Corticothalamic feedback modulates network state
The neocortex, center of our higher cognitive functions, receives basically all its incoming information through a subcortical structure, the thalamus. The thalamus, however, is more than just a relay station. While during wakefulness it passes incoming information to the cortex, during sleep it produces rhythmic oscillations, visible on the electroencephalogram. The information flow in this system is two-way, the cortex also sending a strong feedback to the thalamus, the function of which is not clear yet. We investigated, how layer 6. corticothalamic (CT) feedback influences the network state during sleep and wakefulness. We found that corticothalamic feedback can induce different types of changes in the thalamocortical network, depending on its activation pattern. Prolonged tonic CT activation can abolish sleep spindles during light sleep, producing a deep sleep-like rhythm. Similar activation on higher intensity, however, produced a desynchronized pattern both in cortex and thalamus. These changes were confined to the affected thalamocortical circuit, and were not accompanied by changes in the animal’s general arousal state, implicating CT as a local modulatory system. Some stimulation patterns, however, produced widespread paroxysmal activity, leading to generalized, tonic-clonic type epileptic seizures. Therefore the corticothalamic feedback system may play an important role not only in regulation of sleep oscillations, but also in the generation of pathological states.