Valproate in Atypical Absence

A 6-year-old boy with Lennox-Gastaut syndrome presents with multiple daily atypical absence seizures lasting 20–40 seconds, atonic drop attacks, and nocturnal tonic seizures. His baseline EEG shows slow spike-and-wave complexes at 1.5–2.5 Hz with a diffusely slow and disorganized background. After six weeks of valproate titration to 30 mg/kg/day, his parents report that absence episodes are noticeably shorter and less frequent. On repeat EEG, the background remains abnormally slow, and residual slow spike-and-wave bursts persist at approximately 2 Hz, but the feature marked **C** in the diagram is now evident. Which of the following best explains the electrographic improvement marked **C** in the context of valproate's mechanism of action?