The neocortex generates rhythmic electrical activity over a frequency range covering many decades. Specific cognitive and motor states are associated with oscillations within discrete frequency bands within this range, but it is not known whether interactions and transitions between discrete frequencies are of functional importance. We have recently identified local circuits generating two frequencies (gamma (30 – 50 Hz) and beta2 (20 – 29 Hz)) which are coexpressed in superficial and deep cortical laminae respectively. During coexpression the ratio of modal peak frequencies ensures minimal temporal interaction between predominantly input (superficial) and output (deep) cortical laminae. However, with changes in network drive these two frequencies can combine to generate a third frequency (beta1 – 13 – 17 Hz) manifest in all cortical layers. The expression of this beta1 frequency rhythm occurred concurrently with the establishment of strong temporal interactions between deep and superficial layer network activity. The process occurs via period concatenation, with basic rhythm-generating microcircuits underlying gamma and beta2 rhythms forming the building blocks of the beta1 rhythm by a process of addition. The mean ratio of adjacent frequency components appears therefore to facilitate two functions – during strong, stable network drive interlaminar temporal interactions are minimised, but changes in network drive permits multiple transitions between frequencies. The resulting temporal landscape – coexpression of multiple network rhythms with a single, unifying ratio - may provide a broad framework for multiplexing – parallel information processing on multiple temporal scales.
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