FAQ: What is the difference between the Centroid and Ellipse tracking modes?

[Sam]

EyeLink systems use two primary modes for tracking the center of the pupil: Centroid and Ellipse.

• Centroid Mode: Calculates the pupil's center as the center-of-mass of the thresholded pupil area.
  
• Ellipse Mode: Fits an ellipse to the thresholded pupil area and uses the center of this best-fit ellipse as the pupil's center. On the Host PC, this is visualized as a green ellipse
  

   

Which Mode Should You Use?
The best choice depends on your experimental conditions and configuration.

• Centroid mode is the default for Head stabilized tracking and situations where occlusion of the eye is unlikely.
  
• Ellipse mode is beneficial when the pupil may be partially hidden or appear non-circular. This often occurs when:
  • The participant's pupil is very dilated and may be partially obscured behind the eyelid.
    
  • The participant has eyelid droop (ptosis), which is common in elderly populations.
    
  • The participant is lying down, for example, in an MRI scanner.
    
  In these cases, the Ellipse mode can more accurately estimate the true center of the obscured pupil. Note that Ellipse mode is always used when operating the system in remote mode as the dynamic nature of head movements can increases the likelihood of partial occlusion
  

Other Considerations

• EyeLink 1000: Ellipse mode is slightly noisier than Centroid mode.
  
• EyeLink 1000 Plus & Portable Duo: The noise levels for Centroid and Ellipse modes are comparable.