Sankhajyoti Saha, M.Optom

Editorial Assistant, Vision Science Academy

Assistant Professor, NSHM Knowledge Campus, Kolkata, India

Vision Science Academy Exclusive

Humans have been blessed with the highest form of sensory development, but some lower carnivores eclipse human abilities in certain aspects.
The word “Refuge” conjures up the image of something hefty, and dynamic and this title perfectly aligns with the image of an EAGLE, a proud family member of Accipitridae, ⁽¹⁾ who solicited this title for its own fascinating visual calibre.

How good is eagle vision?

An eagle is blessed with a bewildering vision of 5/20 to 4/20, which is almost four to five times better than humans. Amid the queer trait of their biological vision, they have two types of foveae. One shallow fovea maneuvered in their binocular vision, and a deep fovea for monocular vision. The deep fovea allows these birds to perceive distant objects, specifically to identify prey. ⁽²⁾ In spite of 30-degree separation of their eyes from the midline of their face ⁽¹⁾, the extra fovea allows them to obtain a high-resolution field of view to the side of their heads as well as directly in front of them. ⁽³⁾

Based upon ophthalmoscopic measurement, the maximum anatomical resolving power of the eagle’s deep fovea was calculated as 140 c/deg and Maximum behavioural acuity between 132 and 143 c/deg, which corresponds closely and gathers highest form of visual information while perching. ⁽⁴⁾

Human photoreceptors (especially cones) are separated by 3µm, whereas it is 2 µm in case of an eagle. This means that more pixels into the small area available in the sensory fovea help them to essentially zoom in on their prey like a rabbit in bushes from 10,000 feet up. Inspired by this biological function, mobile phones with a high megapixel into a small sensor have been optimized. ⁽³⁾

DeepFoveaNet is a two Encoder-Decoder convolutional neural network model to detect moving objects in video sequences along with the combination of magnification of the peripheral vision, to detect very small moving objects, has also been actually inspired by the biological functioning of the deep fovea a model. ⁽²⁾

Refraction across a range of luminance:

While evaluating avian visual performances, certain factors should be considered. Diurnal raptors are basically hyperopic and have specific strategies while perching. When Chromatic aberration is taken into account, this refraction will allow birds to accommodate distant stimuli in blue light; this is something an emmetropic human can’t do. This is the reason they can detect objects against the blue sky prominently. As the luminance decreases, the Photopic visual performance falls off rapidly due to the lower ratio of cone dense area in compare to absorption level of visible wavelength. ^{(5) (6)}

Eagles, in their infancy, can’t track down fish underneath the water, due to the ocular refractive pattern. Refractive development occurs with age naturally and they are able to grab the fishes below the water level. ⁽¹⁾

All of the visual strategies actually optimize the performance of an eagle in their prey search, despite having different levels of visual obstructions.

Can we have better than nature’s vision?

Visual systems consist of different spatial frequencies (e.g., Visual Acuity, Contrast Sensitivity). It also attenuates the sinusoidal gratings at different spatial frequencies and other ocular parameters. Better visual system corresponds to less system attenuation. Cut-off frequency indicates the point at which the transfer becomes zero. The Eagle optics at best cut off is twice the human value and with smaller pupil diameter it maximizes its range. ⁽⁵⁾

From a human optics point of view, the hope that was given enough time was at the viewing aperture, a sampling method to emphasize maximum visual performance. ⁽⁵⁾ For larger pupil sizes (>3 mm diameter) “ideal” corrections improve the optical quality of the retinal image. When perfected, “ideal” corrections will provide for high contrast visual acuity between 20/8 and 20/10. ⁽⁷⁾

Wavefront sensing of the human eye provides the necessary monochromatic aberration corrections with higher contrast and crisper edges for human vision, beyond the limits imposed by photoreceptor spacing. Will these corrections lead towards excellence? If yes, how much we can conquer the visual world with our excellencies is still a wrangle.

References:

1. Eagle. (2023, January 11). In Wikipedia. https://en.wikipedia.org/wiki/Eagle
2. Guzman-Pando, A., & Chacon-Murguia, M. I. (2021). DeepFoveaNet: Deep Fovea Eagle-Eye Bioinspired Model to Detect Moving Objects. IEEE transactions on image processing : a publication of the IEEE Signal Processing Society, 30, 7090–7100. https://doi.org/10.1109/TIP.2021.3101398
3. Daly, I. (2019, September 13). The unique visual system of the eagle. Retrieved January 17, 2023, from https://www.opticianonline.net/features/the-unique-visual-system-of-the-eagle#:~:text=Eagles%2C%20like%20many%20other%20birds,cones%20are%20separated%20by%203%C2%B5m.
4. Reymond L. (1985). Spatial visual acuity of the eagle Aquila audax: a behavioural, optical, and anatomical investigation. Vision research, 25(10), 1477–1491. https://doi.org/10.1016/0042-6989(85)90226-3
5. Shlaer, Robert. (1972). An Eagle’s Eye: Quality of the Retinal Image. Science (New York, N.Y.). 176. 920-2. 10.1126/science.176.4037.920.
6. O’Rourke, C. T., Pitlik, T., Hoover, M., & Fernández-Juricic, E. (2010). Hawk eyes II: diurnal raptors differ in head movement strategies when scanning from perches. PloS one, 5(9), e12169. https://doi.org/10.1371/journal.pone.0012169
7. Kohnen, T., Klaproth, O. K., & Bühren, J. (2009). Effect of intraocular lens asphericity on quality of vision after cataract removal: an intraindividual comparison. Ophthalmology, 116(9), 1697–1706. https://doi.org/10.1016/j.ophtha.2009.03.052