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Perception of Object Temperature
When the hand grasps an object, changes in skin temperature can assist in identifying the object and discriminating between different types of objects. These cues become especially important when objects must be identified without visual feedback, such as when reaching for objects in the dark. The thermal cues that assist in identifying an object arise from the changes in skin temperature that occur when the object and hand are in contact. The thermal properties of the object, such as its conductivity and specific heat capacity, the initial temperatures of the skin and object, the thermal contact resistance between the skin and object, and the object's size and shape all determine the rate at which heat is conducted out of the skin or object during contact. Because of the differences in the thermal properties of materials, when an object made from plastic is held in the hand, skin temperature changes much more slowly than when the hand grasps an object made from stainless steel or copper. This means that the metal object feels “cooler” than one made from plastic, even though both objects are at the same temperature. After 10 seconds of contact with a copper object at room temperature, skin temperature can decrease by as much as 5 °C, whereas it changes by less than 2 °C after 10 seconds of contact with a plastic object (Ho & Jones, 2006). The temperature of the skin is usually higher than the temperature of objects encountered in the environment, and so it is the decrease in skin temperature on contact that is used to identify whether an object is made from metal, wood or plastic. For objects warmer than the hand, such as the water in a shower or the handle of a pot on a stove, increases in skin temperature are typically used to evaluate the temperature of the object, and not to identify it.
The sensory system involved in perceiving the changes in skin temperature begins with free nerve endings found in the dermal and epidermal layers of skin that can be functionally classified as cold and warm thermoreceptors. Warm and cold receptors respond similarly to radiant and conducted thermal energy and are involved in the perception of innocuous (harmless) temperatures. The molecular mechanisms underlying temperature sensation have been extensively studied over the past decade with the result that several temperature-sensitive ion channels of the transient receptor potential (TRP) family have been identified as candidate temperature sensors. These thermoTRP channels are expressed in sensory nerve endings and are active at specific temperatures ranging from noxious cold to burning heat (Dhaka et al., 2006). In addition to responding to changes in temperature, these thermoTRPs are involved in chemesthesis, and so mediate the pungent qualities of stimuli such as capsaicin, the "hot" ingredient in chili peppers and menthol, the "cooling" compound from mint.
Thermoreceptors in the glabrous skin on the palm of the hand are mainly used to assist in identifying objects in contact with the hand, whereas thermoreceptors in hairy skin are particularly important in thermoregulation. Cold receptors respond to decreases in skin temperature over a range of 5-43 °C, and discharge most vigorously at skin temperatures around 25 °C. In contrast, warm receptors signal that skin temperature has increased and are most responsive at approximately 45 °C (Darian-Smith & Johnson, 1977). When the temperature of the skin is between 30-36 °C, which is the normal range for skin temperature, both types of receptor are spontaneously active, but there is generally no awareness of cold or warmth. This is called the neutral thermal region; at higher or lower temperatures, there is an enduring sensation of warmth or coolness, respectively. In contrast to body temperature which varies by less than 1 °C across healthy individuals, skin temperature can vary by as much as 12 °C in normal individuals, particularly on the hands and feet (Parsons, 2003).
In addition to sensing the temperature of objects in contact with the skin, afferent signals arising from cold thermoreceptors have been shown to play a role in the perception of wetness. It appears that thermal cues are used in conjunction with tactile inputs to perceive the wetness experienced when the skin is in contact with a wet surface (Filingeri et al., 2014). These interactions between thermal and tactile inputs presumably account for the illusion of skin wetness that can occur when the skin is exposed to cold-dry stimuli which result in cooling rates similar to those that occur during evaporation of water from the skin surface.
The number and density of thermoreceptors in the skin has been measured by placing small warm and cold stimulators on the skin and recording the sites at which a person detects a change in temperature. The locations at which a thermal stimulus is detected are known as warm and cold spots and are assumed to mark the receptive fields of underlying thermoreceptors. Warm and cold spots are only a few millimeters in diameter, and are distributed independently. There are more cold spots than warm spots, and the density of spots varies across the body. For example, on the forearm it is estimated that there are approximately 7 cold spots and 0.24 warm spots per 100 mm2. In addition to differences in the distribution of cold and warm thermoreceptors across the skin surface, the two types of receptor differ with respect to the conduction velocities of the afferent fibers that convey information from the receptor to the central nervous system. Cold afferent fibers are myelinated and so are much faster than unmyelinated warm afferent fibers with conduction velocities of 10-20 m/s as compared to 1-2 m/s for warm fibers. As would be expected from these differences in conduction velocities, the time to respond to a cold stimulus is significantly shorter than that for a warm stimulus.
The skin also contains thermally sensitive receptors leading to pain sensation known as thermal nociceptors that respond to noxious or harmful temperatures. Nociceptors that are responsive to temperature signal to the central nervous system that tissue damage is imminent and that the affected body part should be withdrawn immediately from the thermal source (e.g. a finger on a hot plate). These receptors are active when the temperature of the skin falls below 15-18 °C or rises above 45 °C. When they are activated, the sensation is one of pain. Although the thresholds for activating heat and cold-sensitive nociceptors are usually described as being greater than 45 °C and less than 15°C, in some individuals mild cooling (25-31 °C) and warming (34-40 °C) of the skin can evoke sensations of burning and stinging as well as innocuous sensations of cold and warmth (Green, 2002). Changes in skin temperature also affect the responses of mechanoreceptors in the skin that signal mechanical deformation, such as pressure or vibration, which is why the hands often seem clumsy when they are cold. However, it is generally accepted that mechanoreceptors do not have sufficient encoding capacity to account for thermal sensations.
The ability to perceive changes in skin temperature depends on a number of variables including the location on the body stimulated, the amplitude and rate of temperature change, and the baseline temperature of the skin. There is a 100-fold variation in sensitivity to changes in skin temperature across the body, with the cheeks and the lips being the most sensitive area, and the feet being the least sensitive region. A common finding in many studies of thermal thresholds is that despite the variability in thresholds across the body, all regions are more sensitive to cold than to warmth. In general, the threshold for detecting a decrease in temperature (cold) is half that of detecting an increase in skin temperature (warmth), and the better a site is at detecting cold, the better it is at detecting warmth (Stevens & Choo, 1998).
The thermal sensory system is extremely sensitive to very small changes in temperature and on the hairless skin at the base of the thumb, people can perceive a difference of 0.02-0.07 °C in the amplitudes of two cooling pulses or 0.03-0.09 °C of two warming pulses delivered to the hand. The threshold for detecting a change in skin temperature is larger than the threshold for discriminating between two cooling or warming pulses delivered to the skin. When the skin at the base of the thumb is at 33 °C, the threshold for detecting an increase in temperature is 0.20 °C and is 0.11 °C for detecting a decrease in temperature.
The rate that skin temperature changes influences how readily people can detect the change in temperature. If the temperature changes very slowly, for example at a rate of less than 0.5 °C per minute, then a person can be unaware of a 4-5 °C change in temperature, provided that the temperature of the skin remains within the neutral thermal region of 30-36 °C. If the temperature changes more rapidly, such as at 0.1 °C/s, then small decreases and increases in skin temperature are detected. However, warm and cold thresholds do not decrease any further if the rate at which temperature changes is faster than 0.1 °C/s.
Spatial Aspects of Thermal Perception
In contrast to the visual, auditory and haptic modalities, the range of sensations evoked by changes in skin temperature is rather limited. In response to thermal stimulation, people report that the skin has been cooled or warmed, and perceive the intensity and duration of the stimulus. They may also note the hedonic nature of the stimulus, that is, whether it is pleasant (e.g. standing near a warm fire when cold) or unpleasant (e.g. standing by an open door on a cold day), and at extreme temperatures, sensations of pain predominate. The spatial features of the thermal stimulus, such as its area and shape, or changes in intensity within an area of stimulation are barely resolved. Spatial resolution is poor because the sensory system involved in processing information from thermoreceptors in the skin summates intensity over the area of stimulation, which means that changes in stimulus area are often indistinguishable from changes in stimulus intensity. This property increases the detection of small changes in temperature that occur over a large surface area, which is important to thermoregulation, that is maintaining core temperature constant. For the thermal senses, the spatial extent of stimulation affects the perceived intensity of the stimulus with the result that as the area of stimulation increases the stimulus is perceived to be more intense, rather than just larger. This characteristic also means that a warm or cold stimulus becomes more detectable (i.e. the threshold decreases) if the area of stimulation increases. Spatial summation does, however, decline as the temperature approaches the threshold of pain.
One of the interesting properties of thermal spatial summation is that the areas stimulated do not have to be contiguous for summation to occur. When two symmetrical sites on the body (e.g. both forearms) are stimulated simultaneously, the thermal stimulus at one site is perceived to be more intense than when only a single site is stimulated. As would be expected from this result, both warm and cold thresholds are lower when stimuli are presented bilaterally. However, there is no change in thresholds if the two sites are asymmetric, such as the forehead and the contralateral hand.
A further dimension of spatial processing that is used to characterize sensory systems is spatial acuity, which in the context of the tactile modality refers to the spatial resolution of the skin. As would be expected for a sensory system that displays pervasive spatial summation, the thermal senses are poor at localizing the site of thermal stimulation on the body and at differentiating spatially two thermal stimuli delivered in close proximity. If tactile cues are eliminated by using non-contact thermal stimulation such as radiant heat, the ability to localize the site of stimulation is very poor, particularly if the stimuli are not very intense. The errors of localization are such that warm stimuli delivered to the front or back of the torso can be misperceived as being presented to the other side of the torso. Such errors of localization are never found with mechanical stimulation to the torso and do not occur for thermal stimuli near the pain threshold. Information from tactile and thermal receptors in the skin is conveyed to the brain via different anatomical pathways, and the spatial properties of the tactile and thermal sensory systems reflect this distinction.
Temporal Aspects of Thermal Sensation
The duration of a thermal stimulus and the rate with which it changes can have a marked effect on perception. It is a frequently experienced phenomenon, that the sensation of warmth that is aroused when one steps in the shower gradually diminishes with time. With continuous exposure to a thermal stimulus there is a decrease in neural responsiveness, a process referred to as adaptation. The skin adapts to both warm and cold stimuli over time, and for temperatures close to that of the skin, the rate at which adaptation occurs is rapid, in the order of 60 s for changes of +/- 1°C in skin temperature. It takes much longer for the skin to adapt to more extreme temperatures, and for the forearm complete adaptation occurs within about 25 minutes for temperatures between 28 °C and 37.5 °C (Kenshalo & Scott, 1966). The time required to respond to a thermal stimulus depends on the intensity of the stimulus and the response required. Temperatures close to the thermal pain thresholds are responded to rapidly due to the possibility of tissue damage, but the response to more moderate temperatures is sluggish, when compared to other sensory systems.
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