progressively impaired in a sugar pellet reaching task. Thus, although both the EEG and behaviour indicate wakefulness, local populations of neurons in the cortex may be falling asleep, with negative consequences for performance.
While animals are awake, the eyes are usually open; they move around and respond to their surroundings. During sleep, the eyes close, behaviour stops and animals fail to respond to stimuli. Studies of brain activity also show major differences between an awake state and non-rapid-eye-movement (NREM) sleep, which makes up ~80% of all sleep. While awake, neurons in the cerebral cortex fire irregularly, their membrane potential is tonically depolarized and an EEG shows low-voltage, high-frequency activity. During NREM sleep, neurons become bistable owing to a decrease in the level of neuromodulators: their membrane potential oscillates between a depolarized ‘up’ state similar to that seen in an awake state and a hyperpolarized ‘down’ state during which they cease firing altogether1. These slow oscillations occur in a range between 0.1 Hz and 6 Hz and they are detectable in the form of multi-unit activity (‘on’ and ‘off’ periods) as well as in EEG slow waves.
By staying awake too long, one becomes tired and many studies have demonstrated attention lapses, poor judgement and frequent mistakes in various cognitive tasks, even when the subject may not feel particularly sleepy. Moreover, an EEG shows some trace of the sleep/wake history: the longer a subject has been awake, the higher the spectral power in the slow-wave range (0.5–4 Hz) of the EEG in subsequent sleep, corresponding to larger and more frequent slow waves and to more intense and synchronous neuronal activity. Local variations in cortical activity while awake are associated with local changes during subsequent sleep and with a sleep-dependent increase in task performance.These changes are reversed progressively in the course of sleep. The awake EEG also shows changes that reflect the duration of previous awake states, with power increasing in the theta range (5–7 Hz). Likewise, neuroimaging studies show changes in blood flow and metabolism after sleep deprivation, with some brain regions undergoing decreases in activation and others, increases in activation.
To investigate neuronal activity during a prolonged awake state, we implanted a group of adult rats (n = 11) with 16-channel microwire arrays in deep layers of the frontal motor cortex and recorded both the local field potentials (LFPs) and local multi-unit activity across periods of spontaneous sleep and awake states. As expected, the awake LFP was characterized by low-amplitude fast waves and theta waves, accompanied by irregular, tonic multi-unit activity. This was readily distinguishable from the LFP of NREM sleep, in which high-amplitude slow waves occurred concomitantly with synchronous ‘on’ and ‘off’ periods at the level of multi-unit activity. As expected, the awake LFP was characterized by low-amplitude fast waves and theta waves, accompanied by irregular, tonic multi-unit activity. This was readily distinguishable from the LFP of NREM sleep, in which high-amplitude slow waves occurred concomitantly with synchronous ‘on’ and ‘off’ periods at the level of multi-unit activity.
It is common experience that tiredness after prolonged sleep deprivation can be manifested as ‘microsleeps’: brief episodes of 3–15 s during which a person appears suddenly asleep (eyes closed or closing), may not respond to stimuli and shows sleep-like EEG activity. Clearly, such microsleeps can be dangerous during tasks requiring alertness and the detection of sleep-like behaviour or EEG changes is being pursued to reduce risks.
These findings show that, in animals kept awake beyond their normal sleep time, populations of neurons in different cortical areas can suddenly go ‘offline’ in a way that resembles the off periods of NREM sleep. The main differences are that during sleep, virtually all cortical neurons show on–off oscillations in the slow-wave frequency range, the EEG displays typical sleep slow waves and spindles and the animal is behaviourally immobile and unresponsive, with eyes closed. During a prolonged awake state, however, only subsets of neurons enter off periods, usually for shorter durations; the EEG is typical of an awake state and the animal appears behaviourally awake with eyes open and is responsive to stimuli. Furthermore, the number of off periods increases with the duration of wakefulness, indicating that the likelihood of subsets of neurons going offline in an otherwise-awake cortex increases with sleep pressure. As shown here, the progressive changes observed during sleep deprivation are the mirror image of changes during recovery sleep: neuronal firing rates during on periods, the number and duration of off periods, the number of neurons participating synchronously in off periods and the low-frequency content of the EEG all increase during an awake state just as they decrease during sleep. This supports the concept of homeostatic regulation of the need for sleep.
Perhaps the most striking result of this study is that in the sleep-deprived brain, subsets of neurons may enter an off period in one cortical area but not in another and that even within the same cortical area, some neurons may be off while others remain on. On the basis of this evidence, the wake behaviour of a sleep-deprived subject might be characterized as a covert form of ‘dormiveglia’ or ‘sleep/wake'. Moreover, as shown here using the sugar pellet reaching task, the increasing occurrence of local off periods during a prolonged period of being awake was associated with worsening performance in the task. Paradigms should be developed to associate the occurrence of off periods in specific subsets of neurons more precisely with specific performance failures but these initial findings raise the intriguing possibility that ‘local sleep’ in an awake brain may be responsible for cognitive impairments due to sleep deprivation or restriction. It is especially relevant that cognitive impairments, including defective judgment and irritability, may occur despite an outward impression of wakefulness, the lack of subjective insight and an awake EEG. Sporadic, local neuronal off periods in sleep-deprived subjects may be analogous to the sporadic, local, hyper-synchronous discharges seen in partial epilepsy. Such local events can be detected with careful EEG recordings as interictal spikes and may cause momentary lapses (absence) without overt behavioural signs.
We can only speculate about the mechanisms underlying the local awake off periods. A spontaneous slow oscillation of membrane potentials can occur in the mouse barrel cortex during quiet wakefulness and can affect the amplitude of evoked responses, although it is not clear whether such ‘down’ states occur during active behaviour, are local, affect performance and most importantly, reflect increasing sleep pressure. Although we do not know whether the awake off periods we observed in freely moving rats are associated with neuronal hyper-polarization, their overall similarity to sleep off periods, and the finding that they become more frequent with increasing sleep pressure, indicates that they may be an expression of increasing bistability in neurons. Thus, in addition to the global state of instability that is a hallmark of sleep deprivation, there can also be a local instability, at least in the cerebral cortex. Bistability between on and off periods could be triggered by decreasing levels of arousal-promoting neuromodulators, especially because cholinergic and noradrenergic neurons, for instance, do not always discharge in tight synchrony and presynaptic release can be modulated locally.
Local sleep in awake rats may be either an adaptive or a maladaptive response. In some cetaceans and birds, one hemisphere can remain awake while the other is in slow wave sleep, an adaptive response that permits them to continue swimming, flying or monitoring the environment. The ability to control behaviour actively with some neural circuits while others may be idling could be evolutionarily advantageous. However, dissociated behavioural states, such as sleepwalking, REM sleep behaviour disorder and other parasomnias, are clearly maladaptive. Because local awake off periods are associated with locally increased excitability after intensive training and with failures in performance, it is likely that they represent a form of neuronal tiredness due to use-dependent factors, such as synaptic overload. A question for the future is whether local off periods in the awake state may also serve functional roles, from energy saving, to the initiation of a local restorative process.
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