FAQ: How is EyeLink data filtered?

[Sam]

EyeLink systems use a real-time heuristic filter to reduce noise in the gaze data. This filter has two levels—Standard and Extra—that can be applied independently to the two main data streams (data sent over the link and data written to the EDF File). The core principles behind the filtering algorithm are described in detail in the attached paper:

Stampe, D.M. (1993) Heuristic filtering and reliable calibration methods for video-based pupil-tracking systems. Behavior Research Methods, Instruments, & Computers, 25(2), 137-142.

  Stampe1993_Article_HeuristicFilteringAndReliableC.pdf (Size: 779.47 KB / Downloads: 62)

The filter levels can be set separately for the data being saved to the EDF and the data being transmitted over the link to the Display PC.

• File Data (EDF File)
  This is the data being permanently recorded to the EDF file for post-session analysis.
  • Default Filter: Extra (a two-stage filter with a two-sample delay).
    
  • Reasoning: Since this data is analyzed offline, the slight delay is not a concern, and the higher level of filtering ensures the cleanest possible data for your final analysis.
    
• Link / Analog Data
  This is the live data transmitted to the Display PC (for gaze-contingent tasks) or to an external device via the analog output.
  • Default Filter: Standard (a single-stage filter with a one-sample delay).
    
  • Reasoning: For real-time applications, minimizing delay is crucial. This setting provides a good balance between reducing noise and ensuring the fastest possible data transmission.
    

How to Change Filter Settings
You can change the filter levels before you begin recording using one of two methods:

• Host PC Menu: Navigate to the Set Options screen on the Host PC and adjust the File Sample Filter and Link/Analog Filter settings.
  
• Experimental Script: Send the heuristic_filter command from your experimental script.
  
  The command takes two inputs:
  
  Code:

  heuristic_filter <file filter level> <link filter level> 
  
  The command uses numerical values to set the filter level:
  • 0 = Off
    
  • 1 = Standard (single-stage)
    
  • 2 = Extra (two-stage)
    
  Example Command:
  
  Code:

  heuristic_filter 2 1
  
  This example sets the File Data to "Extra" (2) and the Link/Analog Data to "Standard" (1).
  
  Note: The specific syntax for sending this command will vary depending on your stimulus presentation software (e.g., Experiment Builder, E-Prime, PsychoPy).