Video Stimuli
Version: v1 (current)
Play video content to participants for attention, emotion, or instructional research.
Overview
The Video stimuli task presents dynamic video content using the HTML5 video player. The player always shows native controls, plays inline, and records the participant's playback events (play, pause, seek, ended). Participants advance by clicking the Continue button.
Common research uses include:
- Social cognition: View social interactions or scenarios
- Emotion recognition: Watch dynamic facial expressions
- Instructional videos: Demonstrate procedures before tasks
- Stimuli with temporal dynamics: Present time-varying content
Where to Configure
Study form → Tasks → Stimuli tasks → Video → Configure.
Configuration Parameters
| Parameter | Type | Default | Description |
|---|---|---|---|
Video file (stimuli_media_url) | string | '' | Uploaded video file (any video/* type; MP4 recommended). Set by uploading a file, which stores the download URL |
Main instructions (main_instructions) | string (HTML) | '' | Rich-text instructions shown with the video |
The video is configured by uploading a file (there is no manual URL field). The player always renders native controls; there are no autoplay, loop, mute, require-full-playback, or advance-on-end options.
Participant Flow
- The video is displayed with native player controls; the participant clicks play to start it.
- The participant can play, pause, and seek using the controls. Each of these actions is recorded.
- The participant clicks the Continue button to advance. There is no full-playback gating; the button is always available.
Design Recommendations
- Format: Use MP4 with H.264 encoding for maximum browser compatibility.
- Resolution: 720p minimum; test buffering with your target audience's connection speed.
- Length: Keep clips short and consistent across conditions; long videos risk participant disengagement.
- Captions: Consider adding captions for accessibility.
- Controls: The player always shows native controls, so participants can pause and seek; design your stimuli with self-paced viewing in mind.
- Telemetry: Use the recorded
play_eventsandcompleted_playbackfields to check whether each participant watched the clip to the end.
Common Issues and Solutions
| Issue | Solution |
|---|---|
| Video doesn't play | Check the URL; ensure the video format is MP4/H.264; test on the target browser |
| Video buffering delays | Preload the video; reduce quality; use a CDN; allow time for loading |
| Audio not playing | Check browser autoplay policies; some browsers block playback with sound until the participant interacts. The participant clicks play to start the video |
| Participants skip the video | The recorded completed_playback flag and play_events let you detect participants who advanced without watching to the end |
Data Output
Markers and Responses
Markers (stimulus_shown):
{
"type": "stimulus_shown",
"ts": "2024-01-01T00:00:01.000Z",
"hr": 1234.56,
"data": {
"type": "video",
"url": "https://example.com/videos/clip.mp4"
}
}
Playback markers are recorded as the participant interacts with the player: media_play, media_pause, media_seek, and media_ended. Each carries the playhead position in milliseconds:
{
"type": "media_play",
"ts": "2024-01-01T00:00:02.000Z",
"hr": 2234.56,
"data": {
"position_ms": 0
}
}
Response Data (recorded when the participant clicks Continue):
{
"type": "video",
"stimulus_url": "https://example.com/videos/clip.mp4",
"viewing_time_ms": 30234,
"play_events": [
{ "type": "play", "position_ms": 0 },
{ "type": "pause", "position_ms": 5200 },
{ "type": "ended", "position_ms": 30000 }
],
"completed_playback": true,
"responded": true
}
Summary Artifact
None. The video stimulus task does not generate a summary artifact.
References
- Bradley, M. M., & Lang, P. J. (2007). The International Affective Picture System (IAPS) in the study of emotion and attention. In J. A. Coan & J. J. B. Allen (Eds.), Handbook of Emotion Elicitation and Assessment (pp. 29-46). Oxford University Press.