A 6-year-old girl is brought to the pediatric neurologist by her parents because she has been having brief episodes of staring into space multiple times a day. Each episode lasts about 10 seconds, during which she is unresponsive and does not recall the event afterward. There is no convulsive movement, postictal confusion, or incontinence. Physical and neurologic examinations are normal. Hyperventilation during EEG recording precipitates one of her typical spells. Which of the following findings is most likely during one of these episodes?