“…Moreover, the limited spatial and temporal control of the thermal stimulus in floor plate experiments is problematic. For example, floor plate experiments where rodents can gather thermal information using different body parts, have led to different conclusions about the cortical representation of thermal input, with some lesion studies suggesting that the primary somatosensory cortex is not required for thermal sensation while others have suggested that it is [ 27 , 28 , 29 , 30 ]. Figure 1 Thermal behavioral tasks for rodents .…”
Thermal information about skin surface temperature is a key sense for the perception of object identity and valence. The identification of ion channels involved in the transduction of thermal changes has provided a genetic access point to the thermal system. However, from sensory specific 'labeled-lines' to multimodal interactive pathways, the functional organization and identity of the neural circuits mediating innocuous thermal perception have been debated for over 100 years. Here we highlight points in the system that require further attention and review recent advances using in vivo electrophysiology, cellular resolution calcium imaging, optogenetics and thermal perceptual tasks in behaving mice that have begun to uncover the anatomical principles and neural processing mechanisms underlying innocuous thermal perception.
“…Moreover, the limited spatial and temporal control of the thermal stimulus in floor plate experiments is problematic. For example, floor plate experiments where rodents can gather thermal information using different body parts, have led to different conclusions about the cortical representation of thermal input, with some lesion studies suggesting that the primary somatosensory cortex is not required for thermal sensation while others have suggested that it is [ 27 , 28 , 29 , 30 ]. Figure 1 Thermal behavioral tasks for rodents .…”
Thermal information about skin surface temperature is a key sense for the perception of object identity and valence. The identification of ion channels involved in the transduction of thermal changes has provided a genetic access point to the thermal system. However, from sensory specific 'labeled-lines' to multimodal interactive pathways, the functional organization and identity of the neural circuits mediating innocuous thermal perception have been debated for over 100 years. Here we highlight points in the system that require further attention and review recent advances using in vivo electrophysiology, cellular resolution calcium imaging, optogenetics and thermal perceptual tasks in behaving mice that have begun to uncover the anatomical principles and neural processing mechanisms underlying innocuous thermal perception.
“…Because the LiCl-injected rats reduced intake for 16 °C water and 13° C water, rats apparently generalize from the 10°C water CS across a 3–6° C temperature difference. Thus the range of temperature discrimination for an oral water stimulus appears to be between 3 and 6° C. Only 3°C differences were tested in this experiment, and rats may be able to discriminate smaller differences temperature (e.g., as with their snout [22]).…”
The the influence of temperature on taste cues and the ability to discriminate and learn about different temperatures of foods are important factors regulating ingestion. The goal of this research was to demonstrate that thermal orosensory input can serve as a salient stimulus to guide ingestive behavior in the rat, and also that it interacts with gustatory input during choice and conditioned aversion experiments. A novel apparatus with Peltier refrigerators was used to control the temperature of solutions in 10-minute, 2-bottle tests. It was determined that naive rats preferred cold water (10°C) to warm water (40°). When cold water was paired with a toxic LiCl injection, rats avoided cold water and drank warm water, thus demonstrating that cold water could serve as the conditioned stimulus in a conditioned temperature aversion. Rats conditioned against cold water could discriminate 10°Cwater from 16° C water, but not from 13° C water, thus showing an ability to discriminate orosensory thermal cues to within 3-6° C. Rats also generalized conditioned aversions from cold water to cold saccharin and cold sucrose solutions. However, if rats were conditioned against a compound taste and thermal stimulus (10° C, 0.125% saccharin), the rats could distinguish and avoid each component individually, i.e., by avoiding cold water or warm saccharin. Finally, daily 2-bottle extinction tests were used to assess the strength of aversions conditioned against a taste cue (0.25 M sucrose), a thermal cue (10°C water), or the combination. Aversions to taste or temperature alone persisted for 7-14 days of extinction testing, but the combined taste-temperature aversion was stronger and did not extinguish after 20 days of extinction testing. These results demonstrate that temperature can serve as a salient cue in conditioned aversions that affect ingestion independent of taste cues or by potentiating taste cues.
The temperature of an object provides important somatosensory information for animals performing tactile tasks. Humans can perceive skin cooling of less than one degree, but the sensory afferents and central circuits they engage to enable the perception of surface temperature are poorly understood. To address these questions, we examined the perception of glabrous skin cooling in mice. We found that mice were also capable of perceiving small amplitude skin cooling and that primary somatosensory (S1) cortical neurons were required for cooling perception. Moreover, the absence of the menthol-gated transient receptor potential melastatin 8 ion channel in sensory afferent fibers eliminated the ability to perceive cold and the corresponding activation of S1 neurons. Our results identify parts of a neural circuit underlying cold perception in mice and provide a new model system for the analysis of thermal processing and perception and multimodal integration.An accurate sense of surface temperature helps animals to perceive object structure and identity. Psychophysical experiments have shown that humans are able to perceive tiny changes in skin cooling with a range between 0.4 and 1.8 ˚C 1,2 . It has, however, proved challenging to assess the perceptual ability of rodents to discriminate small temperature steps at threshold levels.Classical paw withdrawal tests cannot differentiate between reflexive avoidance behavior and sensory perception 3 . Two-plate thermal preference arenas have shown that mice avoid cooler floor temperatures [4][5][6] , but this test has limited spatial and temporal control of the stimulus and lacks fine-grained resolution for near threshold perception. We therefore developed a shortlatency, goal-directed thermal perception task using the glabrous skin of the mouse forepaw.A general dogma is that all somatosensory input, including thermal, is integrated by the primary somatosensory cortex (S1) to form a coherent sensory percept. S1 is necessary for tactile somatosensory perception in rodents [7][8][9][10][11][12][13] . The role of S1 in thermal perception, however, is under debate, 3 with three studies concluding that rodent S1 is not involved [14][15][16] and another concluding that it is 17 . This may be because these studies used large cortical lesions with long recovery and retraining periods in freely moving rats that used facial regions to detect temperature [14][15][16][17] .Likewise, very little is known about the underlying cortical neural processing of non-noxious thermal stimuli in rodents. To the best of our knowledge, only one study, conducted in anesthetized rats stimulating scrotal skin, has shown extracellular responses of cortical neurons to thermal stimulation 18 . At the sensory periphery, a range of primary afferents including myelinated Aβ mechanoreceptors 2,19 , thinly myelinated Aδ-fibers and unmyelinated polymodal C fibers, fire during skin cooling 4,20,21 . Although it is thought that thickly myelinated Aδ fibers are responsible for cooling perception, C fibers ...
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