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Flicker Task

Version: v1 (current)

A change detection paradigm using alternating images to reveal attentional limits in visual perception.

Overview​

The Flicker task is closely related to the Change Blindness paradigm. Two versions of an image alternate with a brief blank interval. One contains a change, the other does not. The rapid alternation (flicker) helps detect changes by creating a local motion signal, yet substantial changes often go unnoticed for many seconds, revealing the sparse nature of visual representations.

This task demonstrates that without attention directed to the changing region, even large alterations remain invisible. It's used to study visual attention, scene perception, and the role of transients in change detection.

Scientific Background​

Classic Findings:

  • Flicker Advantage: Easier than one-shot change detection but still challenging
  • Attention Dependence: Unattended changes remain invisible despite flicker
  • Change Magnitude: Larger changes detected faster, but even huge changes can be missed

Seminal Paper:

  • Rensink, O'Regan, & Clark (1997): To see or not to see: The need for attention to perceive changes

Why Researchers Use This Task​

  1. Attention Research: Study role of attention in change detection
  2. Applied Vision: Train observers (radiologists, security) to detect anomalies
  3. Scene Perception: Understand what we encode from visual scenes
  4. Aging Studies: Assess age-related changes in attention and perception

Where to Configure​

Study Form → Tasks → Flicker → Configure.

Configuration Parameters​

ParameterTypeDefaultDescription
Image Duration (ms)number240Duration each image is shown
Blank Duration (ms)number80Duration of the blank between images
Max Cyclesnumber50Maximum flicker cycles before timeout

Participant Flow​

  1. The participant sees the first image displayed on screen.
  2. After the Image Duration, a brief blank screen appears for the Blank Duration.
  3. The second image (with or without a change) is displayed.
  4. The cycle repeats (image A, blank, image B, blank) until the participant detects the change or Max Cycles is reached.
  5. The participant clicks or presses a key when they notice the change. Time to detection is recorded.

Data Output​

Markers and Responses​

The task records high-resolution timestamps in two separate collections:

Markers (stimulus_shown):

{
  "type": "stimulus_shown",
  "ts": "2024-01-01T00:00:01.000Z",
  "hr": 1234.56,
  "data": {
    "trial_index": 1,
    "stimulus_id": "flicker_0_1",
    "image_a_url": "https://example.com/scene1a.jpg",
    "image_b_url": "https://example.com/scene1b.jpg",
    "change_x": 350,
    "change_y": 200,
    "is_practice": false
  }
}

Response Data:

{
  "trial_index": 1,
  "stimulus_id": "flicker_0_1",
  "source": "click",
  "image_a_url": "https://example.com/scene1a.jpg",
  "image_b_url": "https://example.com/scene1b.jpg",
  "change_x": 350,
  "change_y": 200,
  "click_x": 355,
  "click_y": 198,
  "detection_correct": true,
  "detection_time_ms": 5420,
  "flicker_cycles": 8,
  "is_practice": false,
  "block": "main",
  "latency_ms": 5420
}

Summary Artifact​

A JSON file (flicker_summary_<taskIndex>.json) with aggregated statistics:

{
  "task_kind": "flicker",
  "task_index": 0,
  "total_trials": 10,
  "overall": {
    "total": 10,
    "valid_clicks": 9,
    "correct_detections": 7,
    "accuracy": 0.78,
    "mean_detection_time_ms": 5420,
    "mean_cycles_to_detection": 8,
    "timeouts": 1
  },
  "practice": { /* same structure if enabled */ },
  "trials": [ /* per-trial data */ ]
}

Key metrics:

  • detection_correct: Whether click was within the correct change region
  • detection_time_ms: Time from trial start to click
  • flicker_cycles: Number of complete A-B-A cycles before detection
  • accuracy: Proportion of correct detections

Design Recommendations​

  • Image Pairs: Use high-resolution images with clearly defined change regions. The change should be meaningful (e.g., object removal, color shift).
  • Blank Duration: 80 ms is standard; shorter blanks make changes easier to detect, longer blanks increase difficulty.
  • Cycle Limit: Set Max Cycles high enough so most participants can find the change, but not so high that frustrated participants stall.
  • Practice Trials: Include a practice round with an obvious change so participants understand the task.
  • Counterbalancing: Randomize which image (A or B) contains the change across trials.

Common Issues and Solutions​

IssueSolution
Participants cannot find the changeIncrease image duration or reduce blank duration; ensure the change is large enough
Changes detected too quicklyDecrease image duration or increase blank duration; use subtler changes
Timing feels inconsistentEnsure images are preloaded before the trial starts to avoid loading delays
Participants click randomlyAdd instructions emphasizing accuracy; consider requiring participants to indicate the change location

References​

  • Rensink, R. A., O'Regan, J. K., & Clark, J. J. (1997). To see or not to see: The need for attention to perceive changes in scenes. Psychological Science, 8(5), 368-373.
  • Simons, D. J., & Rensink, R. A. (2005). Change blindness: Past, present, and future. Trends in Cognitive Sciences, 9(1), 16-20.

See Also​