Peri-ictal network dynamics of spike-wave discharges: phase and spectral characteristics

The brain is a highly interconnected neuronal assembly in which network analyses can greatly enlarge our knowledge on seizure generation. The cortico-thalamo-cortical network is the brain-network of interest in absence epilepsy. Here, network synchronization is assessed in a genetic absence model during 5 s long pre-ictal->ictal transition periods.

16 male WAG/Rij rats were equipped with multiple electrodes targeting layer 4 to 6 of the somatosensory-cortex, rostral and caudal RTN, VPM, anterior-(ATN) and posterior (Po) thalamic nucleus. Local field potentials measured during pre-ictal->ictal transition and during control periods were subjected to time-frequency and pairwise phase consistency analysis.

Pre-ictally, all channels showed spike-wave discharge (SWD) precursor activity (increases in spectral power), which were earliest and most pronounced in the somatosensory cortex. The caudal RTN decoupled from VPM, Po and cortical layer 4. Strong increases in synchrony were found between cortex and thalamus during SWD. Although increases between cortex and VPM were seen in SWD frequencies and its harmonics, boarder spectral increases (6-48Hz) were seen between cortex and Po. All thalamic nuclei showed increased phase synchronization with Po but not with VPM.

Absence seizures are not sudden and unpredictable phenomena: the somatosensory cortex shows highest and earliest precursor activity. The pre-ictal decoupling of the caudal RTN might be a prerequisite of SWD generation. Po nucleus might be the primary thalamic counterpart to the somatosensory-cortex in the generation of the cortico-thalamic-cortical oscillations referred to as SWD.