## 3 The Dataset Built from a User Experiment
### The User Experiment Setup  
This was a controlled experiment in which participants were recruited to conduct music searching and listening with a web-based music retrieval system in a quiet room. To encourage participants to interact with music, they were asked to create a playlist for the songs they liked during the experiment. Participants' peripheral physiological signals during music listening were recorded by a research-grade wristband. Moreover, we also collected their self-reported emotional responses to music via short pop-up surveys, which were taken as the ground truth labels for the classification experiment. The experiment was conducted with a novel web-based music retrieval system, Moody (version 4), which supported music searching and browsing, online music streaming, playlist management, and interactive online surveys. In the meantime, users' interactions with Moody (e.g., play/pause music, skip a track) were logged as timestamped user events. The music collection hosted in the system is the one used in the MIREX Grand Challenge on User Experience (GC14 UX) [38], which consists of 10K full tracks obtained from a free music service, Jamendo, under the Creative Commons (CC-BY) license. The collection also contains album cover images and metadata of the tracks originally obtained from Jamendo (e.g., title, album, artist, genre), which are displayed in the user interface to facilitate searching and browsing. Similar to many streaming music services, the album cover and metadata were displayed when a song was played (see [33] for a screenshot of the interface). A pop-up survey subsystem was implemented in Moody to collect participants' self-reported emotional responses to music they listened to, which serve as ground truth for training and evaluating MER classifiers. Specifically, for each music piece listened to for more than 30 s, a question would be prompted to solicit participants' emotion induced by the song (Figure 1). The question asked participants to give scores of arousal and valence levels [39] on a continuum of \(-10\) (very low energy, very negative) to 10 (very high energy, very positive). To probe the role of personal characteristics in predicting music-induced emotion, a pre-experiment questionnaire was administered to gather information on participants' demographics and their personality traits based on the Big-Five model (i.e., openness to experiences, conscientiousness, extraversion, agreeableness, emotional stability) measured by the Ten Item Personality Inventory (TIPI) [40]. The instructions of the Moody system and search task were subsequently presented to participants in detail. Following that, participants were asked to interact with the Moody system for no less than 40 minutes, looking for at least 10 songs they liked and adding them to their personal playlists. They were encouraged to explore music in different genres for a more diverse listening experience. During music listening, the Empatica E4 wristband, one of the few research-grade wearable devices, was used to measure participants' physiological signals. This device supports real-time data streaming and provides a Web API for accessing raw physiological data. It has been employed in emotion-related studies in neuroscience, human-computer interaction, etc., with high reliability (e.g., [41]). For signal stabilization and baseline acquisition, the wristband was mounted on each participant's wrist two minutes before the pre-experiment questionnaire. After completing the tasks of the music search and listening, participants were asked to fill out a post-experiment questionnaire concerning their satisfaction with the music collected, emotional states at that moment, and their feedback on the Moody system and the experiment process. Informed consent forms were signed at the beginning of the experiment session, and a nominal remuneration was paid upon the completion of the experiment to compensate participants' time.