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Abstract
Color vision is one of the characteristics that are recognized as having an effect on careers and human interaction. This research establishes the effects of high altitude on color vision. It also investigates the color systems that are affected more in relation to others.
The research methodology mainly involves the analysis of materials on research done in the same area together with the data from various researchers. The findings of this research will reveal whether there is a reduction in color perception in high altitude with regard to the three axes in an effort to conclude the one that is most affected. Are the effects reversible with acclimatization and resumption to normal altitudes? The research also answers this question.
Introduction
Color vision is one of the characteristics that are thought to differentiate human beings as a distinct species from some of the other species that are not able to perceive color. Man has always depended on this trait in several ways to survive. In the present day where careers have become complex, the importance of the trait has only increased. Some of the careers that demand an assessment of the trait in an individual include military profession, professions in the aviation industry, and transport industries among others.
It has been noted that there is a special group of people who are unable to perceive some colors, and hence the intense scrutiny of shortlisted candidates where color discrimination is crucial. The absence of the inability to differentiate between colors may mean life and death in some of the aforementioned careers. Accidents are also predisposed by the presence of inability to differentiate between colors.
Despite the innate inability to differentiate colors that are inborn in some individuals, some other factors have been found to affect people’s ability to perceive colors. This research focuses on the effects of high altitude on color vision. It examines some of the findings from other studies. There is an apparent relationship between high altitude, hypoxia, and color vision.
The many researchers who applied different methods to evaluate the relationship have provided this inference. Vision and light discrimination form a crucial part of people’s mission at high altitude. People in the aviation industry are tasked with the responsibility of making decisions, which are crucial to their missions in navigation. A slight variation may be a determinant of their operational success. Hence, studying this topic will be crucial in terms of knowing the link between color vision and high altitude.
Literature Review
There are a number of literary works detailing the effects of high altitude on eye physiology. These materials have investigated the relationship between colour vision and high altitude. Their findings, which are stated below, will help in determining the answer to the question under study.
Color Vision at Hypoxia
High altitudes have been associated with reduced oxygen concentrations. Individuals visiting them often experience reduced concentrations of oxygen within their bloodstream. This condition, known as hypoxia, has a number of effects on body systems. According to Karakucuk and his colleagues, one of the effects of hypoxia is on vision where normal functioning is impaired in a number of ways (1).
In many of the jobs that people perform on high altitudes, vision is a very important characteristic. Some of the personnel such as land troops, mountaineers, and flight operators need a working vision at any altitude to perform their tasks (1).
Karakucuk and his colleagues also state that although most of the commercial passenger flights are pressurized at high altitudes between 2100m and 3000m, some of the flights such as small planes such as helicopters are not pressurized (1) despite the fact that some of these flights are capable of reaching those altitudes.
Their pilots and their companion operators have an equal reliance on color vision in making critical navigational decisions in the course of their flights. If vision and any aspects of it such as color discrimination fail at these altitudes, navigational crews will have a difficult task in their missions. As such, accidents may occur. Some other researchers have also cited effects of hypoxic conditions on normal vision.
According to Vingrys and Garner, some of these effects include visual fields, acuity, flicker sensitivity, and color vision (2). These researchers however cited the absence of evidence showing moderate hypoxia as it might be experienced in high altitude on color discrimination.
However, they discredited the findings of researchers with these conclusions based on the methodological approach that they had taken in their respective studies (2). Evidence from some of the credible research findings and their own research was sufficient to make a conclusion that there was evidence of deranged color processing systems although this was in an already compromised color vision (2).
Vingrys and Garner also stated some of the methods used to measure color discrimination, with Farnsworth-Munsell 100-Hue (FM 100-Hue) being one of the tests that their research cited as yielding results of presence of color discrimination problems at high altitude (2). Different researchers who found this association between high altitude, hypoxia, and impairment in color vision cited the most common perceptions impaired as being blue-yellow and red-green.
The reasons for requirements of an intact color vision in army personnel and aircraft controllers are emphasized by the introduction of gadgets that employ color discrimination to operate efficiently such as the CRT displays common in aircraft control and monitoring systems (2). Hypoxia is recognized as causing impaired color discrimination by the researchers above.
High Altitude Exposures and Ocular Physiology
Ocular physiology is important to understand before evaluating the effects that high altitude and the resultant hypoxia have on normal color vision. There are a number of changes in the physical environment as one ascends in high altitude. One of these changes is the atmospheric pressure, which according to Buttler is about 760 mmHg at sea level, with the concentration of oxygen here being the standard 21% of the total volume of air (3).
The partial pressure of oxygen in the body is determined by the concentration of the gas in the inspired air. At sea level, this partial pressure is high and appropriate to support the basic body functions. Buttler states that the calculation of partial pressure of oxygen and other gases in the body can be done through the multiplication of the concentration of the gas in the air by the by the atmospheric pressure in the area (3).
The units of the atmospheric pressure in this calculation have to be in absolute atmospheres or in some cases ATA. The calculation of the partial pressure of oxygen at sea level, by taking into consideration the above values, provides a partial pressure of 0.21 ATA for the gas.
Ascending in altitude causes a reduction in the atmospheric pressure. When this phenomenon is applied in the equation, a resultant reduction in the partial pressure of oxygen is observed, despite the concentration of the gas remaining constant throughout the change in altitude.
Mountain climbers who venture onto Mount Everest have a reduced oxygen partial pressure in their system. Some of the effects of this case include loss of consciousness due to impaired metabolism in the brain. The effects are recognized due to the resultant reduction in atmospheric pressure, a situation referred to as hypobaric hypoxia. Research shows that the condition occurs with rapid exposures to high altitudes and sudden changes in altitude (3).
A gradual change in the altitude, on the other hand, has been associated with reduced effects on the partial pressures of oxygen in climbers, as acclimatization allows them to hyperventilate (3). An observed effect of hyperventilation is the reduced concentration of carbon dioxide in the bloodstream, a condition known as hypocapnia.
Major effects that Karakucuk observed in the eye and vision in relation to increase in altitude include changes in the diameter of the arteries supplying blood to the optic disc, the tortuosity of these vessels, and the concentration of the vessels on the optic disc (3). These changes are common to all individuals who ascend suddenly and those who took the time to acclimatize.
The increased vessel diameter, tortuosity, and optic disc hyperaemia resulted in decreased scotopic and photopic vision as well as the activity of retinal ganglion cells (3). This may explain the observed changes in the perception of colors at high altitudes.
Physiology of the Eye Color Discrimination at High Altitude
The eye is a complex sensory organ. Many researchers have discussed it as an extension of the brain in relation to its embryological development. With the observed effects of high altitude and its changes on the normal eye physiology, many theories have been fronted on the exact factor among these changes that cause impaired color perception at high altitude. Some researchers hold varying explanation for the physiological effect.
Some of them include changes in atmospheric pressure, the decrease in blood pressure, availability of carbon monoxide, and motor vibrations (4). In their part, they state that the administration of oxygen in individuals who experience impaired color perception and discrimination at high altitudes have resulted in the reversal of these changes (4). They therefore conclude that the main reason for color and visual impairment is due to hypoxia.
The perception of light and color in the eye is facilitated by the ganglion cells, which are of different sizes based on the function that each serves. Some studies have shown that the nature of vision and color perception is related to the functionality of these ganglions. Some of the electrophysiological studies done show that the smaller ganglion mediates color vision cells, which are slower in the conduction of impulses from the eye (2).
According to Vingrys and Garner, the same electrophysiological studies also demonstrate that anoxia mainly affects these small ganglion cells, sparing the rapidly conducting larger ganglion cells (2). The observation may provide an explanation to the physiological changes that take place in high altitudes resulting to impaired color discrimination. Hypoxia therefore causes changes on the neural channels in the vision pathway. This suggestion differs from that of other researchers who held that the effects were mainly in the photoreceptors (2).
The different visual systems are affected differently and at different altitudes in the visual pathway and photoreception. Vingrys and Garner state that photopic vision mediated by the cones is unaffected in slight increases in altitude. It occurs much later after scotopic vision that is mediated by the rods is interfered with (2).
The implication of this situation is that the scotopic vision is more susceptible to the effects of hypoxia compared to photopic vision, which is more resistant to the change (2).
An observable result of the findings is the requirements to have workers in the aviation industry, especially the cabin crew that has oxygen supplemented at night and during the day for flights above 10,000 feet. Some of the research done on the differences in altitude for the two visual systems concluded that there is no difference on the effects at mild to moderate levels of hypoxia (2).
The processing of vision has been studied in the neural channels that are followed, with researchers stating three channels (2). One of these channels is non-opponent while the rest two of these are color opponent.
Each of these models has its own class of cells mediating its functions. Achromatic mechanisms have been shown to have a major role in the visual detection of small dots. Reduced detection would be because of depression in achromatic mechanisms (2). The findings are therefore inconsistent with the effects of hypoxia on color vision.
The major aim of Vingrys and Garner’s study included the establishment of hypoxic effects on the photoreceptors (2). The researchers cited some of the previous studies that found an association between the impairment of color vision with hypoxia. The reception of the color green was apparent in these studies, with that of red perception being intact in the conditions that the studies were conducted (2).
An explanation made to the observed differences includes that the two color perception systems had a varying vascular supply, with this difference mainly being in the photoreceptor portions. In their own study, however, Vingrys and Garner believed that the vascular supply difference for the photoreceptors developed by the proponents above was defective since the differences could be explained by their observed losses in post-receptor achromatic channels (2).
The researchers believed that the reason for the differences observed was the variation in the absolute threshold that the different channels have for the different colors (2). This led to the conclusion that the effects of hypoxia are on the different channels and that the effects on the photoreceptors cannot be established.
The inner retinal layer, or the neural tissue as it is commonly referred, was cited as an important influence in the perception of vision. Optic neuritis is a condition that can be used to show the effects of the pathways in the color vision. A generalized color vision loss is found in these patients indicating that the neural tissue, and not the receptors, plays an important role in color perception (2).
The findings above are consistent with the electrophysiological findings stated earlier in terms of serving to strengthen the theory that photoreceptors are not the source of color vision impairment in high altitude hypoxia. Studies done on rabbits were also cited in support of the observations. According to Vingrys and Garner, these studies showed that ganglion cell layers, which constitute the visual pathways were the ones affected by hypoxic deficits and not the photoreceptors, which experienced slight effects much later (2).
In the study, the photoreceptors survived for a longer time in the presence of anoxic conditions, as opposed to the ganglion cells that survived for only a few minutes in the same conditions. Ganglion cells are associated with late receptor potentials while the photoreceptors are associated with the early potentials. In another study cited by Vingrys and Garner, the early potentials were reportedly affected only late after the onset of anoxia in monkeys while the late receptor potentials were affected early in the onset of hypoxia (2).
The findings presented by Vingrys and Garner are consistent with the findings of other researchers on the topic. The bipolar cells that are found in the photoreceptor zone are not responsible for color vision impairment in hypoxic conditions such as high altitudes since the ganglion cells are the ones that are affected (2).
Color Vision
It is important that we understand what color vision is and what it entails so that the objectives of the study are fulfilled. The different wavelengths of visible light are responsible for the perception of different colors perceived by the human eye.
Specialized cells in the photoreceptor zones called cones are responsible for the perception of these colors after the light wavelengths strike them. The impulses generated are transmitted to the visual cortex (5). According to Davies and his team of researchers, three different types of cones have been established as exhibiting peak sensitivity to lights of different wavelengths (5).
Color vision is genetically determined. Deficiencies in the genetic compositions result in the color vision impairment that is commonly seen in individuals. Another important thing about the genetic determination of color vision is association with sex that the characteristic is linked with. The X chromosome is thought to code for the long and medium cones in the eye.
Defects on the chromosome that lies in the region that is coding for the same leads to color blindness that is common in male patients (5). On the other hand, cones that serve the function of perceiving short wavelength lights are different from the ones above. They are genetically coded for in the chromosome number 7, with their distribution on the retina also being sparse (5). They are also reported to be fewer in number, accounting for only a significant proportion (10%) of the total population of cones on the retina.
Relation between Color Vision and High Altitude
Most of the studies cited above were done to evaluate the effects of hypoxia as the main aim. Few of studies have restricted themselves to the effects that high altitude has on perception of color. One of the studies that have been conducted includes that by Davies and colleagues who studied the effects that high altitude has on several visual axes (5).
In this study, 28 eyes were subjected to the changes in high altitudes, with the results of the same recorded against each participant (5). Davies and colleagues found no effects on color vision at an altitude of 4000m in the deutan axis (5).
At the altitude of 4000m, the changes in color vision that Davies and colleagues observed included deductions in the protan axis for 4% of the eyes. They also observed some associated reductions in the tritan axis for a large number (72%) of the eyes being (5).
Davies et al. continue to state, “Further on, at 5400m, all eyes had normal protan and deutan axis, while three quarters had minimally reduced, and one quarter moderately reduced, tritan axis” (5). Some previous studies had also demonstrated deterioration in one of the tritans that Davies’ et al. study investigated at an altitude of just 3000m. This was a cause of inconsistency in the two studies, which were both conducted in photopic conditions (5).
Color Vision and Disease
Several diseases affect the eye. The most effects that these diseases have are on vision, which is in most instances reduced. The severity of these diseases and their duration is important in the determination of the effects that they may cause on the eye. Some of the most common diseases affecting the eye include diabetes and glaucoma. These conditions are known to cause a corresponding deterioration in color vision early in their course, with the tritan axis being affected earlier than the others (5).
The reason that Davies et al. use to explain why blue is affected first is the relatively fewer number of the cones associated with the perception of this axis and the fragility that they have been associated with (5). The other reason provided is that the peak sensitivities of green and red are closer together compared to blue, which as a result has a larger receptive field as shown in the figure below (5).
Figure showing different wavelengths of the different colors (Source Davies et al., 2009)
Davies et al. found that even in the presence of disease, the ability to discriminate colors improves in patients who are provided with a means of increasing their partial pressure of oxygen (5). This implies that hypoxia that is associated with the disease conditions in the eye such as diabetes is a key factor in the observed deterioration in color vision.
Some of the other conditions that are associated with decreased perception of color vision include the inherited conditions that are not commonly associated with major deficiencies as color blindness. These conditions have also exhibited the characteristic of sparing the red and green color perceptions by first affecting blue (tritan) axis (6). The conditions include optic atrophy that is inherited as an autosomal dominant condition (6).
Materials and Methods
The research was carried out through first determining the best databases where research materials on the topic would be readily available. With the location of an appropriate database, the determination of key words was done so that the results would be used for accurate analysis. Some of the key words included high altitude, color discrimination, photoreceptors, ganglion cells, hypoxia, ocular physiology, color vision, protan, deutran, and tritan.
The results of the search were later refined, with those that are appropriate for the research being selected. The references of these literatures were later searched. Analysis followed to evaluate whether they would be applicable to the research. The initial search produced 36 results.
After refining the search, the number of literature remaining was 19. These materials were then subjected to a criterion to finally evaluate those that were applicable for review and research.
The inclusion criteria included that the literature had to be written in English, with most of the materials being peer review articles. The exclusion criteria also included the articles that were not written in English. These articles were excluded from consideration. This strategy saw the number of researches utilized drop to 15, as these were relevant for evaluation.
Results
Color Vision at Hypoxia
Some researchers have observed no relationship between high altitude and color vision loss. In one study where the researchers applied the desaturated D15 test to investigate this relationship, the hypoxia developing because of exposure to high altitude did not lead to any major deterioration in color perception (7).
The study also involved examination of some of the other factors that may lead to color vision loss. These factors included the time of exposure to hypoxia, mountain sickness, and the physical exertion resulting from an increase in altitude (7).
In the above factors investigated by Leid and champagne, there was no observed change in the desaturated D 15. Hence, the researchers concluded that increase in altitude has little, if any, effects on color vision (7). The methods used in any study are crucial in the results that are expected.
Researchers should use methods that have been applied in a similar research or an improvement of the same to ensure reproducibility of results. Poor methodology leads to poor results. In the case of the research above, the tool used in the research (desaturated D15 measurement) was not standard. The researchers also stated that it might not have been a reliable tool for them to use based on its weak sensitivity in one of the axis (7).
High Altitude Exposures and Ocular Physiology
Wilmwe and Berens cite the requirement of good eyesight in the aviation industry and for pilots to have led to the large number of studies on color vision and its relation to high altitudes (4). In their research, they found that hypoxia causes a reduction in the perception of color in the eye (3).
In the studies that they cited, the methods used included stilling’s plate that is used in high altitudes. At 20000 feet with reduced pressure, there were no color changes observed (4). This research is among the studies that showed no reduction in color vision with increased attitude and low pressures.
Some authors also cited researchers who had earlier suggested no change in color vision with increased altitude (3) despite their independent results and those of other researcher showing a clear reduction in color vision with a tremendous increase in altitude.
Buttler states that although some researchers suggested no reduction or impairment in color perception, the studies they conducted, as supported by other studies, showed a reduction in color perception with altitudes above 12,000 feet (3). The basic conclusion made therefore is that there is a marked reduction in perception of color vision, with large and sudden increases in height.
Some of the researchers also found a reduced color perception in the presence of hypoxia. Some of the effects could be observed in the visual fields as well as visual acuity. They cited the unavailability of data supporting the reductions in color perception in hypoxic conditions (2).
Many researchers faulted the research showing no relationship between high altitude and color discrimination for poor methodologies that had been used, with one of the researchers using Nagel anomaloscope, which was more advanced (2). This tool facilitated the production of results supporting reductions in color perception in hypoxic conditions, but only for eyes with an already compromised color perception system (2). Moderate hypoxia was then regarded as a source of impairment in color perception because of the study.
Some of the researches that strongly supported the reduction of color perception with increased altitude included Leid and champagne’s research (7). In this research, the authors cited that only one of the researches that they evaluated had data supporting reductions in color perception in hypoxic conditions.
The study had used an anomaloscope (7). The results of this initial study indicated a reduction in the sensitivity to the color green, while Leid and champagne’s research found an association with all axes basing their findings on the artificial conditions used (7).
The results that the researchers used vary due to the tests that each used. The tests have their own specificity and sensitivities, which are a major influence on the analysis that is produced. Some studies have also been conducted on these tests. It is therefore possible to categorize them into the most sensitive to the least sensitive. One of these tests is the Farnsworth-Munsell test.
According to Karakucuk and his colleagues, this test, “is the preferred color testing method used in many investigations at high altitude or laboratory environments” (1). Some researchers have opted to use other less sensitive tests, which provide data that is not reliable or strong in making the required conclusions. Some of the other researchers who also support the use of the Farnsworth-Munsell test include Vingrys and Garner who state that the test provides reliable results in the estimation of color loss in high altitude (2).
Color Vision decreases with High Altitude
The review of literature provided some articles that supported the decrease in color sensitivity with an increase in altitude. Among these researchers are Vingrys and Garner who found that there was a loss of color discrimination when the FM 100-Hue tool was used in combination with the anomaloscope at a height of 12,000 feet (2).
The study done on two individuals showed a decrease in color perception from the level that was recorded at sea level. In another of the studies that Vingrys and Garner looked at, the same tool was applied in the estimation of the effects of color vision on five individuals at an altitude of 18,000 feet. It showed a marked reduction in the color discrimination in these individuals (2). The reductions were in the blue-yellow spectrum. The loss of blue vision was consistent with the results of other studies done using the same methods.
Although Vingrys and Garner found a decrease in the perception of color at high altitude, they differed with some of the results from the literature that they had examined in the axis that is mainly lost in high altitude hypoxia (2). They reported a decrease in color perception in the blue-yellow and red-green combinations. They stated that the reason for the differences might have been the methods used to attain hypoxia (2). The results therefore supported the existence of diminished color perceptions with an increase in altitude.
The results of most research articles examined show that there is a relationship between color vision and high altitude and that a reduction in color discrimination occurs with an increase in altitude. It is however important to establish the colors that are mostly affected with the hypoxia produced by an increase in altitude, as different researchers have given different colors that are affected.
One of the researches carried out with the specific aim of determining the colors that are affected is the one carried by Tekavcic and Igor (8). This study utilized the Mollon-Reffin Minimalist test at an altitude of 5400m. The color vision axes tested included tritan, deutan, and protan (8). The tool was apparently chosen based on the relative ease in administration to the participants. It was also a quick way of carrying out the investigation (8).
Tritan axis (Blue): The most affected
Among the factors that were under study, hypoxia was reported to have a significant relationship with color vision. Tekavcic and Igor also reported that the correlation between color vision and hypoxia was mainly in the tritan axis. High altitude and associated hypoxia can therefore result in defects in the tritan axis whose main perception is the blue color.
In other studies done with the aim of establishing the color deficiencies affected by high altitude, Willmann and colleagues measured the color discrimination thresholds for the different axes in two male participants (6). In this study, participants were tasked to ascent to Mt Everest, with the colors measured at different heights during the expedition (6). The tests applied in this study were quantitative and psychophysical tests, whose analysis applied the use computers (6).
The results of the study were consistent with other studies that found a reduced perception of color in at high altitudes. The major finding, which fulfilled the research objectives, is that the color axis that is mostly affected is the tritan axis (6). Another major finding is that this effect on the tritan axis may be reversed once the individual has acclimatized or returned to lower altitude (6).
In the above research, the researchers found no evidence supporting reduction in color perception for deutan or protan axes. This inference was supported in the data that they got from the research (6).
In support of their findings, they state that there was, “no correlation between altitude increase and protan (r2 linear¼1.64 E_5) or deutan (r2 linear¼0.025) thresholds for observer A and B” (6). Other researchers have also supported the findings that there is a reduction in color perception with an increase in altitude, and that this impairment in color perception is mainly in the tritan (blue) axis.
In another of researches supporting the involvement of tritan axis in the observed reduction in perception of color with increased altitude, Tekavcic and Igor state that their research and that of others managed to implicate the tritan axis as the main cause of the color vision impairment (8). In the research, they demonstrated that reduction in color perception in the tritan axis is further worsened by an increase in altitude, with higher altitudes having markedly higher effects (8).
The color impairment in the tritan axis was also observed in other studies looking at the effects of high altitude on the perception of color. Some of these had the same conclusion that the axis that is mostly affected is the tritan axis. They also supported the stated increase in the impairment with an unprecedented increase in altitude (1).
In another study, the effects of lowering the partial pressures of oxygen through decreasing the atmospheric pressure were evaluated in eight subjects (1). The researchers simulated the lowering of atmospheric pressure observed while climbing a mountain. They proceeded to look at the effects that this had on color vision (1). The results indicated that the blue and red ranges were the main ones affected, thus supporting the other work by previous researchers (1).
The studies therefore indicate that an increase in altitude has the effect of lowering the partial pressure of oxygen in the bloodstream, with the result being hypoxia. The resulting hypoxia is responsible for the impairment in color perception.
The studies also supported the theory that the reduction in the tritan axis perception of color was the main impairment in color perception with an increase in altitude. Some of the studies that found different results can be explained by the relatively weak methods that they utilized to get their results, as it can exemplified by Richelet and colleagues in their study (9).
Discussion
In this section, a discussion of the results will be made, including the reasons behind high altitude affecting particular colors. An explanation will also be provided for the observation that tritan is the main color affected, and whether the effects are dependent on the age of an individual.
There has been a belief that acclimatization has an effect on the outcome of impaired color perception with increased altitude. This will be analyzed to ascertain its relevance. Some of questions to be discussed from the results include whether the time of day influences color vision in high altitudes. The differences that different tests produce in this research will also be discussed, along with the difficulties in carrying out the study.
Physiology behind Tritan Axis being the mostly affected
Studies carried out in photopic conditions have demonstrated dysfunctions in the tritan axis when hypoxia is induced by an increase in altitude. Other studies conducted in mesopic (dim light) conditions showed different results (5).
In the study conducted by Davies et al., mesopic conditions were provided, with the results showing that the color vision of the participants was not affected by their increase in altitude under these conditions (5). Studies done by some of the other researchers in photopic vision also yielded different results. Karakucuk and his colleagues indicate that their study showed deterioration in the blue-yellow color vision range in photopic conditions (1).
The above study demonstrated only the effects of high altitude in photopic conditions. There were suggestions that more changes could be observed if the scotopic and mesopic conditions were provided. Some researchers who have conducted research on the topic have suggested that hypoxia may cause impaired color perception because of depression in ganglion cell activity, which affects both scotopic and photopic vision (2). This means that a reduction in color perception is not only in day light, but also in dark and dim light.
Several explanations have been advanced for why the tritan axis is affected more than the other axes in response to increase in altitude. One of the explanations, which have gathered the support from most researchers, is that the cells that are responsible for the perception of the triton axis are more vulnerable to the effects of hypoxia.
Tekavcic and Igor state that S cones that are relatively smaller compared to the L (large) and M (medium) are responsible for color perception in the tritan axis. These cones are affected adversely by small reductions in partial pressure of oxygen (8). Some of the other researchers that support the observation include Willmann and colleagues. They state that the S cone pathway is more vulnerable to hypoxia compared to the L and M pathways that can withstand severe hypoxia for a longer period (6).
An explanation as to why color vision is affected however is that the cones responsible for this color vision are fewer in number and concentration in the retina. Common conditions affecting the eye affect the cones more often and severely because of this reduction in abundance compared to the other receptors (6).
The L and M cones are also more compared to the S cones. Any reduction in partial pressure of oxygen causes earlier reductions in the S cone’s absolute responses (6). This may be the reason behind the tritan axis being affected earlier compared to the other axes that are mediated by the larger L and M cones.
Another observation that relates to the effects of high altitude on the tritan axis is that supplementation of oxygen and reversal of the impairment.
Higher altitudes were found to have reduced perception in the tritan axis irrespective of supplementation (6). Because of these observations, a high altitude above the one measured in the experiments and studies above will produce marked reductions in the perception of the color blue. Other axes would follow. Supplementation at these high altitudes will have little if any benefits to the perception of color
The results can be summed up in Willmann and colleagues’ conclusion, “high- and extreme-altitude hypoxia adversely affects color vision predominantly along the tritan axis” (high- and extreme-altitude hypoxia adversely affects color vision predominantly along the tritan axis.).
The implications of the research findings are important for patients and aviators who stand to have the most effect from any deterioration in perception of colors. Some of the devices that should have the above taken into consideration in their design and painting include compasses, GPS receivers, and altimeters (6).
Older Individuals have a Higher Score in Tritan Axis
The other effect that was evaluated in most of the studies is the relationship between the age of participants and the impairment in vision produced by high altitudes. In one of the studies, which constituted of mainly young participants, the results of previous studies with older subjects were compared (1).
The young population for the study was meant as a way of eliminating the effects of age on the photoreceptors. This was thought to provide a better way of evaluating the relationship between altitude and perception of colors.
In the study, the results indicated that the population of young participants was affected by altitude, with a reduction in the perception of the same colors as gotten in other studies (1). This study showed that there were no differences in the photoreceptors reaction to high altitude. All individuals are likely to experience the same effects irrespective of their age.
Studies done with a relatively higher mean age of participants produced the same results as the ones done with a younger population. The only difference is that the scores on the tritan axis were higher in older individuals who participated in the studies (6), with this observation being held by several other researchers such as the one by Smith, Earnerst, and Pokorny, and Mollon, Astell, and Reffin (10, 11).
The studies however were all consistent with reductions in color perception in high altitude irrespective of the ages of the participants. This means that individuals of any age will be affected the same way with an increase in altitude. Tritan axis is more likely to be affected compared to the other axes. However, the degrees in the visual reductions for the tritan axis are different, with older individuals having higher scores at any altitudes (6).
Acclimatization improves Color Perception
Acclimatization is reported to have a significant influence on the studies performed on the subject. In one of the studies that used students, the rate of ascending a mountain was slower relative to most individuals. This provided them with a chance to acclimatize (1). The researchers however stated that for people that ascend to high altitudes at a faster rate, there is little chance of acclimatization. Hence, the findings of Rowe’s research may not be valuable for this population (12).
The conclusion was that acclimatization has little help in the prevention of the impairments in color for people ascending mountains. In another study by Dean, Arden, and Dornhorst, however, data collected on the threshold levels for the tritan axis showed improvements with increased stay in high altitude as the participants acclimatized (13).
The researchers suggested that with acclimatization, the physiological adaptation of the eye and the body in general allows the visual pathways to recover, thus reducing the effect of impaired color perception in high altitudes (6).
Davies et al. also observed that with time during their experiment, the subjects had improvements in the color perception. This can be attributed to the acclimatization that they had experienced as Wong, Khan, Adewoyin, Sivaprasad, Arden, and Chong confirmed (5, 14). When using the very sensitive desaturation D15 test, Leid and champagne found that well-trained individual and those who had time to acclimatize experienced little effects of high altitude in their color vision (7).
The disturbances were also reversible with a return to the normal altitudes. This means that if individuals have time to acclimatize, the effects discussed in this study will not be experienced. This does not however apply to individuals such as the air force and commercial airline pilots who ascend to high altitudes suddenly inside a non-pressurized aircraft such as a helicopter (13).
Color Impairment at High Altitude Reversible
In the question of whether the effects of high altitude on color perception are reversible, Tekavcic and Igor managed to show that the effects are reversed only three days after the study for the participants. Normal color vision was restored in a year (8). Some of the other researchers who observed an improvement in color perception when the participants returned to their normal altitudes include Willmann and colleagues (14).
This resulted in the conclusion that the effects of high altitude to color perception are transient in nature (6), thus indicating that the effects of high altitude on the perception of color are reversible and are only present when the individual is not well acclimatized to the high altitude.
According to Gibson and Mckenna, there were some apparent differences in carrying out the study above, including the low availability of literature detailing the effects of altitude on color vision (15). Few researchers had carried out this research. This claim was also crucial because it provided the needed knowledge gaps. The methodologies used in the reviewed literature were also different, hence providing the varying results that were obtained for different studies.
Another of the problems encountered included the estimation of the best ways to synthesize the data from the results of the study. In the studies, some of the problems that the researchers faced included mountain sickness and physical fatigue. It also took a longer time as compared to other types of researches.
Conclusions
The analysis of literature was meant to determine the effects of high altitude on perception of color in human beings. Most of the literature evaluated indicated that there was a relationship between the reduction of color vision perception and increase in altitude. Altitudes above 12,000 feet are the mainly affected by the observed increase in the impairment if color perception. There are also differences in the degree to which the different axes in color vision are affected by an increase in altitude.
The most severely affected is the tritan axis. Acclimatization and return to lower altitudes is observed to reduce the effects of altitude on color vision. However, these effects are reversible. There is therefore need to put in place measures to ensure that personnel involved in intricate activities requiring perfect color vision at high altitudes are protected from these effects. Some of the measures already in place include pressurized aircrafts, supplemental oxygen, and regular color vision check-ups for these personnel.
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