Ms. Chen is a 60-year-old widow with a 6-month history of episodic chest tightness, shortness of breath, and numbness that "moves all over my body." These symptoms, which occur once or twice weekly, occur suddenly, and reach peak intensity within a few minutes. During these episodes she has the sensation of intermittent "hot" air coming from her abdomen to her throat, making her believe that she is being choked. She also describes feeling as if she is in a closed room or small space. Ms. Chen is anxious and frustrated about her symptoms and thinks she might have a serious medical problem. She has had frequent medical evaluations by her primary care physician and second opinions from various specialists but refuses to see a psychiatrist. A magnetic resonance image of Ms. Chen taken during one of these episodes would most likely reveal increased activity in which of the following structures?