User:Isamelia6/Thermoception

Introduction

In physiology, thermoception or thermoreception is the sensation and perception of temperature. It deals with a series of events and processes required for an organism to receive a temperature stimulus, convert it to a neurochemical signal, and characterize the signal in order to trigger an appropriate response (see Thermoregulation).

It's important to note that while heat, temperature, and thermoception are closely related, they are not the exact same thing. Heat refers to the transfer of kinetic energy; temperature refers to the degree of intensity of heat . Thermoception, specifically, is the perception or cognitive processing of temperature. This term aims to address the phenomenon of individuals emotionally and physiologically processing the same temperature differently.

In animals

In snakes

A particularly specialized form of thermoception is used by Crotalinae (pit viper) and Boidae (boa) snakes, which can effectively see the infrared radiation emitted by hot objects. The snakes' face has a pair of holes, or pit organs, lined with temperature sensors. The sensors indirectly detect infrared radiation as a result of a heating effect on the skin inside the pit. They can determine which part of the pit is the hottest, and therefore the direction of the heat source. This mechanism likely evolved as a way to hunt and capture warm-blooded prey animals in a variety of environments including darkness and camouflage, but proved more helpful in predator avoidance . By combining information from both pits, snakes can also estimate not only the direction, but the distance between itself and the heat source.

In bats and other mammals

The Common vampire bat has specialized infrared sensors in its nose-leaf. Vampire bats are the only mammals that feed exclusively on blood. The infrared sense enables Desmodus to localize homeothermic (warm-blooded) animals (cattle, horses, wild mammals) within a range of about 10 to 15 cm. This infrared perception is possibly used in detecting regions of maximal blood flow on targeted prey.

Dogs, like vampire bats, can detect weak thermal radiation with their rhinaria (noses), as this area's particularly low temperature makes it more sensitive to heat. Functional magnetic resonance imaging (fMRI) revealed that the left somatosensory association cortex, involved in both goal setting and predating, became active during this process .

In insects

This mechanism has evolved in a variety of insects as well. Forest fire seeking beetles (Melanophila acuminata), also use this mechanism to sense previous and current heat, as much of their offspring has been harmed due to laying their eggs in conifers eventually or previously killed by forest fires. Darkly pigmented butterflies Pachliopta aristolochiae and Troides rhadamantus use specialized heat detectors to avoid damage while basking. The blood sucking bugs Triatoma infestans may also have a specialised thermoception organ; they also have changing optimal heat preferences dependent on their current environment, suggesting the importance of this system to their survival .

In humans

To best understand thermoception in humans consider the following vignette:

Alex stood outside on a hot day to watch a total solar eclipse. As the moon covers the sun, the environment gets darker, and also colder. In fact, it gets so chilly so suddenly, it feels like night time. As the eclipse ends, the sunlight comes out again and the ambient environment feels hot again just as it did before the eclipse.

In humans, sensation and perception is often discussed within psychophysics. Fechner's Law refers to the mathematical relationship between the intensity of a stimulus, sensation of that stimulus and the perception built based of sensational information. In this scenario, the decrease in heat intensity making contact with your body is drastic and more obvious. Ambient temperature changes are often difficult to detect as they happen incrementally. In thermoception, humans use internal "set points" of ideal internal temperature compared to the outer skin temperature and deploys homeostatic mechanisms accordingly . As thermal stimuli intensity increased, thermoreceptor activation increased logarithmically .

In humans, thermoception can be mediated by "top-down" processes such as attention , where internal guidance of attention would be based on prior knowledge.

Physiology

Importance of Homeostasis and Thermoregulation

Thermoception is closely related to Thermoregulation. Thermoregulation refers to the homeostatic measures taken by an organism to maintain an optimal internal temperature. For humans this is 98.6F or 37C. Within our homeostatic functions is a constant surveillance of our internal body temperature and the ambient environmental temperature known as exteroception and interception. Classic models of homeostatic thermoregulation emphasizes the importance of feedback from thermoreceptors to determine an ideal internal temperature set point . However, recent studies in thermoregulatory systems have found that there are "interneurons" of the brain that are highly involved for determining a homeostatic set point as well, suggesting that our perception of thermal stimuli is vastly more complex than previously thought.

Additionally, when the body is in duress such as in the cases of injury or disease, thermoreceptor sensitivities change, which can also alter the perception of pain (also known as hyperalgesia) . Furthermore the sensation of temperature changes can be difficult to consciously perceive as our autonomic nervous system responds to changes in temperature by vasodilation or vasoconstriction.

Transduction of Heat Stimuli

Transduction refers to the general process, across sensory modalities, of converting sensory information (such as temperature, sound frequencies & amplitude, light frequencies etc) to neurochemical action potentials . Because heat sensation engages the entire body, primary somatosensory neurons, or thermoreceptors, are spread across the entirety of the skin . Our body compares our internal temperature to our outer skin temperature as it comes in constant with air

Sensation

In humans, temperature sensation begins with thermoreceptors. Thermoreceptors, such as Ruffini corpuscle, are widely activated to sense changes in ambient temperature, due to their large and slow adapting receptive fields. Krause corpuscles are superficial to Ruffini corpuscles and sense cold . Krause corpuscles being superficial, or not as deep in the skin, as Ruffini corpuscles allows humans to sense cold faster than hot. This is advantageous as most things in the world are cooler than our skin. Krause corpuscles are activated when the outer skin temperature drops below 95°F (35°C) and work optimally at 77°F (5°C) .

Transmission

After the thermoreceptors in the skin are activated by environmental stimuli, the signal is sent to the spinal cord along the axons of Lissauer's tract that synapse on second order afferent neurons in grey matter of the dorsal horn. The dorsal horn of the spinal cord is a conglomeration of afferent neurons that send sensory information from the body to the brain for further processing

The axons of these second order neurons then decussate, joining the spinothalamic tract as they ascend to neurons in the ventral posterolateral nucleus of the thalamus. A study in 2017 shows that the thermosensory information passes to the lateral parabrachial nucleus rather than to the thalamus and this drives thermoregulatory behaviour.

The insular cortex has implicated in higher-order thermoreceptive processing in noxious and innocuous thermal stimulation studies . The implication of the insular cortex in thermoception is significant because as sub cortical areas responsible for various types of sensory processing emotional processing reward and motivational Behavior. All of which can modulate attention, which in turn, modulates the human ability to accurately predict the intensity of heat stimuli . The posterior insular cortex is implicated in processing pain relating to thermal stimulation, whereas the anterior insula processes innocuous thermal stimuli .

Perception

Attention

To study the impact of contextual factors such as attention, it was found that the predictive model humans use to perceive pain caused by heat stimuli can be distorted by contextual factors that lead to prediction errors. Specifically, neural oscillatory activity, specifically alpha and beta waves in the insular cortex corroborated that people tend to make more intensity prediction errors when first cued with incongruent signals preceding the stimulus .Being cued to what intensity of heat significantly alters aversiveness and ultimately, prediction errors, related to the pain elicited from the given heat stimuli.

Future Research

Thermoception and Nociception in Augmented Reality

In the further development of augmented reality (AR), understanding the sensation and perception of heat and pain in order to program life-like physics within AR has become an essential direction for future research . As people spend more time interacting with immersive technology, it's important to understand how to simulate heat or pain of a game without making it so life like that it could have negative psychological consequences . AR experiences can significantly modulate the hyperalgesic or analgesic perception of a hand burning. Perception of pain and thermal stimuli varies by AR experiences, specifically, when participants can embody limbs, play as disembodied, or have 3rd person avatars.

Novel Methods of Administering Hot or Cold Stimuli and Measuring Thermoception

Ambient heat/cold designs

Little is known about possible research confounds related to mechanically administering heat conditions versus exposing participants to ambient heat conditions that don't require touch. Novel methods of research include new mechanisms of applying thermal stimuli without mechanical force, such as using carbon dioxide emitted from dry ice . This is important because tactile sensation and perception might obscure significant information about how we sense temperature in the ambient environment .

Thermal- imaging

Another novel method of studying thermal stimuli without the confounds of mechanoreception is using thermal imaging and indirect heat sources to measure changes in skin temperature. Infrared thermal imaging (IRT) creates surface level heat maps of various parts of the body. It is often utilized as a diagnostic aid to measure inflammation and brain issue abnormalities, as these both correlate with higher than average temperatures .

References

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• Ezquerra-Romano, I., Chowdhury, M., Leone, C. M., Iannetti, G. D., & Haggard, P. (2023). A novel method to selectively elicit cold sensations without touch. Journal of Neuroscience Methods, 385, 109763. https://doi.org/10.1016/j.jneumeth.2022.109763
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