Cortical activation in generalized seizures

Abstract

Objective: Patients with generalized epilepsy exhibit different epileptiform events including asymptomatic interictal spikes (IS), absence seizures with spike-wave discharges (SWDs), and myoclonic seizures (MS). Our objective was to determine the spatiotemporal patterns of cortical activation in SWDs, IS, and MS in the Gabra1^+/A322D juvenile myoclonic epilepsy mouse.

Methods: We fabricated affordable, flexible high-density electroencephalography (HdEEG) arrays and recorded spontaneous SWD, IS, and MS with video/HdEEG. We determined differences among the events in amplitude spectral density (ASD) in the δ/θ/α/β/γ frequency bands at baseline (3.5-4.0 seconds before the first spike time, t₀ ) and the prespike period (0.1-0.5 seconds before t₀ ), and we elucidated the spatiotemporal activation during the t₀ spike.

Results: All three events had an increase in ASD between baseline and prespike in at least one frequency band. During prespike, MS had the largest δ-band ASD, but SWD had the greatest α/β/γ band ASD. For all three events, the ASD was largest in the anterior regions. The t₀ spike voltage was also greatest in the anterior regions for all three events and IS and MS had larger voltages than SWD. From 7.5 to 17.5 msec after t₀ , MS had greater voltage than IS and SWD, and maximal voltage was in the posterior parietal region.

Significance: Changes in spectral density from baseline to prespike indicate that none of these generalized events are instantaneous or entirely unpredictable. Prominent engagement of anterior cortical regions during prespike and at t₀ suggest that common anterior neural circuits participate in each event. Differences in prespike ASD signify that although the events may engage similar brain regions, they may arise from distinct proictal states with different neuronal activity or connectivity. Prolonged activation of the posterior parietal area in MS suggests that posterior circuits contribute to the myoclonic jerk. Together, these findings identify brain regions and processes that could be specifically targeted for further recording and modulation.

Keywords: GABAA receptor; electroencephalogram; generalized; signal analysis.

Figure 1:. HdEEG recording of generalized seizures

A) Scale diagram of a mouse dorsal brain indicating the location of the HdEEG electrodes. Electrodes with odd and even numbers are over the left and right hemispheres, respectively, and electrode names indicate the region of functional cortex; supplementary motor (sM), primary motor (M), somatosensory (S), barrel somatosensory (B), parietal association (A), and visual (V). Reference (R) and ground (G) electrodes are over cerebellum. B) Template for left and right complementary halves of HdEEG array. The small circles are the electrode pads (EP) where the electrode connectors are soldered. The lines from the electrode pads are the wires that connect to the zero-insertion force pads (ZPs) that connect to the headbox. C) Examples of a spike wave discharge (SWD, blue), interictal spike (IS, green), and myoclonic seizure (MS, red) recorded from the same mouse. Each discharge exhibits frontally-predominant generalized spikes. MS resemble IS on visual inspection but are associated with a myoclonic jerk; time of jerk marked by dotted line (47 ± 31 ms after t₀).

Figure 2:. Baseline spectral density.

Amplitude spectral density (ASD) at baseline (−4.0 to −3.5s relative to t₀) was averaged for all events within each subject. A) average inter-event differences (SWD-IS, SWD-MS, and IS-MS) in baseline density are plotted topographically for each frequency band (δ/θ/α/β/γ). Blue colors indicate that the ASD of the second event type is greater than the first, red colors indicating that the ASD of the first event type is greater than the second, and green signifies no ASD difference (0 ± 4% full scale). White circles mark clusters with CBP test p-values < 0.05. B) Scatter plots depict within-subject averaged baseline δ-, α-, and β-band ASD grouped by event type (upper) or brain region (lower, MO = motor, SS = somatosensory, PA = parietal association, V = visual). Data depicted with the same color and symbol shape were obtained from the same subject and the lines connecting the symbols show the within-subject ASD changes among event type (upper) or brain region (lower). Asterisks represent statistically-significant post-hoc pairwise contrasts between event types (upper) or brain regions (lower) (* p < 0.05, ** p < 0.01, *** p < 0.001, N = 6 subjects).

Figure 3:. Pre-spike spectral density.

For each frequency band (δ/θ/α/β/γ), amplitude spectral density (ASD) was determined at baseline (−4.0 to −3.5s relative to t₀) and pre-spike time (−0.50, −0.25, −0.15, −0.10, −0.10s relative to t₀ for δ, θ, α, β, γ-bands, respectively). Within-subject averaged differences between pre-spike and baseline ASD were determined for each event type, separately (A-B), as well as within-subject differences in pre-spike ASD among the three type of epileptiform events (C-D). A,C) Averaged differences in pre-spike and baseline ASD for each event type (A) or inter-event differences in pre-spike density (C) are plotted topographically for each frequency band. Blue colors indicate decreased pre-spike ASD relative to baseline (A) or that the second event type has greater pre-spike ASD than the first (C) and red colors signify an increase in pre-spike ASD relative to baseline (A), or that the first event type had greater pre-spike ASD than the second (C). Green signifies no difference (0 ± 4% full scale) between the conditions. White circles mark clusters with CBP test p-values < 0.05. B,D) Scatter plots depict B) within-subject averaged baseline (b) and pre-spike (p) θ- and β-band ASD or D) pre-spike ASD (δ-, θ-, α-, and β-band). LMEM testing demonstrated that SWD, but not IS or MS had significant interactions between brain region (MO = motor, SS = somatosensory, PA = parietal association, V = visual) and baseline/pre-spike grouping (B) and that, in the α-, and β-bands, there were significant interactions between brain region and event type in and thus these data are plotted separately for each brain region. Data depicted with the same color and symbol shape were obtained from the same subject and the lines connecting the symbols show the within-subject ASD changes. Asterisks represent statistically-significant post-hoc pairwise contrasts (* p < 0.05, ** p < 0.01, *** p < 0.001, N = 6 subjects). Positive voltage corresponds to surface negative EEG potentials.

Figure 4:. Spatiotemporal analysis of initial spikes.

The initial epileptiform spikes from each event were time-locked. Within-subject time-locked voltages from each event type were averaged from −20 ms to 20 ms with a 2.5 ms time resolution. A) Average inter-event differences (SWD-IS, SWD-MS, and IS-MS) in voltage are plotted topographically from −20 ms to 20 ms relative to t₀ in 5 ms bins. Blue colors indicate that the voltage of the second event type is greater than the first, red colors indicating that the voltage of the first event type is greater than the second, and green signifies no voltage difference (0 ± 4% full scale) between the two event types. White circles mark clusters with CBP test p-values < 0.05. B) Scatter plots depict within-subject averaged voltages averaged into three time points relative to t₀, early (−17.5 to −7.5 ms), middle (−5 ms to 5 ms) and late (7.5 ms to 17.5 ms) and pooled from four brain regions (MO = motor, SS = somatosensory, PA = parietal association, V = visual). Data depicted with the same color and symbol shape were obtained from the same subject and the lines connecting the symbols show the within-subject voltage-differences among event type (upper) or brain region (lower). Asterisks represent statistically-significant post-hoc pairwise contrasts (* p < 0.05, ** p < 0.01, *** p < 0.001, N = 6 subjects).

Figure 5:. Common and differentiating features of epileptiform events.

Following the baseline, all three epileptiform events exhibit the greatest spike voltage in the anterior regions as well as significant increases in β-spectral density from baseline to pre-spike. IS and MS spikes had significantly greater voltage than SWD spikes and MS and SWD events and significantly increased θ and γ spectral density from baseline to pre-spike. SWDs had increased δ- and α-band ASD from baseline to pre-spike, had the largest pre-spike α-, β-, and γ-band ASD, and had the greatest pre-spike α- and β-band ASD located in motor cortex (MO). MS had increased latent spike voltage 7.5–17.5 ms after t₀.