A given stimulus can induce a pleasant or unpleasant sensation depending on the subject's internal state. The word alliesthesia is proposed to describe this phenomenon. It is, in itself, an adequate motivation for behavior such as food intake or thermoregulation. Therefore, negative regulatory feedback systems, based upon oropharingeal or cutaneous thermal signals are peripheral only in appearance, since the motivational component of the sensation is of internal origin. The internal signals seem to be complex and related to the set points of some regulated variables of the "milieu interieur," like set internal temperature in the case of thermal sensation (15). Alliesthesia can therefore explain the adaptation of these behaviors to their goals. Only three sensations have been studied- thermal, gustatory, and olfactory, but it is probable that alliesthesia also exists in such simple ways as in bringing a signal, usually ignored, to the subject's attention. For example, gastric contractions, not normally perceived, are felt in the state of hunger (16). Since alliesthesia relies on an internal input, it is possible that alliesthesia exists only with sensations related to some constants of the "milieu interieur" and therefore would not exist in visual or auditory sensations. As a matter of fact, luminous or auditory stimuli can be pleasing or displeasing in themselves, but there seems to be little variation of pleasure in these sensations, that is, no alliesthesia. There may be some esthetic value linked to these stimuli but it is a striking coincidence that they are in themselves rather neutral and that it is difficult to imagine a constant of the "milieu interieur" which could be possibly modified by a visual or an auditive stimulus-such as light of a certain wavelength or sound of a given frequency. In the light of this theory, it is possible to reconsider the nature of the whole conscious experience. The existence of alliesthesia implies the presence of internal signals modifying the concious sensations aroused from peripheral receptors. It is therefore necessary to question the existence of sensations aroused by direct stimulation of central receptors, such as hypothalamic temperature detectors, osmoreceptors, and others. Does their excitation arouse sensations of their own, or does the sensation have to pass through peripheral senses? Only human experimentation could answer this question. In the same way, it is possible that selfstimulation of the brain is pleasant, not by giving a sensation in itself, but because the electrical stimulus (17), renders peripheral stimuli pleasant.

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In response to a stimulus, a sensation is tridimensional: qualitative, quantitative, and affective. The affective part of sensation, pleasure or displeasure, depends on the qualities of the stimulus. Within a narrow range of intensity, chemical, thermal, and mechanical stimuli are able to arouse pleasure. In addition, pleasure depends on the internal state of the subject. This is easily observed in the case of temperature: pleasure is aroused by a warm stimulus in a hypothermic subject and by a cold stimulus in a hyperthermic subject. This property of a given stimulus to arouse pleasure or displeasure according to the internal state of the subject is termed alliethesia. Alliesthesia is also produced by chemical and mechanical stimuli. Acquired preferences or aversions for alimentary stimuli represent a case of alliesthesia. In the same way, the capacity of any indifferent stimulus to become rewarding, or punishing, by association with some reward or punishment, is also a case of alliethesia. In all cases, pleasure is a sign of a stimulus useful to the subject; displeasure a sign of danger. Usefulness and danger are judged by the central nervous system with reference to homeostasis and the set point of the implied regulation. Pleasure and displeasure thus appear to motivate useful behaviors.

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Humans have higher ventilation when they are hyperthermic but it is not known whether core temperature thresholds for ventilation exist, nor has a physiological rationale been presented for this response. To examine this question, ventilation was studied in relation to core temperatures in humans rendered hyperthermic in a warm bath. Seven subjects [mean (SE), 23.3 (1.4) years] wearing only shorts and a thick felt hat with ear flaps were immersed to the neck in a bath at 41 (0.5) degrees C for 25 min. Tympanic (Tty), esophageal (Tes), thigh skin and forehead skin temperatures, heart rate, inspired minute ventilation (V1 at body temperature and pressure, saturated), ventilation frequency and oxygen consumption (VO2 at standard temperature and pressure, dry) were recorded at 30-s intervals. At immersion V1 briefly increased to 18.6 (3.0) l.min-1, returned to about the pre-immersion value, and significantly increased to 19.3 (3.0) l.min-1 by the end of immersion. VO2 increased significantly from the pre-immersion value of 0.27 l.min-1 to 0.67 l.min-1 by the first 0.5 min of immersion, but then returned to its pre-immersion value. Tty increased to 38.7 (0.2) degrees C and Tes increased to 39.0 (0.2) degrees C by the end of immersion. Core temperature thresholds for increases in V1 were evident at 38.1 degrees C when expressed against Tty and at 38.5 degrees C when expressed against Tes. The results indicated that during body warming core temperature thresholds for V1 are reached and subsequently a hyperpnea was evident, despite VO2 remaining at a resting value.(ABSTRACT TRUNCATED AT 250 WORDS)

A mechanism that selectively cools the brain during hyperthermia is a well-accepted fact in animals. Selective brain cooling (SBC) during hyperthermia has also been proposed in humans, but this suggestion has met with considerable debate. Several authors have rejected the idea of human SBC for the following reasons: 1) SBC is illogical because this mechanism removes the error signal activating the defense against hyperthermia; 2) unlike other animals, humans do not pant and thus do not possess a powerful heat sink at a short distance from the brain; 3) humans do not have a carotid rete, the countercurrent heat exchanger between the arterial and venous bloods flowing in and out of the brain; 4) the high and constant arterial blood flow of the brain is sufficient to cool the brain under all conditions; and 5) the relatively low tympanic temperature (Tty) recorded in hyperthermic humans is not a sign of SBC, but rather is the sign of contamination of Tty by a low head skin temperature. These arguments are reviewed and rejected and results of several recent experiments are summarized. Finally, recent experimental articles that contradict the existence of human SBC or the validity of Tty are discussed and their conclusions refuted. This review points to overwhelming evidence in favor of human SBC.

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The value of a regulated variable in the absence of external perturbation stabilizes at the set point of the system. This set point is an information input that may be determined by an external signal to which the regulated variable is compared or may be determined by the structural characteristics of the system itself. In the case of temperature regulation the actual internal temperature is compared with the set point "wanted" by the organism. The activating signal for the regulatory responses, the "error signal," is the difference between the actual temperature and the set point. When an error signal is detected, the organism produces the available corrective responses. Yet, the notion of thermoregulatory set point has been challenged recently. Such a questioning entails that both fever and anapyrexia are useless concepts. This minireview examines the available arguments and data and concludes that to abandon the concepts of set point, fever, and anapyrexia is premature, at best.