Time and the brain

Temporal synchrony and gamma-to-theta power conversion in the dendrites of CA1 pyramidal neurons

Sachin Vaidya & Daniel Johnston, University of Texas :: Nature Neuroscience, Vol. 16, No. 12, December 2013

Sbr-800epi1ummary and review of the above paper
Gamma and the slower theta oscillations combine to synchronise neural activity related to the hippocampus. The relative timing of this neural firing is regarded as a temporal code. A neuron in the CA1 region of the hippocampus can receive inputs via as many as 30,000 synapses, some spatially distant from axo-somatic integration.

The paper discusses how CA1 pyramidal neurons may correct for the resulting temporal distortion. A gradient of inductive reactance, involving ion channels, can compensate for the delay involved in receiving more distant inputs. In spite of the variable distance from the inputs, signals can still be coincident at the integration zone.

Gamma & theta

Gamma and theta frequencies are shown to be particularly suited to this arrangement. Gamma frequencies are converted into theta frequencies at the synapse because these are more suitable for transmission to the cell body. Correlation of gamma and theta is seen as facilitating oscillatory synchrony and signal transfer, involving particular frequencies, to the cell body. It is the lower frequency theta oscillation that actually transmits the signal to the cell body. A higher amplitude synaptic response can compensate for the attenuation of greater distance, and the soma or cell body is rendered less sensitive to whereabouts an input is located in the dendrites of the neuron.

hippofunctionNeural assemblies have been identified in the hippocampus that are essential for the laying down of new memories and also spatial navigation. Downstream neurons identify upstream neural assemblies by their synchrony, but the switch from gamma to theta appears here as an adaption, in particular circumstances, for the synchrony of signals within a single neuron.

NOTE: The ability of neurons to adjust for temporal delays in processing information may hint at how the brain’s perception also manages to adjust for the up to half-second (Libet’s half second) delay in external stimuli becoming conscious without there being awareness of such a delay.

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