The goal of this observational study is to compare the perioperative EEG characteristics and the incidence of short-term cognitive dysfunction in patients with postoperative delirium and non-postoperative delirium after elderly (> 65 years old) patients undergoing major gastrointestinal surgery under general anesthesia. The main question it aims to answer are: • The correlation between postoperative cognitive dysfunction and postoperative EEG features was evaluated.• To analyze the correlation between EEG characteristics and clinical risk factors of delirium after major abdominal gastrointestinal surgery under general anesthesia in elderly patients.Participants will collect EEG before and after operation and collect the incidence of postoperative cognitive function to explore the mechanism of postoperative delirium and predict postoperative cognitive dysfunction.

Inclusion Criteria:

1. Education level in primary school or above;
2. Age ≥65 years old, gender is not limited;
3. Good communication ability, language expression ability, mental state and consciousness state;
4. Preoperative simple mental state scale score ≥27 points;
5. no history of craniocerebral diseases;
6. For patients who plan to choose abdominal surgery under general anesthesia, the operation time is expected to be about 2h-4h;
7. ASA anesthesia grade I ~ III;
8. No general anesthesia contraindications;
9. Sign informed consent voluntarily;

Exclusion Criteria:

1. Relative contraindications of general anesthesia: patients with serious heart and lung function diseases, drug allergy history, etc.
2. Patients with a history of mental illness or long-term psychiatric drugs (dementia, schizophrenia), chronic analgesic drug use history, alcoholism history and cognitive dysfunction;
3. any cerebrovascular accident occurred within 3 months, such as stroke, transient ischemic attack (TIA), etc.
4. Diabetic patients with severe diabetic complications (diabetic ketoacidosis, hyperosmolar coma, various infections, macrovascular disease, diabetic nephropathy);
5. Chronic hypoxia and carbon dioxide storage, such as chronic obstructive emphysema;
6. a long history of alcohol abuse;
7. Unable to cooperate to complete the test, the patient or family rejected the participant.

Patients who met the inclusion and exclusion criteria for gastrointestinal surgery admitted to the Second Affiliated Hospital of Nanchang University.