# Subtle Gaze and Pupil Dynamics: Detecting Recognition of Familiar Faces with Moving Serial Visual Presentation

## TL;DR

This study introduces a new method to detect recognition of familiar faces using eye movements and pupil dilation, showing improved detection rates compared to previous techniques.

## Contribution

The novel MSVP paradigm combines serial visual presentation with lateral movements to enhance pupil-based detection of familiar information.

## Key findings

- Pupil dilation and its rate of change increased significantly in response to familiar faces.
- Classification based on pupil size change achieved higher detection rates than previous RSVP-based methods.
- Eye movement measures contributed little to detecting familiar-face processing.

## Abstract

A major limitation of the traditional concealed information test (CIT) is its susceptibility to countermeasures. Rapid serial visual presentation (RSVP) paradigms improve resilience against countermeasures, and pupil dilation has been shown to indicate recognition of personally familiar information in the RSVP paradigm. Here, we introduce a novel variation of the RSVP paradigm, Moving Serial Visual Presentation (MSVP), that aims to improve individual detection by incorporating eye gaze and pupil dilation. We combined serial visual presentation with lateral movements to capitalize on pursuit eye movements and pupil size for detecting involuntary recognition and goal-driven suppression of familiar information. Across two experiments, either a target face, a personally familiar face (the participant’s parent), or one of two control faces appeared in a stream. In Experiment 1, participants were required to maintain fixation at a central dot and suppress gaze shifts except toward the task-relevant target face. In Experiment 2, participants were free to move their gaze and responded via keypress. Results indicated that while eye movement measures contributed little to detecting familiar-face processing, both pupil dilation and its rate of change exhibited a noticeable increase in response to familiar faces. At the individual level, classification based on the rate of pupil size change yielded detection rates of 55.2% (Experiment 1) and 33.3% (Experiment 2), exceeding those of previous RSVP-based approaches (22.6%). These findings indicate that MSVP, particularly when eye movements are constrained, enhances the diagnostic value of pupil-based measures for detecting task-irrelevant familiarity, though further work is needed to reach applied standards.