Vision, hearing, taste, smell, and touch: these are the five major senses humans are accustomed to. Our understanding of the world has been shaped by the information we are accessing with these senses. But while these are the only senses humans perceive consciously, they are not the only senses that we have. For example, the semicircular canals of the inner ear contribute to our sense of balance. Similarly, we know when our legs are stretched out or flexed because receptors inform about stretch and load on our muscle fibers and tendons. (See “Proprioception: The Sense Within.”) We also receive sensory feedback on the filling of our bladders and stomachs. Such internal senses are essential for daily life, and we are rarely aware of them as we are of visual or auditory stimuli.
Outside of humans, species across the animal kingdom harbor different—sometimes more powerful—sensory capabilities. Some animals can see infrared or ultraviolet, for example, and many species hear pitches well out of the range of human hearing. Some snakes are sensitive to heat, “seeing” the temperature of their environment. Many fish and salamanders can sense small electric discharges generated when muscle fibers contract, and some insects, birds, and mammals appear to use the Earth’s magnetic field to orient and navigate. (See “Sensory Biology Around the Animal Kingdom.”)
As we continue to learn about the diverse sensory capabilities that exist in nature and to develop technologies that enable detection of a broad range of sensory input, the logical next step is to put new senses into old (human) brains. Devices that replace lost senses already exist. Cochlear or vestibular implants convert auditory and balance input into nervous impulses sent to the user’s brain, and analogous optical devices that can give sight back to the blind are close to coming online. (See “The Bionic Eye,” The Scientist, October 2014.)
Devices that provide humans with senses outside of the traditional five are on the horizon. The brain has proven extraordinarily plastic. It can, for example, interpret sound stimuli even if the signal reaches areas of the brain dedicated for image processing. Initial experimental work was done in animals, rewiring the brain’s pathways for processing sound, vision, etc. However, it now appears that such sensory cross-talk can happen naturally. For example, some blind people describe that they “see” around them, using sound (produced by their stick or tongue) in a manner similar to that of an echolocating animal. Thus, we have reason to believe that we can integrate detection technology with human biology in a way that allows us to at least subconsciously perceive and process stimuli outside of humans’ natural capabilities.
As we continue to learn about the diverse sensory capabilities that exist in nature and to develop technologies that enable detection of a broad range of sensory input, the logical next step is to put new senses into old (human) brains.
Imagine this: infrared information is directly fed into your visual cortex. You can now “see” warm-blooded animals during the night, much like a snake hunting a mouse. You might even be able to tell who around you has a fever by simply glimpsing their infrared temperature. Imagine seeing ultraviolet light as added color, or using polarized light to help you orient yourself in an unknown area, the way an ant does. Imagine having a dog’s sense of smell, or the sense of ultrasound hearing that would enable you to listen to bats. Imagine being equipped with sensors to detect magnetic or electric fields.
Such technology is not far off. The bionic eyes being developed could easily have expanded wavelength ranges, covering infrared and ultraviolet. Cochlear implants could be tuned to expand the range to ultra- and infrasound to hear bat and elephant communications. Emerging “smart skin” technologies offer touch and temperature senses—to furnish sensation to prosthetic limbs, for example—and we could soon add magnetoreception to our array of sensory modalities. (See “Smart Skin Enables Magnetoreception.”)
As these technologies continue to advance, researchers will expand our world beyond the limits of the traditional five senses. In the more distant future, we may even perceive radio signals that permanently connect us with larger networks, thus allowing us to tap into the multitude of sensors beyond those in and on our bodies. Even sensors too large to be incorporated into the human body—think of the vast laser arrays needed to detect disturbances in gravitational waves—could be fed into our consciousness. Then we’d truly have transgressed the boundaries imposed on our worldview by the limited sensory capacity of our species.
Bernd Fritzsch is a member of the German National Academy of Sciences and director of the Aging Mind and Brain Initiative at the University of Iowa in Iowa City. His research aims to retain nerve function for use with cochlear and vestibular implants.