8 Neuronal Oscillations Scale Up and Scale Down the Brain

Progress in understanding the global brain dynamics has remained slow to date in largepart because of the highly multiscale nature of brain activity. Indeed, normal braindynamics is characterized by complex interactions between multiple levels: from themicroscopic scale of single neurons to the mesoscopic level of local groups of neurons,and finally to the macroscopic level of the whole brain. Among the most difficult tasksare those of identifying which scales are significant for a given particular function anddescribing how the scales affect each other. It is important to realize that the scales oftime and space are linked together, or even intertwined, and that causal inference is farmore ambiguous between than within levels. We approach this problem from theperspective of our recent work on simultaneous recording from microand macro-electrodes in the human brain. We propose a physiological description of thesemultilevel interactions, based on phase–amplitude coupling of neuronal oscillationsthat operate at multiple frequencies and on different spatial scales. Specifically, theamplitude of the oscillations on a particular spatial scale is modulated by phasicvariations in neuronal excitability induced by lower frequency oscillations that emergeon a larger spatial scale. Following this general principle, it is possible to scale up orscale down the multiscale brain dynamics. It is expected that large-scale networkoscillations in the low-frequency range, mediating downward effects, may play animportant role in attention and consciousness.