Sensory Processing in the Chilean Brush-tailed Mouse (Octodon degus): A Diurnal Precocial Rodent as a New Model to Study Visual Receptive Field Properties of Superior Colliculus Neurons
Academic Background
The visual system is a crucial component for animals to perceive their external environment, and research on its development and function is essential for understanding the sensory mechanisms of mammals. However, traditional studies have predominantly relied on nocturnal or crepuscular laboratory rodents, such as mice, rats, and hamsters. These animals possess relatively simple visual systems that significantly differ from those of humans. To broaden the scope of research and identify animal models closer to the human visual system, researchers have turned to the Chilean degu (Octodon degus). This species is diurnal, precocial, and has a cone-enriched retina with a more developed visual system. Therefore, the Chilean degu may serve as an ideal model for studying the development and function of the visual system.
The primary goal of this study is to explore the receptive field (RF) properties of visual neurons in the superior colliculus (SC) of the Chilean degu and compare them with those of traditional nocturnal rodents to uncover the unique characteristics of its visual system. Through such comparisons, the study aims to provide new insights into the visual systems of diurnal mammals.
Source of the Paper
This paper was co-authored by Natalia I. Márquez and others from the Department of Biology at the University of Chile, the University of Massachusetts Amherst, and other institutions. It was published in 2025 in the Journal of Neurophysiology.
Research Process
1. Experimental Subjects and Preparation
The study used 13 Chilean degus, which were housed under specific conditions, including a 12-hour light/dark cycle, with ample access to food and water. All experiments adhered to ethical standards for animal research and were approved by relevant institutions.
2. Anatomy and Neuronal Tracing
The study first employed neuronal tracing techniques (Cholera Toxin Subunit B, CTB) to label retinal projections and observe connections between the retina and the SC. CTB was injected into the retinas of four animals, and after 5-7 days, the animals were perfused, fixed, and brain sections were prepared to examine the distribution of retinal projections.
3. In Vivo Electrophysiological Recordings
Electrode recording techniques were used to monitor neuronal activity in the SC. Nine animals were anesthetized and fixed in a stereotaxic apparatus, and microelectrodes were used to record electrical activity in SC neurons. Four types of visual stimuli were employed: a moving white square, sinusoidal gratings, an expanding black circle (simulating an approaching object), and a stationary black circle.
4. Data Analysis
Neuronal activities were sorted using clustering algorithms, and methods such as Gaussian fitting were used to analyze the size and shape of receptive fields. The researchers also analyzed neuronal responses to contrast using the Naka-Rushton function and spatial frequency modulation using Fourier transforms.
Key Findings
1. Retinal Projections and SC Laminar Structure
The study confirmed the laminar structure of the Chilean degu’s SC and found that retinal projections primarily target the superficial layers (sgs and sz layers), with minimal input to the deeper layers (sgi and sgp layers). This aligns with previous findings in rodents.
2. Receptive Field Size and Shape
The study revealed that the receptive fields of SC neurons in the Chilean degu increase in size with depth, and neurons in deeper layers exhibit more horizontally elongated fields. Compared to nocturnal rodents, the receptive fields in the Chilean degu are smaller, likely reflecting its higher visual acuity.
3. Spatial Frequency Tuning
The study also found that SC neurons in the Chilean degu exhibit a broad range of spatial frequency tuning, with some neurons showing a clear preference for high-frequency stimuli (0.24 cycles/degree). This suggests that the Chilean degu’s visual system has high spatial resolution.
4. Contrast Response
Approximately half of the neurons showed linear responses to contrast, which is relatively rare in nocturnal rodents, further supporting the idea that the Chilean degu’s visual system is more adapted to diurnal activity.
5. Response to Approaching Objects
The study also found that SC neurons respond significantly differently to approaching objects compared to stationary ones, showing stronger activity in response to approaching stimuli. This indicates that the SC in the Chilean degu may be involved in detecting and responding to rapidly approaching objects, such as predators.
Conclusion
This study provides the first comprehensive characterization of the visual properties of SC neurons in the Chilean degu, demonstrating significant differences in receptive field size, spatial frequency tuning, and contrast response compared to nocturnal rodents. These findings highlight the Chilean degu as a suitable model for studying diurnal visual systems, particularly for exploring visual perception mechanisms more akin to those of humans.
Research Highlights
- Novel Animal Model: The Chilean degu, as a diurnal and precocial rodent, offers a new perspective for studying the visual system.
- Comprehensive Electrophysiological Analysis: Through diverse visual stimuli and detailed data analysis, the study thoroughly elucidates the visual properties of SC neurons in the Chilean degu.
- Potential Research Applications: The Chilean degu’s visual system is more similar to that of humans, making it a valuable model for future studies on the development and function of the visual system.
Additional Valuable Information
The study also suggests future research directions, such as exploring the developmental processes of the Chilean degu’s visual system and the role of visual experience in its maturation. These investigations may provide new insights into the the