Phasic loss of constraints in timing of neuronal spikes in the feline centrum medianum during EEG alpha-like activity.

In unrestrained cats, temporal patterns of single neuronal firing in the centrum medianum-parafascicular complex (CM-Pf) of thalamus were studied during a state of motionless quiet wakefulness. The spike trains from each neuron were electronically divided into episodes that occurred during desynchronized EEG and those that occurred during bursts of 6-14 Hz EEG spindles or alpha-like activity over the parieto-occipital cortex and in the CM-Pf. Contrary to expectations based on the theory of inhibitory phasing of neuronal activity, the episodes of synchronized quiet wakefulness (S-QW) were associated with independent and random distribution of spike intervals, although they tended to occur in clusters. During episodes of desynchronized quiet wakefulness (D-QW), significant temporal patterns were emitted by most neurons studied. The results suggest that: (a) during D-QW and increased levels of vigilance, temporal patterns are generated by cognitive processes and enhanced specific connectivities between CM-Pf neurons and other systems; (b) if connectivity is defined as increased certainty of synaptic transmission and iterative activation of the same pathways, then the assumption that the gross EEG thalamo-cortical synchronization represents increased connectivities between the CM-Pf neurons and other systems, may be erroneous; and (c) since temporal patterns of single neuronal discharges are determined by specific spatio-temporal distribution of synaptic drive, therefore during EEG spindles, most EPSP-IPSP sequences impinging upon CM-Pf neurons are conveyed by randomly varying polysynaptic pathways and have random spatio-temporal distribution on the soma and dendrites. In light of other observations, such a process is equivalent to an active introduction of uncertainty or 'entropy' into the information processing system, a state which may be important in preserving plasticity of operational modes of CM-Pf neurons and those with which they directly and/or indirectly interact.