Study of Visual Behavior in Freely Moving Primates: Development and Application of an Innovative Eye-Tracking System

Academic Background

The visual system is one of the most extensively studied areas within the primate nervous system, particularly concerning the mechanisms of visual pathways in the cerebral cortex. However, current research on how primates function visually while freely moving and exploring real-world environments is remarkably limited. This gap in knowledge primarily stems from the lack of technology capable of accurately tracking eye movements in freely moving individuals with the speed and resolution required to quantify primate vision. Traditional methods typically require head-fixation in laboratory settings, which limits our understanding of how the visual system operates during natural behaviors. Therefore, developing a technology that can precisely record eye movements without restricting free movement has become an important research direction.

Paper Source

This study, titled “Active vision in freely moving marmosets using head-mounted eye tracking,” was conducted by Vikram Pal Singh, Jingwen Li, Kana Dawson, Jude F. Mitchell, and Cory T. Miller. The authors are affiliated with the University of California San Diego (UCSD) and the University of Rochester. The research was published in the journal Proceedings of the National Academy of Sciences (PNAS) on February 3, 2025.

Research Process

Development of the Innovative Eye-Tracking System

To investigate visual behavior in freely moving primates, the research team developed a wireless head-mounted eye-tracking system called “Cerebro.” The system consists of two main modules: the headpiece module and the backpack module. The headpiece includes a camera assembly fixed to the animal’s head for recording eye movements and the scene in front. The backpack contains electronics for synchronizing cameras, acquiring images, and storing data. The entire system weighs approximately 60 grams, minimally impacting the animal’s activity.

To address the challenge of varying illumination in natural environments, the researchers developed a pupil detection algorithm based on artificial neural networks (ANN). They used a semantic segmentation network named U-Net to accurately detect the animal’s pupil position under different lighting conditions.

Experimental Subjects and Methods

Two young common marmosets were used as experimental subjects, tested under both head-fixed and freely moving conditions. In the head-fixed condition, animals were restrained in chairs and presented with visual stimuli such as flashing dots or drifting gratings to study the tuning properties of visual neurons. In the freely moving condition, animals were placed in an open rectangular arena where they could move freely and explore their environment. The Cerebro system recorded the animals’ eye movements, head movements, and body movements.

To validate the system’s accuracy, calibration experiments were conducted. In the head-fixed condition, animals were asked to fixate on marmoset faces presented at various positions on a screen to calibrate the eye-tracking system. In the freely moving condition, researchers used a laser to draw geometric shapes on a screen, simulating natural visual targets, to assess the system’s precision.

Data Analysis

The study analyzed eye movement trajectories, head movements, and body movements to quantify the characteristics of visual behavior in freely moving states. Specifically, the researchers focused on visual stability in two behavioral states: stationary (when the animal is seated or standing and visually scanning the environment) and locomotion (when the animal is physically moving through the environment). By calculating the correlation between eye movements and head movements, the team evaluated the role of the vestibulo-ocular reflex (VOR) in maintaining visual stability.

Main Results

1. Performance of the Eye-Tracking System
   The Cerebro system achieved an eye-tracking accuracy of 0.05 degrees in the head-fixed condition, comparable to traditional head-fixed eye-tracking systems like the Arrington eye tracker. In the freely moving condition, the median error remained under 1 degree, demonstrating its feasibility for use in natural environments.

2. Characteristics of Visual Behavior
   In the freely moving condition, marmosets exhibited a significantly larger range of eye movements compared to the head-fixed condition, suggesting that the restricted eye movement range observed in head-fixed conditions reflects a motor preference rather than a physiological limitation. Moreover, despite increased head and eye movements during locomotion, visual stability did not deteriorate and even improved. This indicates that animals enhance VOR gain during movement to maintain visual stability.

3. Role of the Vestibulo-Ocular Reflex
   The study showed that VOR plays a crucial role in freely moving conditions, especially during locomotion. Eye movements correlated negatively with head movements, helping to counteract the effects of head motion on retinal stability. This mechanism is essential for stable vision in primates during natural behaviors.

Conclusion

This research, through the development of an innovative head-mounted eye-tracking system, quantified visual behavior and neural activity in freely moving marmosets for the first time. The findings indicate that primates maintain visual stability in natural settings by coordinating head and eye movements, particularly enhancing visual predictability and stability during movement. This discovery not only deepens our understanding of the primate visual system but also provides significant technological and methodological support for future studies on neural mechanisms in natural behaviors.

Research Highlights

1. Technological Innovation: Cerebro is the first device capable of high-precision eye-tracking in freely moving primates, overcoming limitations of traditional methods in natural environments.
2. Scientific Value: The study reveals key aspects of visual behavior in freely moving primates, particularly the critical role of VOR in visual stability.
3. Application Prospects: The system can be applied to study visual behavior in other primates or humans in natural settings, offering broad potential applications.

Additional Valuable Information

The research team plans to use this system to further investigate social perception and neural mechanisms in marmosets, especially in natural environments, exploring how the visual system collaborates with other sensory systems. Additionally, the system provides new technical means for studying natural visual behavior in other animals, such as mice and rats.

Through this study, we not only gain a better understanding of the primate visual system but also open new avenues for future neuroscience research.