Cortically-induced coherence of a thalamic-generated oscillation
Alain Destexhe, Diego Contreras and Mircea Steriade
Neuroscience 92: 427-443, 1999
Abstract
Oscillatory patterns in neocortical electrical activity show various degrees
of large-scale synchrony depending on experimental conditions, but the exact
mechanisms underlying these variations of coherence are not known. Analysis
of multisite local field potentials revealed that the coherence of spindle
oscillations varied during different states. During natural sleep, the
coherence was remarkably high over cortical distances of several millimeters,
but could be disrupted by artificial cortical depression, similar to the
effect of barbiturates. Possible mechanisms for these variations of coherence
were investigated by computational models of interacting cortical and thalamic
neurons, including their intrinsic firing patterns and various synaptic
receptors present in the circuitry. The model indicates that modulation of
the excitability of the cortex can affect spatiotemporal coherence with no
change in the thalamus. The highest level of coherence was obtained by
enhancing the excitability of cortical pyramidal cells, simulating the action
of neuromodulators such as acetylcholine and noradrenaline. The underlying
mechanism was due to cortex-thalamus-cortex loops in which a more excitable
cortical network generated a more powerful and coherent feedback onto the
thalamus, resulting in highly coherent oscillations, similar to the properties
measured during natural sleep.
In conclusion, these experiments and models are compatible with a powerful
role for the cortex in triggering and synchronizing oscillations generated in
the thalamus, through corticothalamic feedback projections. The model
suggests that intracortical mechanisms may be responsible for synchronizing
oscillations over cortical distances of several millimeters through
cortex-thalamus-cortex loops, thus providing a possible cellular mechanism to
explain the genesis of large-scale coherent oscillations in the
thalamocortical system.
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