Hippocampal theta rhythm synchronizes visual neurons in sleep and waking

Effects of the muscarinic agonist pilocarpine on vigilance states and locomotor activity in ring doves

Cholinergic systems play a significant role in regulating a variety of behavioral functions in mammals and birds. The aim of this work is to study the effects of the muscarinic agonist pilocarpine on behavioral states by visual inspection and electroencephalographic recording; also, locomotor activity was continuously recorded by infrared interruption system in ring doves. The current results in birds demonstrated that the muscarinic agonist pilocarpine (1 and 3mg/kg, i.p.) primarily induced theta activity in addition to promote passive waking, while diminished active waking, the EEG slow wave rhythm and REM sleep in ring doves. The locomotor activity recorded continuously in ring doves diminished after pilocarpine treatment, which was in good agreement with the observed reduction of active waking derived of the EEG study. Altogether, the current results are similar to the effects of pilocarpine previously reported in mammals. In conclusion, hippocampal theta rhythm in birds suggests that this rhythm is an ancestral property of hippocampal function and similar cholinergic mechanisms regulate vigilance states and theta generation in mammals and birds.

From a face to its category via a few information processing states in the brain

Cognitive neuroscience assumes a correspondence between specific spatio-temporal patterns of neural activity and the states of a mechanism that processes cognitive information. Mechanistic explanations of cognition should therefore translate patterns of neural activity into the components of a formal mechanism: a set of information processing states and their transitions. For the first time, we carried out this research programme with four naive observers instructed to categorise randomly presented face information. With classification image techniques, we revealed the diagnostic features that the brain requires to produce correct behaviour (i.e., two eyes for gender categorisation in one session; the mouth for expression in the other session). With the same techniques applied to brain signals, we revealed the features processing states associated with modulations of oscillatory EEG energy (measured on occipito-temporal face-sensitive electrodes). Here we show how transitions between distinct feature processing states in the theta/alpha [4-12 Hz] oscillatory bands implement two face categorisations. On the left and right occipito-temporal electrodes of each observer, processing of the contra-lateral eye precedes bilateral integration of the features required for behaviour. For the first time, we relate stimulus information to behaviour via sequences of categorisation-specific feature processing states in the brain.

Effects of pilocarpine on the cortical and hippocampal theta rhythm in different vigilance states in rats

It has been suggested that theta rhythm gates the flow of information between the hippocampus and cortex during memory processes. The cholinergic system plays an important role in regulating vigilance states and in generating theta rhythm. This study aims to analyse the effects of the muscarinic agonist pilocarpine (120 and 360 microg, i.c.v.) on hippocampal and frontal cortical theta rhythm during several vigilance states in rats. Pilocarpine injection increased the duration and number of episodes with theta activity, particularly when theta rhythm appeared during waking states in the cortex and hippocampus simultaneously. It seems that the effects of pilocarpine are related to the appearance of cortical theta activity in waking states, and suggest that pilocarpine could modify the transference rate of information from the hippocampus to cortex in rats during wakefulness states, in relation to the postulated effect of cholinergic system modulating memory consolidation.

Novelty-induced correlation between visual neurons and the hippocampal theta rhythm in sleep and wakefulness

Various rhythms have been shown to affect sensory processing such as the waking-sleep cycle and the hippocampal theta waves. Changes in the firing of visual lateral geniculate nucleus neurons have been reported to be dependent on the animal's behavioral state. The lateral geniculate extracellular neuronal firing and hippocampal field activity were recorded in chronically implanted animals to analyze the relationship during quiet wakefulness and sleep associated with stimulation shifts that may introduce novelty. During wakefulness, a change in light flash stimulation pattern (stimuli frequency shift, stimuli on and off) caused an increment in the theta band power in 100% of the cases and a phase-locking of the spikes in 53% of the recorded neurons. During slow wave sleep, there were no consistent changes in the theta power notwithstanding 13% of the neurons exhibited phase-locking, i.e., novelty may induce changes in the temporal correlation of visual neuronal activity with the hippocampal theta rhythm in sleep. The present results suggest that visual processing in slow wave sleep exists, while auditory information and learning were reported during slow wave sleep in animals and newborn humans. The changes in the theta power as well as in the neuronal phase-locking amount indicate that in slow wave sleep, the ability of the hippocampus to detect/process novelty, although present, may be decreased. This is consistent with the noticeable decrease in awareness of the environment during sleep.

Prefrontal Phase Locking to Hippocampal Theta Oscillations

The interactions between cortical and hippocampal circuits are critical for memory formation, yet their basic organization at the neuronal network level is not well understood. Here, we demonstrate that a significant portion of neurons in the medial prefrontal cortex of freely behaving rats are phase locked to the hippocampal theta rhythm. In addition, we show that prefrontal neurons phase lock best to theta oscillations delayed by approximately 50 ms and confirm this hippocampo-prefrontal directionality and timing at the level of correlations between single cells. Finally, we find that phase locking of prefrontal cells is predicted by the presence of significant correlations with hippocampal cells at positive delays up to 150 ms. The theta-entrained activity across cortico-hippocampal circuits described here may be important for gating information flow and guiding the plastic changes that are believed to underlie the storage of information across these networks.

Sleep protects excitatory cortical circuits against oxidative damage

Activity in excitatory cortical pathways increases the oxidative metabolism of the brain and the risk of oxidative damage. Oxyradicals formed during periods of activity are mopped up by neural pools of nuclear factor kappa-B resulting in their activation and translocation to cell nuclei. During waking hours, glucocorticoids inhibit transactivation by nuclear factor kappa-B, increase central norepinephrine release, and elevate expression of prostaglandin D2. The build-up of nuclear factor kappa-B and prostaglandin D2 produces sleep pressures leading to sleep onset, normally gated by circadian melatonin release. During slow wave sleep nuclear factor kappa-B induces transcription of synaptogenic and antioxidant products and synaptic remodeling follows. Synaptically remodeled neural circuits have modified conductivity patterns and timescales and need to be resynchronized with existing unmodified neural circuits. The resynchronization process, mediated by theta rhythm, occurs during rapid eye movement sleep and is orchestrated from pontine centers. Resynchronization of remodeled neural circuits produces dreams. The waking state results upon successful resynchronization. Rapid eye movement sleep deprivation results in a lack of resynchronization and leads to cognitive inefficiencies. The model presented here proposes that the primary purpose of sleep is to protect cortical circuits against oxidative damage by reducing cortical activity and by remodeling and resynchronizing cortical circuits during this period of reduced activity to sustain new patterns of activation more effectively.

In vivo approach to the cellular mechanisms for sensory processing in sleep and wakefulness

1. The present review analyzes sensory processing during sleep and wakefulness from a single neuronal viewpoint. Our premises are that processing changes throughout the sleep-wakefulness cycle may be at least partially evidenced in single neurons by (a) changes in the phase locking of the response to the hippocampal theta rhythm, (b) changes in the discharge rate and firing pattern of the response to sound, and (c) changes in the effects of the neurotransmitters involved in the afferent and efferent pathways. 2. The first part of our report is based on the hypothesis that the encoding of sensory information needs a timer in order to be processed and stored, and that the hippocampal theta rhythm could contribute to the temporal organization. We have demonstrated that the guinea pig's auditory and visual neuronal discharge exhibits a temporal relationship (phase locking) to the hippocampal theta waves during wakefulness and sleep phases. 3. The concept that the neural network organization during sleep versus wakefulness is different and can be modulated by sensory signals and vice versa, and that the sensory input may be influenced by the CNS state, i.e., asleep or awake, is introduced. During sleep the evoked firing of auditory units increases, decreases, or remains similar to that observed during quiet wakefulness. However, there has been no auditory unit yet that stops firing as the guinea pig enters sleep. Approximately half of the cortical neurons studied did not change firing rate when passing into sleep while others increased or decreased. Thus, the system is continuously aware of the environment. We postulate that those neurons that changed their evoked firing during sleep are also related to still unknown sleep processes. 4. Excitatory amino acid neurotransmitters participate in the synaptic transmission of the afferent and efferent pathways in the auditory system. In the inferior colliculus, however, the effects of glutamate's mediating the response to sound and the efferent excitation evoked by cortical stimulation failed to show differences in sleep and wakefulness. 5. Considering that neonates and also infants spend most of the time asleep, the continuous arrival of sensory information to the brain during both sleep phases may serve to "sculpt" the brain by activity-dependent mechanisms of neural development, as has been postulated for wakefulness.