The instinct of thirst was a cardinal element in the successful colonization by vertebrates of the dry land of the planet, which began in the Ordovician period about 400 million y ago. It is a commonplace experience in humans that drinking water in response to thirst following fluid loss is a pleasant experience. However, continuing to drink water once thirst has been satiated becomes unpleasant and, eventually, quite aversive. Functional MRI experiments reported here show pleasantness of drinking is associated with activation in the anterior cingulate cortex (Brodmann area 32) and the orbitofrontal cortex. The unpleasantness and aversion of overdrinking is associated with activation in the midcingulate cortex, insula, amygdala, and periaqueductal gray. Drinking activations in the putamen and cerebellum also correlated with the unpleasantness of water, and the motor cortex showed increased activation during overdrinking compared with drinking during thirst. These activations in motor regions may possibly reflect volitional effort to conduct compliant drinking in the face of regulatory mechanisms inhibiting intake. The results suggestive of a specific inhibitory system in the control of drinking are unique.fMRI | swallowing T he colonization of the planetary dry land by vertebrates emerging from the fresh water rivers and swamps over the Ordovician and Silurian period was dependent in major part on the development of the instincts of thirst and sodium appetite.The genetically programmed neural organization determining the behavior embodied in these instincts is protective of the chemical integrity and volume of the circulating milieu intérieur: the extracellular fluids. With thirst, for example, in some manner presently only partly understood, the stimuli provided by sensory input from the dry mouth and desiccated pharynx, increased sodium concentration in the cerebrospinal fluid, the osmotic pressure of the carotid arterial blood, activation of the stretch receptors in the heart, and of the concentration of hormones in arterial blood is centrally integrated to contrive the intensity of the specific subjective state of thirst, and the compulsive intention to acquire water. The intensity of this primordial emotion presumptively determines the quantitative intake during drinking (1, 2).Accurate rapid gratification over 5-10 min of body deficit occurs in species including the camel, burro, sheep, Saharan donkey, goat, cow, dog, or horse depleted of 4-8% of body weight. However, for other species, such as rat, guinea pig, hamster, and for particular consideration human, drinking is much slower. With human, other variables are operative in the gratification process, termed "the consummatory act" (3). When attesting to thirst as a result of experimental dehydration, these species may drink initially more than 50-80% of deficit (4) but then slowly complete intake to repletion. This behavior was termed "voluntary dehydration" by Adolph (5), an early pioneer in this field. Adolph studied man in the desert and was dubious ...

Subjective visual experience depends not only on the spatial arrangement of the environment, but also on the temporal pattern of stimulation. For example, flickering and steady light presented in the same location evoke a very different perceptual experience due to their different temporal patterns. Here, we examined whether the availability of processing resources affected the temporal resolution of conscious flicker perception--the ability to distinguish rapid changes in light intensity, detecting visual temporal patterns. Participants detected flicker in a fixated LED that flickered at or around the individually adjusted critical flicker fusion (CFF) threshold while searching for a target letter presented in the periphery either on its own (low perceptual load) or among other letters (high load). Physically identical flickering stimuli were more likely to be perceived as "fused" under high (compared to low) load in the peripheral letter search. Furthermore, psychophysical measures showed a reduction in flicker detection sensitivity under high perceptual load. These results could not be due to criterion or stimulus prioritization differences or to differential likelihood of forgetting the correct response under different load conditions. These findings demonstrate that perceptual load influences conscious perception of temporal patterns.

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In humans, drinking replenishes fluid loss and satiates the sensation of thirst that accompanies dehydration. Typically, the volume of water drunk in response to thirst matches the deficit. Exactly how this accurate metering is achieved is unknown; recent evidence implicates swallowing inhibition as a potential factor. Using fMRI, this study investigated whether swallowing inhibition is present after more water has been drunk than is necessary to restore fluid balance within the body. This proposal was tested using ratings of swallowing effort and measuring regional brain responses as participants prepared to swallow small volumes of liquid while they were thirsty and after they had overdrunk. Effort ratings provided unequivocal support for swallowing inhibition, with a threefold increase in effort after overdrinking, whereas addition of 8% (wt/vol) sucrose to water had minimal effect on effort before or after overdrinking. Regional brain responses when participants prepared to swallow showed increases in the motor cortex, prefrontal cortices, posterior parietal cortex, striatum, and thalamus after overdrinking, relative to thirst. Ratings of swallowing effort were correlated with activity in the right prefrontal cortex and pontine regions in the brainstem; no brain regions showed correlated activity with pleasantness ratings. These findings are all consistent with the presence of swallowing inhibition after excess water has been drunk. We conclude that swallowing inhibition is an important mechanism in the overall regulation of fluid intake in humans.thirst | drinking | swallowing | inhibition | fMRI F luid depletion leads to drinking, an important evolutionary behavior that satisfies the physiological need to replenish lost fluid. The motivation to begin drinking is normally provided, in humans at least, by the presence of a subjective state of thirst. At some point after drinking has commenced, the sensation of thirst disappears and is replaced by the experience of satiation, along with the cessation of drinking. Studies performed in humans and animals indicate the regulatory mechanisms that have evolved to govern the cessation of drinking appear to be tightly calibrated, with the amount of fluid ingested commensurate with the degree of fluid depletion, even though some variation occurs between species regarding the time taken to conclude drinking (1-3).Several factors have been implicated in the regulation of fluid intake, with the majority relating to thirst and the initiation of drinking. These include signals produced by osmoreceptors in the lamina terminalis (4-6), which respond to cellular dehydration and the resulting increase in sodium concentration within the cerebral spinal fluid (2), and signals produced in response to extracellular dehydration, such as those associated with the renin-angiotensin system, which is activated as a result of changes in vascular pressure and volume (7,8). In comparison, the mechanisms responsible for terminating drinking are less well understood. Oropharyngeal metering ...

In humans, activity in the anterior midcingulate cortex (aMCC) is associated with both subjective thirst and swallowing. This region is therefore likely to play a prominent role in the regulation of drinking in response to dehydration. Using functional MRI, we investigated this possibility during a period of "drinking behavior" represented by a conjunction of preswallow and swallowing events. These events were examined in the context of a thirsty condition and an "oversated" condition, the latter induced by compliant ingestion of excess fluid. Brain regions associated with swallowing showed increased activity for drinking behavior in the thirsty condition relative to the oversated condition. These regions included the cingulate cortex, premotor areas, primary sensorimotor cortices, the parietal operculum, and the supplementary motor area. Psychophysical interaction analyses revealed increased functional connectivity between the same regions and the aMCC during drinking behavior in the thirsty condition. Functional connectivity during drinking behavior was also greater for the thirsty condition relative to the oversated condition between the aMCC and two subcortical regions, the cerebellum and the rostroventral medulla, the latter containing nuclei responsible for the swallowing reflex. Finally, during drinking behavior in the oversated condition, ratings of swallowing effort showed a negative association with functional connectivity between the aMCC and two cortical regions, the sensorimotor cortex and the supramarginal gyrus. The results of this study provide evidence that the aMCC helps facilitate swallowing during a state of thirst and is therefore likely to contribute to the regulation of drinking after dehydration.n humans, drinking is commonly motivated by the state of thirst that accompanies dehydration. This motivation needs to be tightly regulated, as underdrinking or overdrinking can lead to serious health effects and potentially even death (1-4). This suggests an intimate relationship exists between the state of thirst and a volitional drinking response. Both the satiation of thirst and the conclusion of drinking also occur before changes in blood volume and osmolality signal the restoration of fluid balance. The volume of fluid drunk nevertheless approximates the fluid deficit, raising the possibility that a reduction in subjective thirst could help regulate the drinking response in the absence of this interoceptive signal from the circulatory system.Valuable insight into the relationship between thirst and drinking has been provided by the results of recent animal studies. These studies have demonstrated neurons in the subfornical organ, the median preoptic nucleus, and organum vasculosum of the lamina terminalis integrate information regarding a thirst stimulus and drinking responses (5-7). This integration represents a potential allostasis mechanism (8) that allows these neurons to anticipate the correction of fluid balance in advance and adjust drinking behavior accordingly (6). Such "anticipator...

In response to dehydration, humans experience thirst. This subjective state is fundamental to survival as it motivates drinking, which subsequently corrects the fluid deficit. To elicit thirst, previous studies have manipulated blood chemistry to produce a physiological thirst stimulus. In the present study, we investigated whether a physiological stimulus is indeed required for thirst to be experienced. Functional MRI (fMRI) was used to scan fully hydrated participants while they imagined a state of intense thirst and while they imagined drinking to satiate thirst. Subjective ratings of thirst were significantly higher for imagining thirst compared with imagining drinking or baseline, revealing a successful dissociation of thirst from underlying physiology. The imagine thirst condition activated brain regions similar to those reported in previous studies of physiologically evoked thirst, including the anterior midcingulate cortex (aMCC), anterior insula, precentral gyrus, inferior frontal gyrus, middle frontal gyrus, and operculum, indicating a similar neural network underlies both imagined thirst and physiologically evoked thirst. Analogous brain regions were also activated during imagined drinking, suggesting the neural representation of thirst contains a drinking-related component. Finally, the aMCC showed an increase in functional connectivity with the insula during imagined thirst relative to imagined drinking, implying functional connectivity between these two regions is needed before thirst can be experienced. As a result of these findings, this study provides important insight into how the neural representation of subjective thirst is generated and how it subsequently motivates drinking behavior.

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