It is known that the frequency of human sleep spindles varies across the cortical regions. In addition, it is reported that spindles of the relatively higher frequency occur preferentially at the positive phase of slow waves, whereas those of the lower frequency ride on the negative phase. However, it remains unclear whether this phase relation is constant within a cortical area or not. Because spindles and slow waves probably correlate with memory processing, studying the organization of spindles and slow waves might be a clue to understand the mechanisms of memory.
For that purpose, we implanted electrodes in various cortical areas of the monkey and recorded cortical field potentials during natural sleep using a telemetry system. We analyzed the frequency of spindles and the phase relation between spindles and slow waves.
Frequency of spindles tended to be faster in the dorsolateral prefrontal cortex than in the parietal and medial prefrontal cortices. The most preferred phase of slow waves at the spindle occurrence was different among the cortical areas: it was at the positive phase in the dorsolateral prefrontal cortex, while it was at the negative phase in the parietal and medial prefrontal cortices. These results indicate that the frequency of spindles and the phase relation with slow waves are dependent on the cortical areas in which the spindles appear, and that there is a cross-frequency coupling between spindles and slow waves.
The topographical organization of the spindle frequency may suggest that spindles in each cortical region have their own rhythm generator for that area. The difference in the phase relations may reflect the different properties of the thalamocortical projections through which slow waves come. The cross-frequency coupling between spindles and slow waves may serve as a mechanism to link spindles in different cortical areas, and thus it may play a role for information processing such as memory consolidation.