Ohtahara Syndrome

A full-term male infant born at 39 weeks presents with stereotyped clusters of brief tonic extension seizures beginning on day of life 12, occurring both in wake and sleep. He is unresponsive to multiple antiepileptic drugs. MRI brain shows left hemimegalencephaly. Rapid whole-exome sequencing identifies a de novo pathogenic variant in STXBP1. An interictal EEG at 5 weeks of life is obtained during both active wakefulness and quiet sleep. The structure marked **A** in the diagram represents the characteristic EEG pattern observed. Which of the following best describes the key distinguishing feature of this pattern in Ohtahara syndrome?