Evoked oscillations in unit recordings from the thalamo-cortical auditory system: an aspect of temporal processing or the reflection of hyperpolarized brain states?

Since the beginning of the 90’s several lines of research have brought new insights concerning the temporal aspects of sensory processing. Initial observations showing that sensory stimuli can trigger evoked, or induced, oscillations has generated a large number of studies where, explicitly or implicitly, the large-scale rhythmic activity of thalamo-cortical neurons was viewed as a key factor to synchronize neurons responding to different dimensions of a given stimulus, and therefore to solve the so-called "binding problem" (for reviews see Eckhorn 2000, Singer 1990, 1999). This line of research contrasts with the rate coding concept which has been used over the last 40 years to describe the functional properties of neurons in sensory systems and the plasticity of sensory systems both during development and in adulthood. We review here results obtained in the thalamo-cortical auditory system. After a characterization of the oscillatory events, we determined their origins by inactivating each component of the thalamo-cortical loop. Then we compared, within the same animals, the oscillations obtained in anesthetized and unanesthetized conditions. Our data strongly suggest that to clarify the functional roles of evoked oscillations, and more generally of any aspects of temporal coding, we need to study them on awake animals.