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Snip & Stitch: a simple and accessible correction for the pupil foreshortening error
Koert H Stribos1, Damian Koevoet1, Yuqing Cai1
1Experimental Psychology, Helmholtz Institute, Utrecht University, Utrecht, The Netherlands.
Behavior Research Methods
|March 30, 2026
Summary
Pupil foreshortening error (PFE) hinders eye-tracking studies. A new method, Snip & Stitch, corrects PFE by 71-81%, enabling more flexible cognitive research using pupil size measurements.
Area of Science:
- Cognitive science
- Neuroscience
- Psychology
Background:
- Pupil size is a physiological marker for cognitive processing.
- Pupil foreshortening error (PFE) is a significant challenge in pupillometry.
- Existing PFE correction methods have limitations in applicability and flexibility.
Purpose of the Study:
- Introduce 'Snip & Stitch', a novel and accessible method to correct PFE.
- Evaluate the effectiveness of Snip & Stitch in reducing PFE.
- Enhance the utility of pupil size as a measure in diverse eye-tracking research.
Main Methods:
- Developed 'Snip & Stitch' algorithm to correct PFE.
- The method involves 'snipping' pupil size changes during saccades and 'stitching' by subtracting pre- and post-saccadic differences.
- Applied the method to eye-tracking data to assess PFE reduction.
Main Results:
- Snip & Stitch significantly reduced PFE by an estimated 71-81%.
- The method is straightforward, accessible, and easy to implement.
- Demonstrated applicability for experiments with up to five saccades.
Conclusions:
- Snip & Stitch offers a practical solution to the PFE problem.
- This method expands the use of pupillometry in various experimental designs and setups.
- Facilitates more reliable cognitive research utilizing pupil size measurements.
