The participants were categorized into two groups using the Patient Health Questionnaire (PHQ-9) and the 17-item Hamilton Depression Rating Scale (HAM-D). PHQ-9 is a self-report scale. PHQ-9 scores of 5 or higher represent mild and severe depression, while PHQ-9 scores of lower than 5 mean normal condition. HAM-D is a scale administered by a healthcare professional. The cut-off point of the HAM-D scale is 8, which divides normal conditions from depressive conditions. The participants first completed PHQ-9. For those with PHQ-9 scores of 5 or higher, the trained researchers interviewed them and rated their severity of depression on the HAM-D. The participants first completed PHQ-9.

For those with PHQ-9 scores ≥ 5, the trained researchers interviewed them and rated their severity of depression on the HAM-D. Those with HAM-D scores > 8 were classified into the depressed group.

Those with PHQ-9 scores < 5 or HAM-D scores ≤ 8 were classified into the non-depressed group.

Participants went through four stages. In the rest stage, they were instructed to relax. In the neutral control stage, they watched an informative video about plastic injection molding. In the sadness induction stage, they viewed sad clips from either Hachi: A Dog's Tale or My Brother and Sister, with the order randomized across participants. In the music intervention stage, they listened to the cheerful Cuckoo Waltz. Each stage lasted for about 6 minutes.

The interventions for the two groups were exactly same.

PS: On the basis of the PHQ-9, this HAMD interview was conducted to further group the participants. So, following the experimental protocol, participants who scored ≥ 5 on the PHQ-9 measure underwent the HAMD interview, while the remaining individuals were simply interviewed about their feelings during the experiment.

The SAM scale is a brief and widely used tool for assessing emotional states. The scale is a nonverbal self-report measure of emotion, using a set of cartoon-like manikins. The manikins illustrate nine intensity levels for valence (1 = unpleasant; 9 = pleasant), arousal (1 = calm, 9 = excited), and dominance (1 = controlled; 9 = controlling).

The PANAS is another widely used scale to measure mood or emotion. The Chinese version of PANAS is comprised of 18 items, with 9 items measuring positive affect (e.g., joyful, inspired) and 9 items measuring negative affect (e.g., sad, fearful). Because participants were required to report their feelings repeatedly in this study, 18 items would be too long and tiresome. The 18 items plus "calmness" were used as 19 options for participants to choose. They needed to select one of the emotion words to tag their strongest emotion for the stage that they had just experienced. This revision of PANAS allowed us to capture the specific emotion type.

MeanNN refers to the average duration between consecutive normal heartbeats, also named inter-beat interval. A higher MeanNN indicates a lower heart rate.

SDNN measures the overall variability of heart rate. Higher SDNN values indicate greater variability.