Some of these "senses" are largely cerebral (sense of agency), others are more instinctive (reflexive senses) or intuitive (sense of effort), while yet others are evolutionary survival mechanisms (sensing another animal's heartbeat). Others are extraordinarily combinative (sense of balance). Many of these senses and sensory response faculties overlap (e.g., magnetoreception and polarized light).

❯❯ Agency

The sense of knowing/understanding one's ability to act on one's own accord. Pop psychology on mindfulness will frame this as "self-advocacy" or of "taking control of one's life," but such views don't provide a fully nuanced perspective. Instead, consider how studied social psychologists may frame a sense of agency as a sense of ownership-accountability over the mind and the body, but monitored and influenced by the variables, constraints, and controversies that manifest in one's environment, real or perceived. In this context, agency represents voluntary control over one's thoughts and actions to reach a desired, experiential state. An important caveat, however, rests behind the descriptors "perceived" and "desired"; agency can also be dangerously misleading, as "priming thoughts" about forthcoming events often "foster [an] illusory sense of agency" over said actions or events, notes an article in Frontiers in Psychology. A sense of agency is affirmational, yes, but it can also make one delusional.

In storytelling, agency regards characters and the level of control they exert over their engagement with the narrative reality. On the micro level, agency can also refer to a character's control, attempted control, or accountability concerning specific experiences. What environmental pressures would force a character to act in her self-interest? What dangers may be present, and how intense might they be, to force a character to work against her self-interest? Human history is rife with political machinations that have resulted in individuals who feared greater punishment for doing what was right than for their acquiescing to that which was wicked.

The differing theories of agency and cognitive causation are intensely layered. But for writers determined to validate this on the page, it may help to recall German philosopher Thomas Metzinger's self-model theory of subjectivity. Metzinger's theory holds that for one's self-representation to be fully experienced, it must be transparent, and a conscious self-representation can only be fully transparent if its internal properties are accessible.

❯❯ Auditory Hallucinations

Gothic literature is bursting with auditory hallucinations, whether from the sound or voice of an "other" that has gained a sense of autonomy, from a valorized voice meant to warn against danger, from a word of the defiant who ardently resists entrapment, or from a disturbed and narcissistic, disembodied entity.

An auditory hallucination can manifest as either incoherent sounds, echoic memories of traumatic experiences, or distinct voices. In humans, such hallucinations might arise or occur in various disorders (paracusis), as a result of post-traumatic stress, or in a patient with psychosis. Auditory hallucinations occur in the general population ranging from 5% to 28%, according research appearing in the journal World Psychiatry. Altered or damaged brain connectivity (cognitive processing) is the subject of much research. But in some cases, scientists suggest the origin stems from spontaneous activation of an individual's auditory network; that is, the spontaneous firing of sensory neurons in the absence of appropriately functioning inhibitory mechanisms (i.e., the limbic system). From the perspective of the individual, the source of the hallucination varies, as does the quality and intensity of said hallucination.

❯❯ Chemoreceptors

Sensory cells or organs that interact with chemicals in the blood; or more specifically, chemical controls for stimulating or inhibiting respiration. The amount of respiration depends on this neuronal network's response effectiveness. Peripheral chemoreceptors detect large changes in arterial blood oxygen, notably as it relates to the respiratory rate (allowing oxygen into the blood), blood flow (sensitivity to hypoxia), and cardiac output (supplying oxygen to the body). Central chemoreceptors detect changes in arterial carbon dioxide, notably concerning brain blood flow and metabolism, lung ventilation, and pH control (for optimal protein structure and function). A simple example would be to imagine a fantasy novel in which a character or adventurer is impervious (or not) to a gaseous poison that would inhibit proper breathing.

From a more practical standpoint, abnormally enhanced peripheral chemosensory inputs result in an overactivation of the sympathetic nervous system. According to the journal Biological Research, onsetting pathologies can include "hypertension, heart failure, obstructive sleep apnea, chronic obstructive pulmonary disease (e.g., systemic inflammation, muscle dysfunction, and/or cachexia, which is when the body literally wastes away), and metabolic syndrome (e.g., sympathetic hyperactivity, impaired blood-pressure sensitivity)."

❯❯ Effort

Prior to undertaking an activity, physical or mental, most humans make an assessment of the energy required to successfully complete said effort. The "sense of effort" deemed necessary to effectively conduct a task can occur consciously or subconsciously, depending on one's familiarity with the event at hand. Cognitive fatigue, muscular fatigue, cost-benefit analyses, and more, all affect the perception of effort.

In physical/behavioral terms, some scientists call the sense of effort a "judgment of force," because of how an individual must accurately estimate the ratio of vigor to fatigue, assign various motor commands (intuitively or peripherally), and dynamically assess how sustained the effort must be (duration of force application). However, for individuals with disabilities, a sense of effort is tangibly skewed; practice doesn't always make perfect; local, social, and environmental factors influence one's quality of concentration, feelings of strain, and stimulus sensitivity (or insensitivity).

On a related note, in psychological terms, self-control is viewed as an aversive mechanism. That is to say, to consciously recognize the costs of exerting effort or to establish a credible perspective on what is or is not a rational level exertion given the scenario at hand. Too much effort? Not enough effort? The right amount of effort, but for the wrong reasons? Self-control will surely have something to say about that. Psychologists frequently debate the extent to which humans are evolutionarily hostile toward effort-contingent rewards (or, conversely, actively assign positive values to effort).

❯❯ Electroreception

At its broadest, electroreception concerns sensitivity to electrical fields. Applied narrowly, electroreception explains a predator's capacity to locate and monitor its prey based on the electrical signals produced by said prey's heartbeat or nerves. For a predator, this means locating one's prey no matter where it hides, as well as at extended distances, depending on the medium through which the electrical field passes (e.g., water, air). Sensors are often delicate (e.g., beneath the skin of a shark's head rest hundreds of highly conductive, ampullary electroreceptors). Fascinatingly, some weakly electric fish have evolved their signal frequencies away from the sensory range of their predators in an effort to increase environmental fitness. Other animals use electroreception for intraspecies communication, identifying mates, or sensing and evading unwanted visitors.

❯❯ Equilibrioception

The sense of balance. A generally unobtrusive physiological sense in humans and animals to prevent them from falling over as they move or stand. It entails a visual system, a vestibular system (spatial awareness via the inner ear apparatus), and proprioception ("kinesthesia," the sense that lets one perceive the location, movement, and action of various parts of the body), all working together to orient the individual to the surrounding environment (and gravity) to achieve balance.

Balance is a quintessentially multi-modal sense. To summarize an array of medical literature on the matter, balance occurs when (1) sensory input (vestibular, visual, proprioceptive) is processed by (2) the cerebellum (coordination and regulation), the cerebral cortex (higher-level thinking), and the brainstem (sorting of sensory information), and is then paired with (3) motor output reflexes, motor impulses, and postural adjustments.

Cognitive or physiological damage, spatial disorientation, illness, or malfunctioning sensory inputs all affect one's sense of balance and one's dependence on it. As cheekily noted in an editorial published in Behavioral Sciences, "It is said that (perfect) balance is the action of not moving."

❯❯ Exoskeletons

Not traditionally thought of as a unique sense, the exoskeleton, while protecting the body, also enhances one's capacity to interpret the surrounding environment. Some animal exoskeletons specialize in providing certain types of sensory enhancements (e.g., stress or pressure sensitivity), some exoskeletons possess environment-particular chemical compositions (e.g., to maintain osmotic balance or inhibit infection).

For writers who are interested in this clever brand of defense and offense curiously bundled into the same package, one recommends researching the differences between exoskeletons, ossified scale exteriors, and for extra credit: whatever the hell turtles are made of (hint: a combination of bony plates, fused scapula, and fused rib bones, blended over countless years of evolution).

❯❯ Homeostasis

To wit, it's "any process that living things use to actively maintain fairly stable conditions necessary for survival," per Scientific American. Achieving homeostasis relies on a convergence of multiple senses.

More critically, achieving homeostasis also means maintaining stability despite an array of conflicting stressors or environmental characteristics (which themselves influence hormone secretion and sensitivity). Hunger? Thirst? Sweat? Blood pressure? In terms of what the body requires, homeostasis concerns regulatory mechanisms or processes that enable one to dynamically maintain steady-state conditions. Anticipatory feedforward mechanisms initiate advantageous and predictive responses to keep the body healthy (or, healthy enough).

Never underestimate the value of negative feedback mechanisms (i.e., change or error signaling; disturbances of the "normal range" of critical feedback), and never overlook the danger of having a time lag in repairing otherwise natural or effective systems once they're damaged (i.e., disturbance or departure from equilibrium).

❯❯ Itch

As the journal Clinical & Experimental Allergy explains, "itch, or pruritus, can be defined as an unpleasant sensation that evokes the desire to scratch. [C]hronic itch originates from [..] [a] serious, unmet clinical need. Broadly, subtypes of chronic itch have been delineated and termed pruriceptive, neuropathic, neurogenic, and psychogenic itch."

Pruriceptive itch follows activation of primary nerve terminals, is inflammatory in nature, and notably follows, not precedes, skin damage. A neuropathic-type itch stems from nerve injury or nerve trauma. The neurogenic type is an itch resulting from central nervous system activation without necessarily activating the sensory nerve fibers (e.g., internal injury results in external, physiological reaction). A psychogenic-type itch comes from underlying mental illness (as with delirium). So, an itch can be caused by something seemingly minor (skin irritation), injurious (nerve trauma), deceptive (overactive nerves), or systemic (internal injury or disease).

❯❯ Magnetoreception

Most commonly, navigation by way of sensitivity to magnetic field intensity. In birds, for example, the optic nerves receive and process the magnetic intensity of their environment and transmit said information to the brain. Sensitivity to magnetic fields is frequently cited when discussing what the greater animal kingdom has but humanity does not (at least, at scale). Salmon, hatchling turtles, honeybees, whales, and bats are all said to use magnetoreception, for navigation or migration, to some extent.

Magnetic fields, unlike other sensory stimuli, pass completely unimpeded through biological tissue. By extension, magnetic-field sensitivity is more ambiguous and under-researched than other senses, as the process of transducing the magnetic stimulus into a cellular response lacks specificity. Three hypotheses dominate: (1) mechanically sensitive magnetoreceptors; (2) light-sensitive, chemical-based mechanisms; (3) an anatomical structure that would enable electromagnetic induction. These concepts are not mutually exclusive, according to research published in PLOS Biology, "animals may have evolved multiple mechanisms to detect different components of the (magnetic) field." Wild.

❯❯ Pain

Neural feedback permitting the central nervous system to detect (or avoid) potentially damaging stimuli, either passively or actively. This is nociception. A StatPearls article on PubMed notes: "Inactive nociceptors provide less-than-conscious nudges that strongly encourage the avoidance of potentially injurious and hazardous exposures." Now, if you want to get technical, then general pain and nociception are not identical; nociceptive pain is more acutely defined according to the locus of sensory activation (e.g., skin, tendons, joints, bones, muscles, internal organs). But to keep the conversation accessible, only a few additional notes remain.

Consider, for example, congenital insensitivity to pain with anhidrosis, a rare nervous system disorder that begets a lack of sensitivity to noxious stimuli (resulting in recurring infections, cuts, bruises, and unintentional self-harm). Following such a diagnosis, "pain-sensing nerves in these patients are not properly connected in parts of brain that receive the pain messages," per the Iranian Journal of Pediatrics. With no cure for this hereditary disease, treatment regimens aim to control body temperature and prevent self-injury.

Consider also, allodynia, a different type of pain. In short, allodynia is chronic pain brought on by extreme sensitivity to touch. Actions or behaviors that are not typically considered painful can be excruciating. Pain and pain sensitivity are essential to survival, but what if one's body is unable to differentiate variations in pressure or temperature? The result is debilitating. In a medical environment, reducing such pain is extraordinarily complex (e.g., nerve-block injections, surgery, opioids, lots of therapy).

❯❯ Polarized Vision

Interestingly, animals with polarized vision can control the amount of light entering their eyes (or, attenuate the orientation at which light waves oscillate). Many animal species have developed superior navigational skills by basing their efforts on the sun's various positions. In other words, navigating the sky using time-dependent light patterns. Some animals use polarized vision (or polarized-light sensitivity) for "contrast enhancement, camouflage breaking, object recognition, and signal detection and discrimination," according to a research article published in Integrative and Comparative Biology.

When perceiving scattered or refracted light, environmental factors, atmospheric factors, perturbations in the medium (e.g., waves in water), medium quality, and pollution all affect an already highly sensitive manner of pattern discernment.

Human-world applications abound, from fancy sunglasses that enable one to increase visual clarity in high-glare environments to increasing the precision of advanced military technology. In one fantastic example, engineers at the University of Illinois at Urbana-Champaign studied the mantis shrimp in extraordinary detail and developed a camera (i.e., a one-inch cube) that mimics the shrimp's use of polarized light as well as the shrimp's capacity to manipulate its detection of light intensity. According to Scientific American, the camera's dynamic emulation of these natural abilities could help cars detect hazards in ambiguous conditions, enable military drones to identify camouflaged or shadowed targets, and help surgeons perform more accurately. It's difficult to state how powerful this new technology is: The engineers' cube camera's light-detection ability was 10,000-times higher than today's commercial cameras (and yes, the tech is already available for cheap, mass production...).

❯❯ Proprioception

The sense that allows an individual to perceive or otherwise intuit the location, movement, and action of various parts of the body (i.e., a limb-position sense). Proprioception is also defined as the unconscious awareness of joint position, for how one absently-mindedly deliberates, identifies, and predicts willed movement. That is to say, knowing what one's body is doing, in the moment (not reactive or reflexive), without really thinking about it. This is all about movement detection and movement-detection thresholds.

Likening proprioception to kinesthesia, either broadly or narrowly, is common, but may not be entirely appropriate when one considers how dependent (and specific) proprioception is regarding sensory nerve endings and their correlation to the particular location, position, and orientation of specific joints, muscles, and limbs (e.g., athletic trainers often focus on soft tissues, such as muscles, tendons, and ligaments). Some researchers have gone so far as to detail the number and type of skin receptors pertinent to signaling limb position. To simplify, proprioception is imperative for precise and fluid movements.

Impairment doesn't simply mean reduced movement-sense and spatial-bodily awareness (kinesthesia), damaged receptors mean the body's physical pathway to communicate with the brain is broken or askew. For example, imprecise sensory interpretation as a result of muscle vibration (e.g., antagonistic conditioning, muscle fatigue) may encourage receptors to inform the brain of the illusion of limb movement or of limb displacement.

❯❯ Spinal Reflexes

Also not considered among the traditional senses, reflexes are important and effective components of sensory stimulus-reaction complexes. Reflexes are involuntary or unintentional (uncontrolled). Each type of reflex response is initiated by sensory stimuli relayed from any of the other major senses. Most importantly, the stimulus itself excites specialized sensory receptors that respond unambiguously to a certain type, quality, or intensity of stimulation.

Interestingly, reflexive actions receive their signals from the spinal cord. This makes them considerably faster than one's normal reactions because they bypass the traditional neural pathway (the brain). Not to say the brain is uninvolved. The brain continuously builds, adapts, and influences spinal circuitry, in both short- and long-term development, and many spinal reflexes operate simultaneously as a result. An overview of the main types of spinal reflexes will include: stretch reflex (muscle contraction), crossed-extensor reflex (opposite limb compensating for loss of support), withdrawal reflex (nociceptive reflex, protecting the body from pain), and autogenic inhibition reflex (negative feedback mechanism to control muscle tension).

"The spinal cord is the simplest and most technically accessible part of the mammalian [central nervous system]. Thus, spinal cord reflexes, the brain's influence over them, and the spinal cord plasticity this influence produces provide the basis of a powerful experimental protocol for studying the mechanisms and substrates of learning." (Encyclopedia of Neuroscience)

❯❯ Thermoception

The brain's recognition and the body's ability to register changes in temperature. Or, put more simply, sensitivity to heat flux and temperature intensity. Animals possess a diversity of temperature sensitivity mechanisms. All thermosensors have activation thresholds and are moderated by various inflammatory mediators (e.g., some proteins are intrinsically heat-sensitive, others are cold-sensitive).

What does this mean? It means thermoception is fundamental to animal survival, as temperature homeostasis is essential to comfort and reproduction. It also means the human body cannot actually determine the absolute temperature of its environment; it must instead regulate its own temperature relative to that of its immediate surroundings.