Cortical feedback controls the frequency and synchrony of oscillations in the visual thalamus.
Thierry Bal, Damien Debay and Alain Destexhe

Journal of Neuroscience 20: 7478-7488, 2000.

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Thalamic circuits have an intrinsic capacity to generate state-dependant oscillations of different frequency and degrees of synchrony, but little is known of how synchronized oscillation is controlled in the intact brain, or what function it may serve. The influence of cortical feedback was examined using slice preparations of the visual thalamus and computational models. Cortical feedback was mimicked by stimulating corticothalamic axons, triggered by the activity of relay neurons. This artificially coupled network had the capacity to self-organize and to generate qualitatively different rhythmical activities according to the strength of corticothalamic feedback stimuli. Weak feedback (1-3 shocks at 100-150 Hz) phase-locked the spontaneous spindle oscillations (6-10 Hz) in geniculate and perigeniculate nuclei. However, strong feedback (4-8 shocks at 100-150 Hz) led to a more synchronized oscillation, slower in frequency (2-4 Hz) and dependent on GABAB receptors. This increase in synchrony was essentially due to a redistribution of the timing of action potential generation in LGN cells, resulting in an increased output of relay cells towards the cortex. Corticothalamic feedback is thus capable of inducing highly synchronous slow oscillations in physiologically intact thalamic circuits. This modulation may have implications for a better understanding of the descending control of thalamic nuclei by the cortex, and the genesis of pathological rhythmical activity, such as absence seizures.