Involvement of the thalamocortical system in epileptic loss of consciousness

Experiments on putative neuronal mechanisms underlying absence seizures as well as clinical observations are critically reviewed for their ability to explain apparent "loss of consciousness." It is argued that the initial defect in absences lies with corticothalamic (CT) neuronal mechanisms responsible for selective attention and/or planning for action, rather than with those establishing either the states or the contents of consciousness. Normally, rich thalamocortical (TC)-CT feedback loops regulate the flow of information to the cortex and help its neurons to organize themselves in discrete assemblies, which through high-frequency (>30 Hz) oscillations bind those distributed processes of the brain that are considered important, so that we are able to focus on what is needed from moment to moment and be aware of this fact. This ability is transiently lost in absence seizures, because large numbers of CT loops are recruited for seconds in much stronger, low-frequency ( approximately 3 Hz) oscillations of EPSP/IPSP sequences, which underlie electroencephalographic (EEG) spike-and-wave discharges (SWDs). These oscillations probably result from a transformation of the normal EEG rhythm of sleep spindles on an abnormal increase of cortical excitability that results in strong activation of inhibitory neurons in the cortex and in nucleus reticularis thalami. The strong general enhancement of CT feedback during SWDs may disallow the discrete feedback, which normally selects specific TC circuits for conscious perception and/or motor reaction. Such a mechanism of SWD generation allows variability in the extent to which different TC sectors are engaged in the SWD activity and thus explains the variable ability of some patients to respond during an absence, depending on the sensory modality examined.