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Clocks tick, bridges and skyscrapers vibrate, neuronal networks oscillate. Are neuronal oscillations an inevitable by-product, similar to bridge vibrations, or an essential part of the brain's design? Mammalian cortical neurons form behavior-dependent oscillating networks of various sizes, which span five orders of magnitude in frequency. These oscillations are phylogenetically preserved, suggesting that they are functionally relevant. Recent findings indicate that network oscillations bias input selection, temporally link neurons into assemblies, and facilitate synaptic plasticity, mechanisms that cooperatively support temporal representation and long-term consolidation of information.
The first human electroencephalographic (EEG) pattern described was an 8 to 12 Hz rhythm, the alpha waves of Berger (1), followed by a barrage of intensive clinical and basic research. From scalp recordings, investigators identified various other oscillatory patterns that were particularly obvious during rest and sleep. However, the scalp EEG during conscious, waking behavior demonstrated lowamplitude, "desynchronized" patterns. This apparent inverse relation between cognitive activity and brain rhythms was further emphasized by the dominance of oscillations in anesthesia and epilepsy, states associated with loss of consciousness (2). Therefore, the motivation to relate these "idling" or even harmful rhythms to complex cognitive brain operations was diminished.
The recent resurgence of interest in neuronal oscillations is a result of several parallel developments. Whereas in the past we simply watched oscillations, we have recently begun creating them under controlled situations (3-8). Detailed biophysical studies revealed that even single neurons are endowed with complex dynamics, including their intrinsic abilities to resonate and oscillate at multiple frequencies (9, 10), which suggests that precise timing of their activity within neuronal networks could represent information. At the same time, the neuronal assembly structures of the oscillatory patterns found during sleep were related to the experiences of the previous awake period (11, 12). These results led to the tantalizing conjecture that perception, memory, and even consciousness could result from synchronized networks (13-17). The synchronous activity of oscillating networks is now viewed as the critical "middle ground" linking single-neuron activity to behavior (2-6, 15). This emerging new field, "neuronal oscillations," has created an interdisciplinary platform that cuts across psychophysics, cognitive psychology, neuroscience, biophysics, computational modeling, physics, mathematics, and philosophy (2-11, 13-22).
A System of Brain Oscillators
Neuronal networks in the mammalian fore-brain demonstrate several oscillatory bands covering...