Pupil Shape in the Animal Kingdom

Methods

In this project, we utilized ISETCam and ISETBio to simulate the imaging process that occurs within an animal's eye.

File:Figure. schematic.png

Figure 1. The flow chart illustrates the simulation process, with the examples below each step.

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.

Figure 2. Horizontal pupil has enhanced features identification along the vertical direction compared with vertical pupil. (A) Horizontal pupil and vertical pupil. (B) Point spread function of each pupil case. (C) Optical images of the grid scene.(D) Cone mosaic absorption of the optical image.

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.

Figure 3. Vertical pupil has enhanced features identification along the vertical direction compared with round pupil with the same size. (A) Round pupils in comparison with the vertical pupils with the same size (B) Point Spread Function of each pupil case. (C) Optical images of the grid scene. (D) Cone mosaic absorption of the optical image and (E) the horizontal integration of the absorption intensity.

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.

Appendix

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