Stimulus-specific oscillatory responses of the brain: a time/frequency-related coding process

Objectives: To review the coherent, rhythmic oscillations above 20 Hz that occur in response to sensory inputs in the firing rate and membrane or local field potentials of distributed neuron aggregates of CNS layered structures. Results: Oscillatory activity at approximately 20–80 Hz occurs in response to either olfactory, auditory and visual (contrast) stimuli; oscillations at frequencies centered on 100–120 Hz or 600 Hz are recorded, respectively, from the visual system (luminance stimulation) and from the somatosensory cortex. Experimental evidence suggests sources/mechanisms of generation that depend on inhibitory interneurons and pyramidal cells and are partially independent from those of conventional (broadband) evoked responses. In the olfactory and visual systems, the oscillatory responses reflect the global stimulus properties. A time/phase correlation between firing rate, spiking coincidence and oscillatory field responses has been documented. The oscillatory responses are postsynaptic both in cortex and in precortical structures (e.g. retina; LGN). Evidence indicates intracortical and thalamocortical interacting mechanisms of regulation as well as GABAergic and cholinergic modulation. In the visual cortex the oscillatory responses are driven by oscillations in the synaptic input. Oscillatory potentials are dependent on resonance phenomena and produce narrow-band synchronization of activated neurons. They may have a role in the ‘binding’ of separate neuronal aggregates into sensory units. Conclusions: Oscillatory responses contribute as a time/frequency coding mechanism to pacing neurons selectively for the physical properties of stimulus and are involved in sensory information processing.