Neural dynamics in a model of the thalamocortical system. I. Layers, loops and the emergence of fast synchronous rhythms

A large-scale computer model was constructed to gain insight into thestructural basis for the generation of fast synchronous rhythms (20-60 Hz)in the thalamocortical system. The model consisted of 65,000 spikingneurons organized topographically to represent sectors of a primary andsecondary area of mammalian visual cortex, and two associated regions ofthe dorsal thalamus and the thalamic reticular nucleus. Cortical neurons,both excitatory and inhibitory, were organized in supragranular layers,infraganular layers and layer IV. Reciprocal intra- and interlaminar,interareal, thalamocortical, corticothalamic and thalamoreticularconnections were set up based on known anatomical constraints. Simulationsof neuronal responses to visual input revealed sporadic epochs ofsynchronous oscillations involving all levels of the model, similar to thefast rhythms recorded in vivo. By systematically modifying physiologicaland structural parameters in the model, specific network properties werefound to play a major role in the generation of this rhythmic activity. Forexample, fast synchronous rhythms could be sustained autonomously bylateral and interlaminar interactions within and among local corticalcircuits. In addition, these oscillations were propagated to the thalamusand amplified by corticothalamocortical loops, including the thalamicreticular complex. Finally, synchronous oscillations were differentiallyaffected by lesioning forward and backward interareal connections.