Coherent oscillations and short-term plasticity in corticothalamic networks

The neocortex and thalamus are a unified oscillatory machine. Different types of brain rhythms, which characterize various behavioral states, are combined within complex wave-sequences. During the stage of sleep that is associated with low-frequency and high-amplitude brain rhythms, the excitatory component of a cortically generated slow oscillation is effective in triggering thalamically generated rhythms and in increasing their spatiotemporal coherence over widespread territories. Thus, the study of coherent oscillations, as they appear naturally during states of vigilance in animals and humans, requires intact-brain preparations in which the neocortex and thalamus engage in a permanent dialog. Sleep oscillations are associated with rhythmic spike-bursts or spike-trains in thalamic and cortical neurons, which lead to persistent excitability changes consisting of increased depolarizing responses and decreased inhibitory responses. These shortterm plasticity processes could be used to consolidate memory traces acquired during wakefulness, but can also lead to paroxysmal (hypersynchronous) episodes, similar to those observed in some epileptic seizures.