Pupil Shape in the Animal Kingdom
Introduction
Animals exhibit a wide variety of pupil shapes, unlike the uniform round pupils in humans. These shapes are closely related to the ecological niches of the animals, which refers to their behavioral and environmental needs. In recent times, researchers are looking into biologically inspired artificial vision systems to achieve novel imaging capabilities. Examples include an artificial vision system with a vertically elongated pupil for object detection and recognition under diverse light conditions, inspired by the cat’s eye, and an artificial vision system with a W-shaped pupil for imaging under uneven light conditions, inspired by the cuttlefish eye. A deeper understanding of how vision systems function in animals could inspire the development of advanced optical systems for various applications such as robotics, surveillance and autonomous vehicles. Hence, the goal of our project is to study the impact of different pupil shapes on vision, particularly focusing on the vertical slit pupil, horizontal slit pupil, W-shaped pupil, and multiple slit pupil, using ISETCam and ISETBio in MATLAB.
Background
Point spread function and Apertures
Point spread function is a measure of how an imaging system responds to a point source of light. It reflects the blurring of light caused by diffraction, aberrations and other imperfections in the imaging system. A circular aperture results in a pattern called an Airy Disk. A smaller aperture size increases diffraction and the spread of the airy disk, causing more blurring. Non-circular apertures, such as square and triangle, produce unique PSFs compared to circular apertures. When the aperture is elongated in one axis, such as a rectangle, the diffraction pattern is extended in the direction perpendicular to the aperture extension. Most species, including humans, have circular pupils as a circular aperture allows symmetrical distribution of light across the retina. This minimizes distortions and provides consistent image sharpness across the entire visual field, making it optimal for general vision tasks. This suggests that species with non-circular pupils must have evolved them for specialized purposes.
Theories for various pupil shapes
Studies have shown that vertical slit pupils are common in ambush predators, as they enable the predators to keep vertical features in focus across different depths of field, while using the amount of defocus blur of horizontal features to estimate distance accurately. Conversely, horizontal slit pupils help terrestrial prey gain panoramic views of their surroundings, helping them detect predators more effectively. Research has also highlighted the role of pupil dynamics in regulating light entry. For example, many species with vertical pupil shapes are nocturnal, so there is a need to control the amount of light entering during daylight. Another example is the cuttlefish, which lives in shallow waters, a high dynamic range environment with the top surface bright due to sunlight and the bottom sea floor dark. The cuttlefish’s W-shaped pupil is said to help balance out the vertically uneven light field entering its eye. Another intriguing aspect of pupil shape is its connection to color vision. Cephalopods, such as cuttlefish, have only one type of photoreceptor. However, their W-shaped pupil is believed to enhance chromatic aberration, allowing them to perceive color by adjusting the lens-to-retina distance.
Methods
In this project, we utilized ISETCam and ISETBio to simulate the imaging process that occurs within an animal's eye.
Scene
We created two types of scenes for our simulations: sceneCreate.m to generate gridlines and sceneFromFile.m to read an image. The gridlines illustrate the effects along different axes, while the image provides a visualization of what the animal might be seeing.
Aperture
The aperture represents the pupil shape and is modeled as a 201x201 matrix. In this matrix, ones indicate the regions where light can pass through (white), while zeros denote the areas where light is blocked (black). For the single-slit elliptical aperture, we can adjust both the size and the orientation of the ellipse (vertical or horizontal). For the multiple-slit aperture, we can modify the slit length, slit width, and the number of slits. For the w-shaped aperture, we intake the w-shape black and white image to generate the aperture matrix, and are able to change the size of it.
Point spread function and Optical image
The point spread function was simulated for each aperture to analyze the impact of aperture shapes on light passing through the aperture. This was achieved by creating a diffraction-limited wavefront structure using wvfCreate.m and computing the point spread function with wvf.Compute.m with the aperture matrix as input. The optical image was generated using oiCompute.m taking in the scene and the aperture matrix as inputs. The simulation process assumes that the optical lens is diffraction-limited.
Cone Mosaic and Absorptions
To simulate how the eye’s sensor processes the scene, we modeled the human cone mosaic using cMosaic.m in ISETbio, and used cm.compute.m to simulate the cone excitations and absorptions. Due to time constraints, we only tried the human cone distribution, but it would be highly valuable to try incorporating known cone distributions from the literature for various animals in the future.
Results
Horizontal goat pupil and vertical cat pupil
The horizontal slit pupil introduces blurring along the vertical direction, evident in the scene by the horizontal gridlines appearing blurred while the vertical gridlines remain sharply in focus. This effect is also replicated in the cone mosaic plot, confirming the impact on visual perception. Conversely, for the vertical slit pupil, the phenomenon is observed along the opposite axis. The horizontal gridlines remain sharply focused, while the vertical gridlines appear blurred. This demonstrates that a vertical pupil causes blurring in the horizontal direction while maintaining clarity along the vertical axis.
These plots align with our understanding of the functional advantages of pupil shapes in animals. For prey animals with horizontal slit pupils, having sharp focus across the horizontal axis is advantageous, as it allows them to effectively monitor their surroundings for predators. Additionally, the laterally placed eyes of these animals provide a wide field of view, further enhancing their ability to detect threats. On the other hand, predator animals with vertical slit pupils benefit from sharp focus along the vertical axis, which aids in keeping their prey in view. This vertical focus is particularly useful when combined with head or eye movements to track prey's horizontal motion, supporting their hunting strategies.
Slit pupil vs. circular pupil
When comparing a slit pupil to a circular pupil of the same area, we observe that for a smaller aperture, the circular pupil leads to blurring across the entire scene. This occurs because diffraction becomes more pronounced in smaller apertures, causing the loss of sharp focus. In contrast, a slit pupil of the same area can retain focus along one axis. Specifically, for a vertical slit pupil, the image remains sharp along the vertical direction while blurring occurs along the horizontal axis.
These plots support our understanding of the behavior of predator animals. Many predators with vertical slit pupils are also nocturnal. During the day, their pupils constrict to a small area to limit the amount of light entering the eye. If their pupils were circular, the small aperture would cause significant diffraction, leading to a loss of visual clarity. However, the vertical slit pupil allows them to maintain good focus along one axis, despite the reduced aperture size, enabling them to retain functional vision even in bright conditions. This ability is particularly beneficial for nocturnal predators, allowing them to effectively navigate and hunt in varying lighting conditions.
Effect of W-shaped pupil on chromatic aberration
multiple-slit pupil
Conclusions
Appendix
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