EEG spindles in the rat: evidence for a synchronous network phenomenon

Whether physiological and pathological electroencephalographic (EEG) spindling phenomena are generated by similar mechanisms remains unknown. We studied four different types of spindling events analysing the behaviour of 19 brain regions during these events in the intact brain of freely behaving rats hypothesising similar generation with a leading role of thalamus or cortex for the dominant rhythm of the spindles with a hierarchy of time delays for propagation throughout the brain led by these structures. Autoregressive analysis revealed that all structures studied were capable of leading a spindle, although some did so infrequently. The highest incidence of significant leadership was shown by hindlimb cortex with a proportion of 0.38 during picrotoxin spindles. Contralateral time delays within bilateral structures were extremely small demonstrating synchronous activity with no dominant hemisphere for any of the spindling activities (range: 0.1+/-0.4 ms for frontal cortex to 10.0+/-1.6 ms for piriform cortex). Between different structures, time delays demonstrated that spindling activity occurred in all structures almost simultaneously (range: 0.2+/-1.4 ms between frontal cortex and piriform cortex to 12.3+/-1.2 ms between hindlimb cortex and amygdala). We conclude that different spindle types appear to be subtle alterations of a single phenomenon, during which many brain regions briefly synchronise.