Perception is the gateway to cognition; it is the difference between awareness and ignorance. Although we are often not consciously aware of how our senses operate as they perceive the world, their physiology nevertheless dictates our understanding of reality. The realm of speculative fiction is filled with characters that possess sensory abilities different from those of an average human, differences which can lead to dramatically altered viewpoints. Whether they are keen-eyed elves, super-powered humans, or visitors from another planet, it is interesting to consider how the way these alien entities sense the world might affect their outlook and behavior.

But what does it mean to say something is or is not a “sense”? For any type of stimulus, such as physical texture or rays of light, if information about that stimulus can be detected, then it is being sensed. A specific class of stimuli is referred to as a modality. The process of detecting and converting a stimulus into information about the environment is referred to as transduction, and in animals, it takes the form of sensory neurons converting that information into neural signals. By transducing sensory information into these signals, all of our different perceptual modalities can be brought into a common computational format. The combined information from all of our senses constitutes the components that build our perceived realities.

The animal kingdom is a great place from which to draw inspiration when imagining the behavioral effects of differing perceptual abilities. Within the zoological diversity of our planet we find fascinating sensory variations in forms that are shared with humans, as well as completely novel forms which humans can only glimpse through the aid of specialized technology.

Before delving into alien forms of perception, it is good to briefly review a human baseline for comparison. Classical thought famously names five senses: sight, hearing, smell, taste, and touch. In actuality this list fails to acknowledge a number of additional senses, including the vestibular system (sometimes referred to as the “inner ear”), which perceives motion of the head and enables balance, and proprioception, which is the sense of posture and body position. Likewise, categorizing sensory modalities is not always cut and dry; pain and temperature sense are sometimes classified as their own separate categories, but other times they are lumped in with touch and proprioception to form one larger category referred to as somatosensation, or “body sense.”

Even within each sensory category, there is typically a limited range of stimuli that can be perceived. For example, in vision, we see only a sub-band of electromagnetic radiation, which we have dubbed the visible spectrum. Likewise, our ears detect vibrations in the air, but only for those frequencies which create resonance within the cochlea. Smell and taste are marvels of chemoreception, but the gamut of possible molecules massively dwarfs the set of those that interact with our receptors. Even somatosensation has its limits, with a wide range in detectable resolution across different body surfaces (for example, fingers can detect much finer details than the skin on the back). It all ultimately boils down to a simple truth: if a stimulus occurs but there is no receptor to detect it, that stimulus is not perceived. For every sensory system we have, its limits and strengths are a product of evolution’s twin facets of selective pressure and stochastic whimsy.

For example, on Earth, visible light is a highly useful subset of the electromagnetic spectrum. However, it is not the only set of radiation that is regularly present in the environment. In addition to visible light, the sun gives off appreciable quantities of both ultraviolet (UV) and infrared radiation (IR). Humans detect light through a set of photoreceptors in our retina; each type of photoreceptor is tuned to a range of wavelengths that interact with the receptor to create a neural signal. Although some human photoreceptors can have limited interaction with the longer wavelengths within UV, we have no dedicated receptors that optimally respond to these wavelengths. Additionally, many materials readily filter out UV, including those that allow visible light to pass through (such as glass and the lens and cornea of our eyes). Thus, most UV never reaches our photoreceptors, restricting even marginal detection. A number of non-human animals have evolved eyes that can detect UV, including some types of birds and bees. However, because UV is invisible to us, it is hard to appreciate its role in the lives of these creatures without dedicated study of each specific species’ behavior and environment. Nevertheless, we have found some important impacts of UV perception. For example, a number of flowers have striking features visible only under UV that makes it easier for bees to forage and pollinate. Likewise, some species of bird have evolved plumage with UV reflective pigments, aiding in gender identification and mating displays. Unusual for mammals, reindeer, though lacking a specific photoreceptor dedicated to UV detection, appear to have evolved an eye structure that does not filter out UV. They are therefore able to see some portions of UV, aiding them both in foraging for food as well as in spotting predators like polar bears and arctic wolves who, even though they are well-camouflaged in the visible spectrum, stand out against a snowy background in the UV spectrum.

In a world occupied by warm-blooded creatures, IR is potentially even more useful than both UV and visible light, as it is frequently present in the environment independent of sunlight. However, seeing in IR is also more difficult for a warm-blooded creature, because any IR receptors would have to be shielded from the emissions produced by one’s own body. Nevertheless, some cold-blooded animals, particularly a subset of snakes, have evolved the ability to detect IR. Interestingly, snakes do not see IR with their eyes, but rather sense it through a separate set of sensory organs known as pit organs, which are usually located further forward along the snout. The pit organ is useful not only for targeting warm-blooded prey, but also for the snakes’ own thermoregulation needs.

When it comes to aliens, it is important to consider what conditions might prevail in their native environments. Would they be exposed to a different set of wavelengths of ambient radiation? Perhaps they have evolved under different conditions pertinent to another sensory modality? If they do see a different range of wavelengths, for example, consider how many different aspects of life this might affect. Aesthetics may be completely different; perhaps goblins and orcs prefer ruddy, earthy tones because their vision, tuned for underground living, is skewed more toward red and IR wavelengths, and they simply do not see blues and purples. Alternatively, an entity whose vision centers on UV might find human dwellings oppressively dark and smothering as the glass of our windows filters out incoming UV and our indoor lighting fails to produce appreciable amounts. Beyond aesthetics and comfort, a different range of detectable stimuli between two factions will also have a profound effect on conflicts between them. Camouflage and stealth suddenly become much less intuitive when one species lacks an understanding of what the other sees.

Even when a creature can see wavelengths invisible to humans, or hear frequencies that we do not, it is still relatively straightforward to imagine what it might be like to sense that information. We know what it is like to see and hear, so we can readily extrapolate to what it might be like to perceive outside our normal range. There are some sensory modalities, however, for which humans have no natural equivalent; these are sensory systems that frequently require advancements in other domains of knowledge before we even know to go looking for them.

Arguably the most famous example of such a system is echolocation, which is an active form of sensation. With echolocation, animals project a wave of sound and then listen for delays and distortions as it strikes objects and reflects. As early as the eighteenth century it was known that bats navigate by hearing rather than sight. However, it wasn’t until 1939, after the development of our own sonar systems, that the mechanism by which bats accomplished this feat was understood. The discovery that bats could perform such a remarkably precise perceptual task, which only a few decades prior had been cutting edge military technology, was met with shocked disbelief by many in the scientific community. Since then, we have discovered a number of additional creatures that utilize echolocation for navigation and hunting, including dolphins and swiftlet birds.

Academic furor aside, animal echolocation is still somewhat intuitive for humans. We have both an auditory sense as well as the ability to vocalize, so it is simply the combination of these two capacities which is novel (in fact, there have been reported cases of humans teaching themselves rudimentary forms of echolocation). There are some senses, however, for which we lack any comparable ability.

Magnetoreception and electroreception refer to the ability to perceive magnetic and electric fields, respectively. Magnetoreception is most commonly used for orientation and navigation via the Earth’s magnetic field, whereas electroreception is most common in aquatic environments in which vision is unreliable. For both of these senses, even after the initial discoveries of electricity and magnetism, it took people centuries of observation and interaction with many animals possessing these senses before they were discovered.

Identifying an entirely new form of sensory perception is an incredibly difficult task. To even get started, it requires a confluence of knowledge about both the existence of the phenomenon producing the stimulus and observance of a behavior that is indicative of information derived from that stimulus. Encountering a wholly alien species, even when non-hostile, is almost guaranteed to be fraught with a disconcerting lack of knowledge about what environmental information the aliens are privy to. A great deal of narrative tension can be built around the sudden discovery of unexpected knowledge on the part of an alien visitor.

Of course, this trope can easily be inverted as well. Aliens who lack the sensory abilities of which we take for granted might miss our own capabilities and be taken off-guard. Take for instance, an alien species that senses its environment predominantly through vision and electrical fields but never evolved audition. They might send in stealth commandos with camouflaged suits sporting thick electrical insulation, only to be completely flummoxed as to how humans keep catching them as they tromp with heavy footfalls through our world. However, in the same way that we will frequently develop specialized tools to supplement our own senses and tap into the secret sensory lives of the other creatures which inhabit our world, an intelligent race of aliens is likely to also actively work to develop their own equipment to circumvent any sensory shortcomings they may have.

One form of interaction which deserves special note is that of communication. While not exclusively a matter of perception, our sensory abilities form a vital component in the communicative exchange. If you cannot sense the information being presented, you cannot successfully interact. However, there is no single perceptual modality that is by default suited to communication. Most people typically think of communication as being an auditory process, but humans frequently utilize visual media to communicate in the form of both writing and sign language. Other forms of less semantically specific communication include pheromones and body language.

For communicating certain concepts such as spatial structure, other animals may actually possess a more natural signal-sensory pair than we do. For both echolocation and active electroreception—meaning the organism projects an electric field and perceives based off the interaction of the field with the environment—an entity possesses the ability to generate stimuli that can be directly received by another animal. Like the old adage of a picture being worth a thousand words, this would be analogous to simply transmitting a picture of an object instead of having to carefully describe its characteristic traits. We have developed tools and skills to do this for each other through art, photography, and computer graphics, but these methods require the appropriate equipment and are frequently much slower than such an inborn system might prove to be.

Although we usually take them for granted, senses define the avenues along which we can interact with and experience our environment. We learn from birth to interpret and rely on the information gleaned from the senses available to us. Because of their fundamental nature to worldly interactions, the specific features of the available modalities will impact all aspects of life, from art to engineering, diplomacy to war. Consideration of these wide-ranging effects can lead to rich and engaging narratives.

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