Wednesday, 18 July 2018
Using Hawkeye from the Avengers to communicate on the eye
A Personal View Barry W. Fitzgerald 17 Jan 2018https://doi.org/10.1152/advan.00161.2017 Abstract Superheroes, such as Iron Man, Captain America, Wonder Woman, Batman, and Hawkeye, have appeared in numerous films, displaying their range of incredible superpowers and abilities. Therefore, it is unsurprising that many people would not only wish to attain these powers, but also to learn about scientific accessibility to these powers. Popular culture characters such as superheroes can provide a unique platform for the communication of difficult scientific concepts. In the classroom, these characters can be used to communicate learning objectives to students in an interesting, fun, and accessible manner by taking advantage of student familiarity with the characters. Hawkeye, a member of the Avengers, is one such superhero who can be utilized by educators. His powers can be attributed in part to his advanced eyesight, which has physiological aspects in common with many birds of prey. Hence, Hawkeye can instigate discussion on the physiology of the human eye, while also allowing for comparison with other species, such as birds of prey, and reflection on advancements related to genetic engineering and wearable technologies. In addition, in my experience, Hawkeye has proven to be a highly suitable popular culture character for use in scientific communication and outreach. INTRODUCTION In recent years, there has been an upsurge in the popularity of the superhero genre, motivated in part by the large number of successful superhero films. Some films focus on superhero teams, such as the X-Men and the Avengers, with team members having a diverse range of complementary abilities. In the Avengers, Iron Man (Tony Stark) wears a powered exoskeleton suit that provides him with increased strength and the ability to fly, whereas Captain America (Steve Rogers) is a World War II super-soldier whose physiology has been permanently altered following treatment with an experimental serum and radiation exposure (3). Other team members are aliens, such as Thor, and the Vision is an android powered by an alien relic known as an Infinity Stone. Nonetheless, the abilities of some Avengers are firmly based on genetics and human physiology. Black Widow, or Natasha Romanoff, is a highly trained Russian spy with Olympic athlete level fitness, agility, and reflexes and an expert in hand-to-hand combat, whereas Hawkeye, or Clint Barton, who also possesses many of Ramonff’s abilities, is an expert archer. In my opinion, Hawkeye is one of the more grounded superheroes. He has not been the subject of an experimental treatment and does not wear an Iron Man-like suit. Just like the average person, Hawkeye fatigues and must eat to replenish his energy reserves. For this alone, he is a superhero to whom the student can relate. Hawkeye’s abilities as an archer can be attributed to his exceptional eyesight, which can act as an innovative platform to introduce the physiology and anatomy of the eye to students (13). The character can also be used to motivate discussion on how current scientific research could be used in the near future to increase the acuity of human eyesight (13). Using superheroes such as Hawkeye as a platform for the communication of learning objectives related to physiology can have a multitude of benefits for educators and students (3, 61, 62). First, superheroes can be used to motivate the student’s initial interest in a new topic or subject. Given the popularity of superhero films, it is likely that the majority of students will have seen, or, at the very least, be aware of the superhero character. Second, the use of superheroes in the classroom represents a departure from traditional analogies used to enforce or support the delivery of learning objectives. Third, the topic of superheroes can be used to inspire educators in the design of laboratory experiments, in-class assignments, student projects, and final examination questions. The use of superhero paradigms can also foster stronger links between educators and students, and between physiological aspects of the human body and the world of the student. In addition, Hawkeye can be used in scientific outreach to help bridge the gap between academic research and the general public. In this article I will discuss the physiology of Hawkeye’s eyes, drawing comparisons with the vision of birds of prey. I will motivate Hawkeye using one scene from the 2012 film The Avengers (2). In addition, I also briefly suggest how Hawkeye can be used to motivate other topics, such as genetic engineering and transgenesis. Finally, I propose pedagogical applications of Hawkeye in the classroom and suggest methods for motivating classroom discussions as well as planning experiments connected to learning objectives. Hawkeye: the Superhero Many people will be familiar with Hawkeye, as portrayed by Jeremy Renner, from films such as The Avengers (2), Avengers: Age of Ultron (1), and Captain America: Civil War (4). The character first appeared in the Marvel comic book Tales of Suspense, no. 57, in 1964 (29), and he joined the Avengers in Avengers, vol. 1, no. 16, from 1965 (28). His two key physiological characteristics are his speed of reaction and superhuman eyesight, which both contribute to his ability as a master archer. To rapidly fire one accurate arrow after another, Hawkeye must be at the peak of human conditioning, with highly developed tendons and ligaments, as well as exemplary eye-hand coordination. However, the focus of this article is solely on his vision. To motivate this topic, I will refer to one particular scene from the 2012 film, The Avengers (2). In the film, the Avengers battle the Chitauri, an alien army, that arrives via a wormhole above Stark Tower in New York City. During the fight scene, Hawkeye is perched on a skyscraper firing arrows at Chitauri gliders flying through the New York City skyline. At one point, Black Widow flies an alien glider with Loki, the main antagonist of the film, in pursuit. Sensing the danger, Black Widow contacts Hawkeye for assistance. Hawkeye calmly loads an arrow in his bow and tracks the gliders of Black Widow and Loki. The camera then zooms in on the pair of gliders, and, in doing so, gives the audience an impression of Hawkeye’s advanced vision. Hawkeye confidently states “I got him” and fires an arrow at Loki. Although Loki catches the arrow, it explodes, destroying Loki’s glider and allowing Black Widow to escape. Hawkeye and Birds of Prey Hawkeye’s accuracy with the bow and arrow is dependent on his eyesight, which is more advanced than the average human eyesight. As indicated previously, Hawkeye has not been enhanced via experimentation, technology, or alien artifacts. He has not been administered an experimental serum like Steve Rogers (3), and he has not been genetically modified after exposure to cosmic rays like the Fantastic Four. Hence, I must conclude that he has inherited the genetic propensity for advanced eyesight from his parents. The nickname “Hawkeye” reflects this and suggests that he has many attributes in common with birds of prey (Fig. 1). There are several aspects of the eye that could be central to Hawkeye’s vision. For brevity, I will highlight what I believe are three key aspects, which are advanced accommodative mechanisms, differing retinal structure, and increased photoreceptor density in the eyes. I will also compare the position and associated visual field of his eyes with those of birds of prey. Fig. 1. Fig. 1. Sketch of the eye of a bird of prey. [Image credit: Jean-Philippe Frimat. From Fitzgerald (13) with permission.] Download figure Download PowerPoint Accommodative mechanism. First, I will consider the accommodative mechanism in the human eye (Fig. 2). The human eye is sensitive to electromagnetic waves, with wavelengths ranging from 390 nm up to 700 nm, extending from violet to red. When light enters the eye, it is refracted by the cornea and the lens to focus the light onto the retina, the light-sensitive part of the eye, where the light excites photoreceptor cells, leading to chemical and electrical nerve impulses (Fig. 2). Neurons, known as retinal ganglion cells, collect the impulses and transmit them via the optic nerve to the brain. The cornea has a fixed spherical shape, acts as a converging lens, and accounts for roughly two-thirds of the refractive power of the eye. The ciliary muscles surrounding the lens change the refractive power of the lens, a process known as accommodation. To resolve distant objects, the lens will assume a thin and flat geometry, whereas, for closer objects, the lens is thicker and rounder. Fig. 2. Fig. 2. The human eye with the cornea, lens, optic nerve, retina, and fovea all indicated. Download figure Download PowerPoint The accommodative process in birds of prey is different than for humans (23, 60) (Fig. 3). While the ciliary muscles connected to the lens change the refractive ability of the transparent lens in a human eye, accommodation is facilitated in birds of prey through the deformation of both the lens and cornea. Corneal accommodation plays a key role in the overall vision of birds. In front of the cornea is a layer of air, and behind the cornea is the aqueous humor, which is a waterlike liquid. With increased curvature, the cornea has greater refractive power and can thus exploit to a greater extent the interface between the air and aqueous humor, which have differing refractive indexes. This process is facilitated by the anterior sclerocorneal muscles, which are striated or striped muscles in the eye. Fig. 3. Fig. 3. An example of the anatomy of an avian eye. [From Whittow (60) with permission from Elsevier.] Download figure Download PowerPoint The refractive power of an optical device is a measure of the degree by which it bends light and is calculated from the reciprocal of the focal length (P = 1/f). It is measured in units of diopters (D) or m−1. For a human eye, the refractive power is ~60 D, with ~40 D due to the refractive power of the cornea. While the refractive power of the eyes in birds of prey is only slightly larger, the corneal and lenticular accommodative processes allow for greater flexibility and adaptivity for focusing images on the retina. In addition, some avian species can be emmetropic and myopic at the same time (23). This means that a bird can simultaneously focus on two objects: one that is distant and one that is closer to the bird. Such accommodative mechanisms can explain why Hawkeye is able to track multiple targets at varying distances in the film The Avengers (2). While Hawkeye temporarily retires from the Avengers in Avengers: Age of Ultron (1) and returns to action in Captain America: Civil War (4), he will eventually have to permanently retire due to degradation of his accommodative abilities. In a normal human eye, the process of accommodation deteriorates over time, leading to the condition of presbyopia, a decrease in the eye’s ability to focus on close objects. While this condition will not affect Hawkeye’s ability to focus on distant objects and could be corrected with advanced surgical techniques (14), Hawkeye will certainly suffer some loss of corneal and lenticular accommodation with age, just like many avian species (23). In fact, Hawkeye is shown as blind in the Old Man Logan comic book (37), where it appears that he has developed cataracts in his eyes and suffered serious eye trauma rather than experienced degradation of his accommodative mechanisms. We have yet to see Hawkeye using his trusted bow and arrow underwater in the Marvel Cinematic Universe films. The human eye loses the refractive ability of the cornea when underwater, because the refractive index of water is approximately the same as the refractive index of the fluids in the eye. As a result, the lens is unable to compensate for the missing refractive power, leading to the projection of an image beyond the retina, known as hyperopia. This explains why someone sees blurry images when underwater. If Hawkeye were trying to incapacitate a villain underwater, he would need additional accommodative mechanisms from certain birds of prey to retain visual acuity. The gannet is a bird of prey that seemingly retains visual acuity when moving from air to water. Gannets are large seabirds that search for fish from above the water before plunging into the water after the prey. They can capture prey during two different phases of the dive. First, they can capture prey in the momentum phase of a dive or when the gannet first enters the water. Second, prey can be acquired during the wing-flapping phase, where the gannet pursues the prey through the water. An experimental study of plunging gannets suggests that they switch from aerial vision to clear underwater vision in <0.1 s (8, 32). The study suggests that the clear aquatic vision is due to the reshaping of the lens against the iris, which controls the amount of light entering the eye. When underwater, the ciliary muscles pull the lens forward rather than deforming the lens in the traditional manner. The lens then slightly protrudes beyond the iris and increases the refractive ability of the lens. For Hawkeye, this process may be sufficient to correct for the loss of refractive power of the cornea while underwater and allow him to retain his advanced accommodative mechanism. Photoreceptor cells and foveae. While Hawkeye’s accommodative mechanisms are crucial for focusing light onto his retina, his visual acuity is also dependent on his retinal structure. In the human eye, there are two main types of photoreceptor cells, referred to as rods and cones. Four genes are associated with these cells: one gene for rods and three genes for cones. The RHO gene leads to the production of the protein rhodospin, which is found in the rods. The other three genes are OPN1SW, OPN1MW, and OPN1LW, which encode for the blue-sensitive opsin protein, green-sensitive opsin protein, and red-sensitive opsin protein, respectively (40). Here SW stands for short wave, MW is medium wave, and LW is long wave. The breakdown of cones in the eye is ~65% red cones, 33% green cones, and 2% blue cones, where the blue cones have the greatest sensitivity (48). Hence, humans, like cats (50), have trichromatic vision, given that we possess cones sensitive to three wavelength ranges. Rods provide lower resolution, but they are very sensitive to lighting conditions. Thus they are important for vision in poorly illuminated conditions. At the lowest luminance, where the rods are primarily active, vision is referred to as scotopic. On the other hand, cones allow for high spatial resolution or visual acuity, while being insensitive to light intensity. For indoor lighting or sunlight, cones mainly contribute to variances in vision, since the contribution from the rods saturates, a vision referred to as photopic (48). In the average human eye, rods and cones are located in differing parts of the retina (22, 48). The human eye comprises between 90 and 120 million rods and, on average, 4.5 million cones. Cones are packed into the fovea centralis in the center of the macula lutea in the retina, whereas rods are found in other regions of the retina. In an investigation on the photoreceptor count in eyes from 21 donors, the density of cones in the fovea region has been shown to be in excess of 125,000 cones/mm2, whereas the density of rods peaks somewhere in a ringlike area 3–5 mm from the fovea at 150,000 rods/mm2 (22). In comparison, an eagle can have up to 1,000,000 cones/mm2 in its fovea (10). In recent years, there has been a notable advancements in relation to noninvasive medical imaging of microvascular flow in the vicinity of the skin (25, 34, 35, 44), imaging of bowels (31), and the imaging of the human eye (21, 46, 59), as well as the eyes of birds of prey (51). For the latter two cases, the noninvasive approach means that the retinal structure of both birds of prey (51) and humans (59) can be studied in a manner that is both accurate and safe for the subject. One approach that is ideally suited for ophthalmic imaging, such as retinal cross-sectional imaging, is optical coherence tomography (OCT) (21, 46, 51). Similar to ultrasonic pulse-echo imaging, OCT is a technique that has been developed to facilitate noninvasive cross-sectional imaging in biological systems. OCT uses interferometry to generate a two-dimensional image of tissue using back-reflected photons from tissue of low-coherence light, with a wavelength of 830 nm. Three-dimensional images of the retinal structure for two diurnal hawks using OCT show that diurnal hawks are bifoveate, meaning that they possess two foveae in the retina, classified as the deep fovea and shallow fovea (51). Just like the human eye, these foveae contain cones and are associated with acute vision. The deep fovea is located in the nasal or central region, which is close to the nasal part of the bird’s skull, whereas the shallow fovea is found in the temporal region, which is closer to the temple of the bird (see Fig. 3). Therefore, the retinal structure in birds of prey is markedly different from the retinal structure of the human eye. Of the two foveae, the deep fovea is more clearly discernible and developed. For a Broad-winged Hawk, the depth of the deep fovea has been shown to be 66% of the total retinal thickness (51), whereas the shallow fovea is imaged as a slight depression in the retina. Conversely, retinal images of another species, the Short-tailed Hawk, clearly demonstrate the presence of the shallow fovea. This suggests that the extent of the shallow fovea could depend on the species or on an individual from that species (51). Typically, the central fovea contains a higher density of photoreceptor cells in comparison to the shallow fovea (Fig. 4). Fig. 4. Fig. 4. Relative receptor density for an ideal bird of prey. [From Tucker (55) with permission.] Download figure Download PowerPoint As shown in the OCT study of Ruggeri et al. (51), some individuals in avian species do not have a noticeable shallow fovea. It may very well be the case that Hawkeye does not have a shallow fovea, given that the human eye ordinarily does not have such a fovea. However, there is credible evidence from the film The Avengers (2) that suggests the contrary. At one instance in the Battle of New York City, we find Hawkeye perched on a skyscraper with an arrow loaded and ready to fire. As he carefully tracks one glider, he appears to focus on a second glider at the same time. Without checking on the position of the first glider, he fires the arrow, hits the target and then prepares to fire at the second glider. As mentioned in the previous section, this multitargeting capability may also be facilitated by the combination of corneal and lenticular accommodative processes. Nonetheless, this multitracking example offers support for the proposal that Hawkeye is bifoveate. The positions of Hawkeye’s foveae would be markedly different from birds of prey as influenced by the skull structure. For birds of prey, the deep fovea has a line of sight that is greater than 30° to the side of the reference line through the skull (Fig. 5). On the other hand, the shallow fovea makes an angle of ~15° with the same reference line. These angles are dependent on the species and on the extent to which the eye can rotate in the socket, as many avian species have limited movement of the eye in the socket (23). As a result, a bird of prey needs to move its head in order for the light from an object of interest to fall on the deep or shallow fovea. For objects at distances >20 m, one study has shown that birds of prey spend most of their time looking at the objects sideways in order for the light to fall on the deep fovea, the region with the highest visual acuity (55), while a recent study has also shown that birds of prey, such as the Northern Goshawk and Red-tailed Hawk, search for prey using random head turns (42). Given that his eyes can move in their sockets, it is unlikely that Hawkeye has to significantly move his head when focusing on targets using either his supposed deep or shallow fovea. Birds of prey do not have the same luxury. When facing forward, light from an object of interest will predominantly fall on the shallow fovea (Fig. 5). While this is in the binocular vision field, it does not provide the bird with the greatest visual acuity. Fig. 5. Fig. 5. An illustration of the frontal section of the head of an idealized bird of prey, such as a raptor, showing the positions of the deep and shallow foveae. The line of sight (LOS) through the pupil to the deep fovea is shown in the diagram. [From Tucker (55) with permission.] Download figure Download PowerPoint Unlike birds of prey, Hawkeye does not appear to search for prey in a random fashion. In The Avengers (2), Hawkeye scans his surroundings in a seemingly methodical fashion, which may indicate that his bifoveate eyes make similar angles with the reference line of his skull. Given the physiology of his eyes, eye rotation in his sockets is apparently not an issue. If he possesses both a deep and shallow fovea, unlike some birds of prey, Hawkeye can rely on both eye and head rotation to allow light for an object to fall on either the shallow or deep fovea. For Hawkeye to possess the visual acuity suggested in the “I got him” scene from The Avengers (2), he would certainly need a deep fovea similar to that of birds of prey, a photoreceptor density similar to that of birds of prey and advanced avian-like accommodative mechanisms outlined in the previous section. In the scene, Hawkeye is perched on a building looking down East 42nd Street toward the East River. At the moment that his vision is shown to zoom in on the gliders of Black Widow and Loki, the gliders are ~700 m from Hawkeye’s position. From the film, I estimate that the gliders are ~3 m in length and 1.5 m in width. The golden eagle is reported to be able to instantly adjust its vision to zoom in on a rabbit with a length of up to 0.5 m that is a distance of more than 3 km from the eagle (15). Therefore, the advanced vision and automatic zooming ability of his eyes attributed to Hawkeye in this scene from The Avengers (2) is representative of the ability of birds of prey. Finally, in this section, I will reflect on the structure and additional cone in the eyes of birds of prey and potentially possessed by Hawkeye. Many avian species, including hawks, are tetrachromats, meaning that their retina contains four different cones, where the additional cone is sensitive to ultraviolet (UV) wavelength. UV vision has been shown to be important for tracking the scent marks of prey while hunting (19). For example, scent marks deposited by voles have been shown to have a UV spectra (57). Another study has shown that the UV cones are magnetoreceptors, used by certain species to enable orientation with the Earth’s magnetic field (41). The structure of cones in birds of prey is further complicated by the presence of oil droplets in the cone. When light interacts with a cone, it passes through the oil droplet before interacting with the photosensitive component of the cone. The droplet, of which there are six different types, acts as a filter for different wavelengths, meaning that the response of each cone is dependent on the cone type and the type of oil droplet. Thus birds of prey can process a much larger range of colors in comparison to the human eye (23). From the films, thus far, there is no evidence to suggest that Hawkeye can detect UV wavelengths through the presence of a fourth cone. To strengthen the association with birds of prey, it might be an idea for writers of future Marvel Cinematic Universe films to reveal that Hawkeye can “see” or detect UV light. Size, position, and visual fields. I now reflect on the size, position, and visual field of the human eye in comparison to birds of prey. One of the most striking aspects of the anatomy of birds of prey is the size of their eyes in comparison to their skull. In the case of humans, the eyes take up ~5% of the volume of the skull, whereas for many birds of prey, the eyes occupy ~50% of the volume of the skull (23). The position of the eyes in humans and birds of prey is also noticeably different. Humans have front-facing eyes, whereas the eyes of many birds of prey are angled away from the midline or reference line of their skulls (Fig. 5). Birds of prey and humans have markedly different visual fields. Figure 6 shows a comparison of the horizontal visual fields of a Red-tailed Hawk and a human. For the Red-tailed Hawk, this is the visual field when their eyes are fully converged or fully forward toward the reference line of the skull (Fig. 5). The Red-tailed Hawk has a binocular field of ~33°, which extends 16.5° left and right of the skull reference line. The lateral or monocular field, shown in white in the figure, extends a further 122° on either side of the binocular field, while their blind spot makes up the remaining 82°, which is more or less directly behind them. The visual field is species dependent, with some having larger or smaller binocular fields and blind spots. For example, the species Cooper’s Hawk has a binocular field of 36° and a blind spot of 66° (45). For the human eye, the binocular field is ~120° with a monocular field of roughly 35° either side of the binocular field. A blind spot of 170° makes up the rest of the human visual field, which is over twice the size of the blind spot for the Red-tailed Hawk. Experiments have found that some birds of prey spend more time looking sideways at objects when the distance to the object is larger than 20 m, which indicates that, to use the acute vision of the deep fovea, birds of prey look at objects using a single eye and the monocular field of vision (55). In a recent study involving a species known as Harris’s Hawks, the birds displayed a high frequency of head movements, and hence sideways head positions, when selecting a target, which may be associated with their increased dependence on their monocular vision field (47). Fig. 6. Fig. 6. Comparison of the horizontal visual fields for a Red-tailed Hawk and a human. The binocular field is shaded in gray, the lateral field is in white, and the blind spot is in black. For the Red-tailed Hawk, this is the visual field when the eyes are converged toward the frontal part of the skull or closer to the reference line (see Fig. 5). Download figure Download PowerPoint Given his appearance, it is clear that Hawkeye, and the actor Jeremy Renner who plays the character for that matter, cannot have the same visual field of a bird of prey such as the Red-tailed Hawk. Hawkeye’s eyes are front facing in his skull and certainly not angled away from the midline of his skull. Even if Hawkeye is bifoveate, possesses a larger density of photoreceptor cells, and has advanced accommodative mechanisms, his only recourse to overcome the limitations presented by the size of his blind spot is to rotate his head. In The Avengers (2), Hawkeye can be seen continually moving his head as he assesses the attack formation and flight paths of the Chitauri gliders in New York City, whether perched above the city on a skyscraper or engaging targets from a ground position. Hawkeye’s head movement is a necessity as he tries to mimic the visual field of a bird of prey. In one scene, it appears that Hawkeye takes down a glider behind him without looking or turning his head. However that scene takes place after a verbal exchange with Iron Man during which the focus of the film moves from Hawkeye on the rooftop to Iron Man flying down a New York street. The film focuses on Iron Man for approximately 4 s, which would have been sufficient time for Hawkeye to quickly look behind himself, identify a target, and then use his experience as an archer to predict the correct moment to blindly fire the arrow at the glider. Just like birds of prey, head movement is important for Hawkeye to identify and focus on targets. Hawkeye normally uses his advanced eyesight to isolate a target from a stationary position and then fire an arrow at the target, such as in The Avengers (2). Thus it is rare that Hawkeye will be traveling at large speeds while firing arrows. Nevertheless, head position is crucial for birds of prey when engaging a prey. As shown by Tucker (55), birds of prey tend to hold their head in a sideways position when viewing a prey that is straight ahead. Birds of prey keep their head in this position to ensure that light from the target falls on the deep fovea, thus providing maximum visual acuity. However, in maintaining a sideways position, the bird experiences increased aerodynamic drag. Tucker suggests that a bird can follow a logarithmic spiral flight path to minimize the effects of aerodynamic drag and still keep its head sideways during flight (55). Despite having to fly a longer path, the bird of prey could, in theory, engage a prey faster due to the associated decrease in aerodynamic drag. While the issue of aerodynamic drag does not affect Hawkeye, the approach could be of interest to Falcon, another member of the Avengers. Falcon wears a flight pack that deploys wings enabling flight. Similar to a bird of prey, Falcon also needs to move his head to observe targets, which could in turn lead to undesired fluctuations in the aerodynamic drag force, and possibly the lift force, he experiences during flight. The hydrodynamic forces experienced by a real falcon have been investigated in the past by placing a live subject in a wind tunnel, where it was noted that, as the air speed increased, the falcon decreased wing span and lift coefficient (56). In the future, it may prove beneficial to model bird-of-prey aerodynamics using hydrodynamic coefficients for nonspherical particles that experience drag and lift forces in industrial processes (33, 36). Creating Hawkeye’s Eyesight From the outset of this article, I have argued that the only feasible biological explanation for Hawkeye’s advanced eyesight is thousands of years of genetic evolution in the form of adaptation, genetic drift, or mutation of his DNA. This suggests that members of Barton’s family also have similar eyesight. In the Marvel comic books, this is the case, as Clint’s brother, Barney Barton, is also an accomplished archer, thanks to his augmented vision. While evolution could lead to the emergence of Hawkeye-like sight in the human species in the future, although it is highly unlikely, we may have to wait millennia and many generations for these genetic changes to take hold. The genetic mutation of the Barton DNA to allow for the family’s advanced vision is associated with precise changes of the nucleotides, the subunits of DNA, in the double-helix structure of the DNA. The three primary means of instigating mutation in DNA is by changing of a nucleotide, deletion of a nucleotide, or the insertion of a nucleotide between two existing nucleotides. Once the DNA changes, it can be passed onto the next generation via genetic inheritance (13). For example, Barton’s DNA, just like our DNA, contains the genes OPN1SW, OPN1MW, and OPN1LW behind the production of the blue, green, and red photoreceptor cones in his foveae. For Barton to have the visual acuity of birds of prey, he will certainly need a density of these cones comparable to birds of prey in his foveae. The production of photoreceptor cones are regulated by transcription factors (TFs), which are proteins that control how often a gene is transcribed into a protein. In the case of the human eye, the TFs for photoreceptor cones include OTX2, NR2E3, and CRX (54), whereas, in birds, the TFs will be slightly different (7). Thus the TFs for birds are in some way linked to the larger density of cones, in comparison to humans, in their foveae. Rather than wait for the TFs in human DNA to mutate and change over many generations to match those of birds, we could speed up the process by using genetic engineering approaches. The field of genetic editing and DNA site-specific modification has been reinvigorated recently following the development of the genetic engineering tool CRISPR/Cas9 (11, 13, 20, 27, 49, 53). CRISPRs, which stands for clustered regularly interspaced palindromic repeats, are DNA chains of base sequences separated by gaps in a genetic sequence, whereas Cas9 is a restriction endonuclease or enzyme that can cut a DNA sequence at defined locations or sites. The CRISPR/Cas9 method can be used to remove or disrupt specific genes, which is why it is popularly referred to as the genetic disruptor (26), or, after a cut is made in the DNA, a new gene can be inserted at the location of the cut. CRISPR/Cas has been used to study the development of fins in zebrafish (39), prevent the browning of mushrooms (58), store images and films in living bacteria (52), and remove the genetic code for muscular dystrophy from human embryos (30). The study on human embryos has implications for the modification of the human germline (18), as CRISPR/Cas is not entirely accurate and can produce “off-target cuts” in a genome (11). In theory, CRISPR/Cas could be used to replace human TFs with those from birds of prey in an effort to replicate the photoreceptor cell density of birds of prey and Hawkeye. This process is known as transgenesis, and in the past it has been used to modify goats to produce spider silk in their milk (6). There are a number of other human genes that would also have to be replaced with avian genes to fully replicate avian vision abilities, such as the genes associated with the development of two foveae and the corneal accommodation mechanism. This represents a difficult biological problem that could in the future be feasible using an advanced implementation of a tool such as CRISPR/Cas. Nonetheless, future geneticists will only be able to explore transgenesis involving avian and human DNA, subject to stringent evaluation and scientific scrutiny (18). Rather than rely on genetic engineering, we could replicate Hawkeye’s eyesight by adapting bionic lenses. Companies such as Ocumetics (43) and Alphabet (or Google) (5) are developing bionic lenses that could replace lenses that have cataracts and provide a patient with 20/20 vision. In the case of the Ocumetics bionic lens, the damaged biological lens in the eye is replaced via cataract surgery, and the bionic lens is connected to the ciliary muscles of the eye. This will provide the wearer with a larger range of vision capabilities that could in theory be significantly greater than 20/20 vision. Alphabet’s bionic lens (patent pending) works in a slightly different way from the Ocumetics lens. The Alphabet lens would be injected into the eye and be integrated with the biological lens, if it is still present. The optical power of the lens can be varied using an external controller, and the lens could also communicate with external devices. These lenses are not designed to enhance a person’s eyesight beyond the limit of human vision. In addition there have been major advancements related to electronic retinas (38) and the stimulation of ciliary muscle response (17). Diseases such as retinitis pigmentosa and age-related macular degeneration can have detrimental effects on the photoreceptor cells in the retina and, as a result, lead to blindness. In recent years, electronic retinal implants have been developed and implanted in patients in an effort to restore photoreceptor functionality (38). One example is the Argus II electronic epiretinal device, developed by the company Second Sight Medical Products, which has been shown to help a number of blind subjects identify letters and words (9). However, an electronic retina such as the Argus II is a long way from fully reversing the effects of blindness caused by retinitis pigmentosa or age-related macular degeneration. Nonetheless, when the technology does reach this level, it offers the potential to be adapted for multiple advanced electronic foveae in the human eye, like those possessed by Hawkeye. In addition, in a recent study, the ciliary muscles of 27 patients with early emmetropic presbyopia were subjected to electrostimulation in an attempt to improve their near-vision capabilities (17). Results from the study showed that electrostimulation of the ciliary muscles did improve the vision of the patients. In the future this could prove to be a valuable course of action when treating the condition. On the other hand, electrostimulation of the ciliary muscles could be used to facilitate greater flexibility and range of motion, thus facilitating larger deformation of the lens. In conjunction with the bionic lens mentioned earlier and the possibility of electronic foveae, these treatments certainly do open up the possibility of a technological route toward creating Hawkeye in real life. This certainly would be an easier path towards creating Hawkeye’s vision in the real world and avoid many of the ethical issues associated with genetic engineering. Pedagogical and Communication Model In the past, I have used popular culture icons to motivate interest among audiences in biological and physical sciences at schools, universities, and public events. From my experience, scientific and engineering concepts can be interwoven with superhero characters such as Hawkeye to create a connection with an audience and enable communication of difficult-to-grasp concepts. Such an approach can be referred to as the “middle-ground hypothesis” (62). The idea to write this article and the related chapter in my book Secrets of Superhero Science (13) stemmed from the positive engagement of audiences with the material. In the classroom environment, popular culture can make the learning process more engaging, invigorate the content linked to key learning objectives, and potentially address issues students have with key concepts (16). There are many ways that a character like Hawkeye can be used in conjunction with learning objectives. First, teachers can focus on differences between standard human eyes and Hawkeye’s eyes to explain the function of the human eye and highlight limitations. Comparison of human vision with that of birds of prey can also be used to emphasize differences in the sensory systems of different animals. Second, the possibility of replicating the eyesight of birds of prey in humans can prompt discussions on DNA, protein synthesis, advanced genetic engineering techniques, such as the CRISPR/Cas9 genetic engineering tool, and transgenetics. These topics can, in turn, motivate dialogue with regard to responsible innovation and proper ethical practice in genetics. Third, it is also possible to examine recent advancements in modern technologies, such as wearable bionic lens, that can give the wearer enhanced eyesight or electronic retinas. Fourth, Hawkeye can inspire student projects. Students could investigate the limitations of Hawkeye’s sight and present strategies for how he might defeat villains under specific circumstances. Hawkeye can form the basis for in-class assignments and final examination questions. Relevant questions that could be used in the classroom include the following: With reference to the The Avengers (2), discuss the limitations, if any, of Hawkeye’s eyesight as portrayed in the final battle scene in New York City. If one of Hawkeye’s eyes were damaged in a fight scene, how would the functionality of his two foveae change? With reference to other muscle fibers in the human body, are there any fibers in the body that could be used to replace the ciliary muscles, such that Hawkeye’s lens could be changed into shapes that are ordinarily inaccessible? If Hawkeye is able to see in the UV spectrum, would this have any effect on his ability to target objects in the visible spectrum? If we could use a genetic engineering tool such as CRISPR/Cas to modify human DNA to create an avian-human hybrid, consider the different aspects of the eye that would have to be changed to duplicate bird of prey vision. In this paper I have motivated the discussion of Hawkeye with reference to one principal scene from the film The Avengers (2). However, there are many film scenes where Hawkeye uses his advanced vision that could be used to motivate a scientific-based discussion. For example in Captain America: Civil War (4), Hawkeye frees Captain America from Spider-Man’s spider web by firing an arrow across an airport terminal. The arrow is seen passing straight through the spider web bound around the Captain’s hands. Based on the vision limitations of birds of prey, an interesting exploratory study would to be estimate how far away Hawkeye must have been when he fired the arrow to cut precisely through a small point in the spider silk. One way of estimating the distance would be to use the average speed of an arrow and count the seconds from the time of release to the time that it cuts through the arrow. However, would hawklike vision allow Hawkeye to resolve a small target only millimeters in size in the spider silk from that distance? For the general public, superheroes have a “wow” factor. Before the release of any superhero film, there is always increased media coverage where actors, the director, and film studios seek to promote their latest production. Marketing of these films uses TV advertisements, sharing of social media content, and promotional posters on billboards and at bus stops. While some people may have never seen a superhero film, such publicity campaigns ensure that most people are at least familiar with the superheroes. At the outset of most presentations or workshops, I ask the audience if they have seen a superhero film. Almost everyone will have seen at least one superhero film, while those that have not seen a superhero film are at least familiar with characters such as Batman, Superman, Iron Man, Spider-Man, and Wonder Woman, mainly due to the superhero film revolution. While the comic books can also be used to communicate difficult scientific concepts (24), I have opted to concentrate on the films in my outreach for the primary reason that most people will not have read the comic books. Hawkeye has become an integral part of my scientific outreach program when discussing the limitations and possible augmentation of human vision. Conclusion In this article, I have discussed the advanced eyesight of the superhero Clint Barton, otherwise known as Hawkeye. Using a scene from The Avengers (2), I have highlighted some of the key aspects behind his vision, all of which can be related in some way to the eyesight of birds of prey. In recent years, popular culture icons such as superheroes have become more prominent in society, due in part to the success and frequency of superhero films in modern cinema. In a superhero world where Thor is an almost invincible god, Tony Stark has developed and wears the advanced Iron Man exoskeleton suit, and Captain America is a genetically modified super soldier, Hawkeye is unique in that he does not rely on technology and has not been genetically engineered. His natural ability is unmatched in the Avengers, a fact that is often overlooked by his peers. Just like many superhero characters (3, 13, 61), and even Santa Claus (12), the character Hawkeye provides a unique platform for the communication of scientific concepts. In the classroom, Hawkeye can be used to highlight the physiological aspects of eyesight while also comparing the physiology of birds of prey. Hawkeye can also motivate discussion in relation to ligaments and tendons, bodily reflexes, and neural networks. He can also be used to discuss the ethical aspects of genetic engineering and allow students to reflect on the emergence and morality of emerging genetic engineering approaches. In addition, Hawkeye can be used to invoke interest in the latest advancements in modern wearable technology, such as bionic lens, and the implications of such technologies for future health care. Outside of an educational environment, Hawkeye can be used to bridge the gap between academia and a nonspecialist audience. E. Paul Zehr has noted that “Pop culture contains many compelling stories, and it makes sense to use these for our purposes in communicating science” (62). While Hawkeye may not have the ability to fly like Superman or the healing powers of Wolverine, his powers and abilities fascinate many people. He can be used as part of focused classroom teaching or in the dissemination of new and innovative research. Without a doubt, Hawkeye can truly be a superhero for educators and scientific communicators. DISCLOSURES No conflicts of interest, financial or otherwise, are declared by the author. AUTHOR CONTRIBUTIONS B.F. conceived and designed research, performed experiments, analyzed data, interpreted results of experiments, prepared figures, drafted manuscript, edited and revised manuscript, approved final version of manuscript. ACKNOWLEDGMENTS I thank Jean Philippe Frimat for stimulating discussions on the subject, sharing my enthusiasm for superheroes, and contributing an artistic image to the article. AUTHOR NOTES Address for reprint requests and other correspondence: B. W. Fitzgerald, Delft University of Technology, Process & Energy Department, Intensified Reaction & Separation Systems, Building 34K, Room 34K-0-100, Leeghwaterstraat 39, 2628 CB Delft, The Netherlands (e-mail: email@example.com; firstname.lastname@example.org).