Neural dynamics in response to binary taste mixtures

Gustatory cortex neurons perform reliability-dependent integration of multisensory flavor inputs

Flavor is the quintessential multisensory experience, combining gustatory, retronasal olfactory, and texture qualities to inform food perception and consumption behavior. However, the computations that govern multisensory integration of flavor components and their underlying neural mechanisms remain elusive. Here, we use rats as a model system to test the hypothesis that taste and smell components of flavor are integrated in a reliability-dependent manner to inform hedonic judgments and that this computation is performed by neurons in the primary taste cortex. Using a series of two-bottle preference tests, we demonstrate that hedonic judgments of taste + smell mixtures are a weighted average of the component judgments, and that the weight of the components depends on their relative reliability. Using extracellular recordings of single-neuron spiking and local field potential activity in combination with decoding analysis, we reveal a correlate of this computation in gustatory cortex (GC). GC neurons weigh bimodal taste and smell inputs based on their reliability, with more reliable inputs contributing more strongly to taste + smell mixture responses. Input reliability was associated with less variable responses and stronger network-level synchronization in the gamma band. Together, our findings establish a quantitative framework for understanding hedonic multisensory flavor judgments and identify the neural computations that underlie them.

Neuronal Sequences and dynamic coding of water-sucrose categorization in rat gustatory cortices

The gustatory system allows us to perceive and distinguish sweetness from water. We studied this phenomenon by recording neural activity in rats' anterior insular (aIC) and orbitofrontal (OFC) cortices while they categorized varying sucrose concentrations against water. Neurons in both aIC and OFC encoded the categorical distinction between sucrose and water rather than specific sucrose concentrations. Notably, aIC encoded this distinction faster than OFC. Conversely, the OFC slightly preceded the aIC in representing choice information, although both cortices encoded the rat's choices in parallel. Further analyses revealed dynamic and sequential encoding of sensory and categorical decisions, forming brief sequences of encoding neurons throughout the trial rather than long-lasting neuronal representations. Our findings, supported by single-cell, population decoding, and principal-component analysis (PCA), demonstrate that gustatory cortices employ neuronal sequences to compute sensorimotor transformations, from taste detection to categorical decisions, and continuously update this process as new taste information emerges using dynamic coding.

Learning enhances representations of taste-guided decisions in the mouse gustatory insular cortex

Learning to discriminate overlapping gustatory stimuli that predict distinct outcomes-a feat known as discrimination learning-can mean the difference between ingesting a poison or a nutritive meal. Despite the obvious importance of this process, very little is known about the neural basis of taste discrimination learning. In other sensory modalities, this form of learning can be mediated by either the sharpening of sensory representations or the enhanced ability of "decision-making" circuits to interpret sensory information. Given the dual role of the gustatory insular cortex (GC) in encoding both sensory and decision-related variables, this region represents an ideal site for investigating how neural activity changes as animals learn a novel taste discrimination. Here, we present results from experiments relying on two-photon calcium imaging of GC neural activity in mice performing a taste-guided mixture discrimination task. The task allows for the recording of neural activity before and after learning induced by training mice to discriminate increasingly similar pairs of taste mixtures. Single-neuron and population analyses show a time-varying pattern of activity, with early sensory responses emerging after taste delivery and binary, choice-encoding responses emerging later in the delay before a decision is made. Our results demonstrate that, while both sensory and decision-related information is encoded by GC in the context of a taste mixture discrimination task, learning and improved performance are associated with a specific enhancement of decision-related responses.

Stereotyped goal-directed manifold dynamics in the insular cortex

The insular cortex is involved in diverse processes, including bodily homeostasis, emotions, and cognition. However, we lack a comprehensive understanding of how it processes information at the level of neuronal populations. We leveraged recent advances in unsupervised machine learning to study insular cortex population activity patterns (i.e., neuronal manifold) in mice performing goal-directed behaviors. We find that the insular cortex activity manifold is remarkably consistent across different animals and under different motivational states. Activity dynamics within the neuronal manifold are highly stereotyped during rewarded trials, enabling robust prediction of single-trial outcomes across different mice and across various natural and artificial motivational states. Comparing goal-directed behavior with self-paced free consumption, we find that the stereotyped activity patterns reflect task-dependent goal-directed reward anticipation, and not licking, taste, or positive valence. These findings reveal a core computation in insular cortex that could explain its involvement in pathologies involving aberrant motivations.

Learning enhances representations of taste-guided decisions in the mouse gustatory insular cortex

Learning to discriminate overlapping gustatory stimuli that predict distinct outcomes - a feat known as discrimination learning - can mean the difference between ingesting a poison or a nutritive meal. Despite the obvious importance of this process, very little is known on the neural basis of taste discrimination learning. In other sensory modalities, this form of learning can be mediated by either sharpening of sensory representations, or enhanced ability of "decision-making" circuits to interpret sensory information. Given the dual role of the gustatory insular cortex (GC) in encoding both sensory and decision-related variables, this region represents an ideal site for investigating how neural activity changes as animals learn a novel taste discrimination. Here we present results from experiments relying on two photon calcium imaging of GC neural activity in mice performing a taste-guided mixture discrimination task. The task allows for recording of neural activity before and after learning induced by training mice to discriminate increasingly similar pairs of taste mixtures. Single neuron and population analyses show a time-varying pattern of activity, with early sensory responses emerging after taste delivery and binary, choice encoding responses emerging later in the delay before a decision is made. Our results demonstrate that while both sensory and decision-related information is encoded by GC in the context of a taste mixture discrimination task, learning and improved performance are associated with a specific enhancement of decision-related responses.

Multisensory integration of orally-sourced gustatory and olfactory inputs to the posterior piriform cortex in awake rats

Flavour refers to the sensory experience of food, which is a combination of sensory inputs sourced from multiple modalities during consumption, including taste and odour. Previous work has demonstrated that orally-sourced taste and odour cues interact to determine perceptual judgements of flavour stimuli, although the underlying cellular- and circuit-level neural mechanisms remain unknown. We recently identified a region of the piriform olfactory cortex in rats that responds to both taste and odour stimuli. Here, we investigated how converging taste and odour inputs to this area interact to affect single neuron responsiveness ensemble coding of flavour identity. To accomplish this, we recorded spiking activity from ensembles of single neurons in the posterior piriform cortex (pPC) in awake, tasting rats while delivering taste solutions, odour solutions and taste + odour mixtures directly into the oral cavity. Our results show that taste and odour inputs evoke highly selective, temporally-overlapping responses in multisensory pPC neurons. Comparing responses to mixtures and their unisensory components revealed that taste and odour inputs interact in a non-linear manner to produce unique response patterns. Taste input enhances trial-by-trial decoding of odour identity from small ensembles of simultaneously recorded neurons. Together, these results demonstrate that taste and odour inputs to pPC interact in complex, non-linear ways to form amodal flavour representations that enhance identity coding. KEY POINTS: Experience of food involves taste and smell, although how information from these different senses is combined by the brain to create our sense of flavour remains unknown. We recorded from small groups of neurons in the olfactory cortex of awake rats while they consumed taste solutions, odour solutions and taste + odour mixtures. Taste and smell solutions evoke highly selective responses. When presented in a mixture, taste and smell inputs interacted to alter responses, resulting in activation of unique sets of neurons that could not be predicted by the component responses. Synergistic interactions increase discriminability of odour representations. The olfactory cortex uses taste and smell to create new information representing multisensory flavour identity.

Activation of D1/D5 Receptors Ameliorates Decreased Intrinsic Excitability of Hippocampal Neurons Induced by Neonatal Blockade of NMDA Receptors

BACKGROUND AND PURPOSE

Dysregulation of dopaminergic transmission combined with transient hypofunction of N-methyl-D-aspartate receptors (NMDARs) is a key mechanism that may underlie cognitive symptoms of schizophrenia.

EXPERIMENTAL APPROACH

Therefore, we aimed to identify electrophysiologic alterations in animals neonatally treated with the NMDA receptor antagonist, MK-801 or with saline solution.

KEY RESULTS

Patch-clamp whole-cell recordings from MK-801-treated animals revealed altered passive and active electrophysiologic properties compared with CA1 pyramidal cells from saline-treated animals, including upregulation of the K⁺ inward-rectifier conductance and fast-inactivating and slow/non-inactivating K⁺ currents. Upregulation of these membrane ionic currents reduced the overall excitability and altered the firing properties of CA1 pyramidal cells. We also explored the capability of cells treated with MK-801 to express intrinsic excitability potentiation, a non-synaptic form of hippocampal plasticity associated with cognition and memory formation. CA1 pyramidal cells from animals treated with MK-801 were unable to convey intrinsic excitability potentiation and had blunted synaptic potentiation. Furthermore, MK-801-treated animals also exhibited reduced cognitive performance in the Barnes maze task. Notably, activation of D1/D5 receptors with SKF-38, 393 partially restored electrophysiologic alterations caused by neonatal treatment with MK-801.

CONCLUSION AND IMPLICATIONS

Our results offer a molecular and mechanistic explanation based on dysregulation of glutamatergic in addition to dopaminergic transmission that may contribute to the understanding of the cognitive deterioration associated with schizophrenia.

Perturbation of amygdala-cortical projections reduces ensemble coherence of palatability coding in gustatory cortex

Taste palatability is centrally involved in consumption decisions-we ingest foods that taste good and reject those that don't. Gustatory cortex (GC) and basolateral amygdala (BLA) almost certainly work together to mediate palatability-driven behavior, but the precise nature of their interplay during taste decision-making is still unknown. To probe this issue, we discretely perturbed (with optogenetics) activity in rats' BLA→GC axons during taste deliveries. This perturbation strongly altered GC taste responses, but while the perturbation itself was tonic (2.5 s), the alterations were not-changes preferentially aligned with the onset times of previously-described taste response epochs, and reduced evidence of palatability-related activity in the 'late-epoch' of the responses without reducing the amount of taste identity information available in the 'middle epoch.' Finally, BLA→GC perturbations changed behavior-linked taste response dynamics themselves, distinctively diminishing the abruptness of ensemble transitions into the late epoch. These results suggest that BLA 'organizes' behavior-related GC taste dynamics.

Multisensory interactions underlying flavor consumption in rats: the role of experience and unisensory component liking

The mechanisms by which taste and odor are combined in determining food choice behavior are poorly understood. Previous work in human subjects has yielded mixed results, potentially due to differences in task context across studies, and a lack of control over flavor experience. Here, we used rats as a model system to systematically investigate the role of experience and unisensory component liking in the multisensory interactions underlying consumption behavior. We demonstrate that taste–smell mixture consumption is best explained by a linear average of component liking. The observed pattern of results was not dependent on prior experience with specific taste–smell combinations, and unique for multisensory as opposed to unisensory mixture consumption. The results are discussed with respect to existing models of flavor integration, and a maximum-likelihood integration model previously described for multisensory judgments in other systems.

Adaptive weighting of taste and odor cues during flavor choice

Colloquially referred to as "taste," flavor is in reality a thoroughly multisensory experience. Yet, a mechanistic understanding of the multisensory computations underlying flavor perception and food choice is lacking. Here, we used a multisensory flavor choice task in rats to test specific predictions of the statistically optimal integration framework, which has previously yielded much insight into cue integration in other multisensory systems. Our results confirm three key predictions of this framework in the unique context of flavor choice behavior, providing novel mechanistic insight into multisensory flavor processing.NEW & NOTEWORTHY The authors demonstrate that rats make choices about which flavor solution (i.e., taste-odor mixture) to consume by weighting the individual taste and odor components according to the reliability of the information they provide about which solution is the preferred one. A similar weighting operation underlies multisensory cue combination in other domains and offers novel insight into the computations underlying multisensory flavor perception and food choice behavior.

Chemospecific deficits in taste sensitivity following bilateral or right hemispheric gustatory cortex lesions in rats

Our prior studies showed bilateral gustatory cortex (GC) lesions significantly impair taste sensitivity to salts (NaCl and KCl) and quinine ("bitter") but not to sucrose ("sweet"). The range of qualitative tastants tested here has been extended in a theoretically relevant way to include the maltodextrin, Maltrin, a preferred stimulus by rats thought to represent a unique taste quality, and the "sour" stimulus citric acid; NaCl was also included as a positive control. Male rats (Sprague-Dawley) with histologically confirmed neurotoxin-induced bilateral (BGCX, n = 13), or right (RGCX, n = 13) or left (LGCX, n = 9) unilateral GC lesions and sham-operated controls (SHAM, n = 16) were trained to discriminate a tastant from water in an operant two-response detection task. A mapping system was used to determine placement, size, and symmetry (when bilateral) of the lesion. BGCX significantly impaired taste sensitivity to NaCl, as expected, but not to Maltrin or citric acid, emulating our prior results with sucrose. However, in the case of citric acid, there was some disruption in performance at higher concentrations. Interestingly, RGCX, but not LGCX, also significantly impaired taste sensitivity, but only to NaCl, suggesting some degree of lateralized function. Taken together with our prior findings, extensive bilateral lesions in GC do not disrupt basic taste signal detection to all taste stimuli uniformly. Moreover, GC lesions do not preclude the ability of rats to learn and perform the task, clearly demonstrating that, in its absence, other brain regions are able to maintain sensory-discriminative taste processing, albeit with attenuated sensitivity for select stimuli.

The neuroscience of sugars in taste, gut-reward, feeding circuits, and obesity

Throughout the animal kingdom sucrose is one of the most palatable and preferred tastants. From an evolutionary perspective, this is not surprising as it is a primary source of energy. However, its overconsumption can result in obesity and an associated cornucopia of maladies, including type 2 diabetes and cardiovascular disease. Here we describe three physiological levels of processing sucrose that are involved in the decision to ingest it: the tongue, gut, and brain. The first section describes the peripheral cellular and molecular mechanisms of sweet taste identification that project to higher brain centers. We argue that stimulation of the tongue with sucrose triggers the formation of three distinct pathways that convey sensory attributes about its quality, palatability, and intensity that results in a perception of sweet taste. We also discuss the coding of sucrose throughout the gustatory pathway. The second section reviews how sucrose, and other palatable foods, interact with the gut-brain axis either through the hepatoportal system and/or vagal pathways in a manner that encodes both the rewarding and of nutritional value of foods. The third section reviews the homeostatic, hedonic, and aversive brain circuits involved in the control of food intake. Finally, we discuss evidence that overconsumption of sugars (or high fat diets) blunts taste perception, the post-ingestive nutritional reward value, and the circuits that control feeding in a manner that can lead to the development of obesity.

Masking the Detection of Taste Stimuli in Rats: NaCl and Sucrose

While psychophysical and neurophysiological assessments of taste sensitivity to single chemical compounds have revealed some fundamental properties of gustatory processing, taste stimuli are rarely ingested in isolation. Arguably, the gustatory system was adapted to identify and report the presence of numerous chemicals ingested concurrently. To begin systematically exploring the detectability of a target stimulus in a background in rodents, we used a gustometer to train rats in a 2-response operant task to detect either NaCl (n = 8) or sucrose (n = 8) dissolved in water, and then tested the sensitivity of rats to the trained NaCl stimulus dissolved in a sucrose masker (0.3, 0.6, or 1.0 M, tested consecutively) versus sucrose, or the trained sucrose stimulus dissolved in a NaCl masker (0.04, 0.2, or 0.4 M) versus NaCl. Detection thresholds (EC50 values) were determined for the target stimulus dissolved in each concentration of the masker. Except for 0.04 M NaCl, all masker concentrations tested increased the target stimulus EC50. Target stimulus detectability decreased systematically as masker concentrations increased. The shift in liminal sensitivity for either target was similar when the threshold for the masker was considered. At least for these prototypical stimuli, it appears that the attenuating impact of a masker on the detection of a target stimulus depends on sensitivity to the masking stimulus. Further study will be required to generalize these results and extend them to more complex maskers, and to discern neural circuits involved in the detection of specific taste signals in the context of noisy backgrounds.

Synaptic Integration of Thalamic and Limbic Inputs in Rodent Gustatory Cortex

Neurons in the gustatory cortex (GC) process multiple aspects of a tasting experience, encoding not only the physiochemical identity of tastes, but also their anticipation and hedonic value. Information pertaining to these stimulus features is relayed to GC via the gustatory thalamus (VPMpc) and basolateral amygdala (BLA). It is not known whether these inputs drive separate groups of neurons, thus activating separate channels of information, or are integrated by neurons that receive both afferents. Here, we used anterograde labeling and in vivo intracellular recordings in anesthetized rats to assess the potential convergence of BLA and VPMpc inputs in GC, and to investigate the dynamics of integration of these inputs. We report substantial anatomic overlap of BLA and VPMpc axonal fields across GC, and identify a population of GC neurons receiving converging BLA and VPMpc inputs. Our data show that BLA modulates the gain of VPMpc-evoked responses in a time-dependent fashion and that this modulation is dependent on the recruitment of synaptic inhibition by both BLA and VPMpc. Our results suggest that BLA shapes cortical processing of thalamic inputs by dynamically gating the excitatory/inhibitory balance of the GC circuit.

Single and population coding of taste in the gustatory cortex of awake mice

Electrophysiological analysis has revealed much about the broad coding and neural ensemble dynamics that characterize gustatory cortical (GC) taste processing in awake rats and about how these dynamics relate to behavior. With regard to mice, however, data concerning cortical taste coding have largely been restricted to imaging, a technique that reveals average levels of neural responsiveness but that (currently) lacks the temporal sensitivity necessary for evaluation of fast response dynamics; furthermore, the few extant studies have thus far failed to provide consensus on basic features of coding. We have recorded the spiking activity of ensembles of GC neurons while presenting representatives of the basic taste modalities (sweet, salty, sour, and bitter) to awake mice. Our first central result is the identification of similarities between rat and mouse taste processing: most mouse GC neurons (~66%) responded distinctly to multiple (3-4) tastes; temporal coding analyses further reveal, for the first time, that single mouse GC neurons sequentially code taste identity and palatability, the latter responses emerging ~0.5 s after the former, with whole GC ensembles transitioning suddenly and coherently from coding taste identity to coding taste palatability. The second finding is that spatial location plays very little role in any aspect of taste responses: neither between- (anterior-posterior) nor within-mouse (dorsal-ventral) mapping revealed anatomic regions with narrow or temporally simple taste responses. These data confirm recent results showing that mouse cortical taste responses are not "gustotopic" but also go beyond these imaging results to show that mice process tastes through time.NEW & NOTEWORTHY Here, we analyzed taste-related spiking activity in awake mouse gustatory cortical (GC) neural ensembles, revealing deep similarities between mouse cortical taste processing and that repeatedly demonstrated in rat: mouse GC ensembles code multiple aspects of taste in a coarse-coded, time-varying manner that is essentially invariant across the spatial extent of GC. These data demonstrate that, contrary to some reports, cortical network processing is distributed, rather than being separated out into spatial subregion.

Impact of precisely-timed inhibition of gustatory cortex on taste behavior depends on single-trial ensemble dynamics

Sensation and action are necessarily coupled during stimulus perception - while tasting, for instance, perception happens while an animal decides to expel or swallow the substance in the mouth (the former via a behavior known as 'gaping'). Taste responses in the rodent gustatory cortex (GC) span this sensorimotor divide, progressing through firing-rate epochs that culminate in the emergence of action-related firing. Population analyses reveal this emergence to be a sudden, coherent and variably-timed ensemble transition that reliably precedes gaping onset by 0.2-0.3s. Here, we tested whether this transition drives gaping, by delivering 0.5s GC perturbations in tasting trials. Perturbations significantly delayed gaping, but only when they preceded the action-related transition - thus, the same perturbation impacted behavior or not, depending on the transition latency in that particular trial. Our results suggest a distributed attractor network model of taste processing, and a dynamical role for cortex in driving motor behavior.

Interaction of Taste and Place Coding in the Hippocampus

An animal's survival depends on finding food and the memory of food and contexts are often linked. Given that the hippocampus is required for spatial and contextual memory, it is reasonable to expect related coding of space and food stimuli in hippocampal neurons. However, relatively little is known about how the hippocampus responds to tastes, the most central sensory property of food. In this study, we examined the taste-evoked responses and spatial firing properties of single units in the dorsal CA1 hippocampal region as male rats received a battery of taste stimuli differing in both chemical composition and palatability within a specific spatial context. We identified a subset of hippocampal neurons that responded to tastes, some of which were place cells. These taste and place responses had a distinct interaction: taste-responsive cells tended to have less spatially specific firing fields and place cells only responded to tastes delivered inside their place field. Like neurons in the amygdala and lateral hypothalamus, hippocampal neurons discriminated between tastes predominantly on the basis of palatability, with taste selectivity emerging concurrently with palatability-relatedness; these responses did not reflect movement or arousal. However, hippocampal taste responses emerged several hundred milliseconds later than responses in other parts of the taste system, suggesting that the hippocampus does not influence real-time taste decisions, instead associating the hedonic value of tastes with a particular context. This incorporation of taste responses into existing hippocampal maps could be one way that animals use past experience to locate food sources.SIGNIFICANCE STATEMENT Finding food is essential for animals' survival and taste and context memory are often linked. Although hippocampal responses to space and contexts have been well characterized, little is known about how the hippocampus responds to tastes. Here, we identified a subset of hippocampal neurons that discriminated between tastes based on palatability. Cells with stronger taste responses typically had weaker spatial responses and taste responses were confined to place cells' firing fields. Hippocampal taste responses emerged later than in other parts of the taste system, suggesting that the hippocampus does not influence taste decisions, but rather associates the hedonic value of tastes consumed within a particular context. This could be one way that animals use past experience to locate food sources.

Retronasal Odor Perception Requires Taste Cortex, but Orthonasal Does Not

Smells can arise from a source external to the body and stimulate the olfactory epithelium upon inhalation through the nares (orthonasal olfaction). Alternatively, smells may arise from inside the mouth during consumption, stimulating the epithelium upon exhalation (retronasal olfaction). Both ortho- and retronasal olfaction produce highly salient percepts, but the two percepts have very different behavioral implications. Here, we use optogenetic manipulation in the context of a flavor preference learning paradigm to investigate differences in the neural circuits that process information in these two submodalities of olfaction. Our findings support a view in which retronasal, but not orthonasal, odors share processing circuitry commonly associated with taste. First, our behavioral results reveal that retronasal odors induce rapid preference learning and have a potentiating effect on orthonasal preference learning. Second, we demonstrate that inactivation of the insular gustatory cortex selectively impairs expression of retronasal preferences. Thus, orally sourced (retronasal) olfactory input is processed by a brain region responsible for taste processing, whereas externally sourced (orthonasal) olfactory input is not.

Sucrose intensity coding and decision-making in rat gustatory cortices

Sucrose's sweet intensity is one attribute contributing to the overconsumption of high-energy palatable foods. However, it is not known how sucrose intensity is encoded and used to make perceptual decisions by neurons in taste-sensitive cortices. We trained rats in a sucrose intensity discrimination task and found that sucrose evoked a widespread response in neurons recorded in posterior-Insula (pIC), anterior-Insula (aIC), and Orbitofrontal cortex (OFC). Remarkably, only a few Intensity-selective neurons conveyed the most information about sucrose's intensity, indicating that for sweetness the gustatory system uses a compact and distributed code. Sucrose intensity was encoded in both firing-rates and spike-timing. The pIC, aIC, and OFC neurons tracked movement direction, with OFC neurons yielding the most robust response. aIC and OFC neurons encoded the subject's choices, whereas all three regions tracked reward omission. Overall, these multimodal areas provide a neural representation of perceived sucrose intensity, and of task-related information underlying perceptual decision-making.

The role of the gustatory cortex in incidental experience-evoked enhancement of later taste learning

The strength of learned associations between pairs of stimuli is affected by multiple factors, the most extensively studied of which is prior experience with the stimuli themselves. In contrast, little data is available regarding how experience with "incidental" stimuli (independent of any conditioning situation) impacts later learning. This lack of research is striking given the importance of incidental experience to survival. We have recently begun to fill this void using conditioned taste aversion (CTA), wherein an animal learns to avoid a taste that has been associated with malaise. We previously demonstrated that incidental exposure to salty and sour tastes (taste preexposure-TPE) enhances aversions learned later to sucrose. Here, we investigate the neurobiology underlying this phenomenon. First, we use immediate early gene (c-Fos) expression to identify gustatory cortex (GC) as a site at which TPE specifically increases the neural activation caused by taste-malaise pairing (i.e., TPE did not change c-Fos induced by either stimulus in isolation). Next, we use site-specific infection with the optical silencer Archaerhodopsin-T to show that GC inactivation during TPE inhibits the expected enhancements of both learning and CTA-related c-Fos expression, a full day later. Thus, we conclude that GC is almost certainly a vital part of the circuit that integrates incidental experience into later associative learning.

Overlapping Representation of Primary Tastes in a Defined Region of the Gustatory Cortex

Both physiological and imaging approaches have led to often-disparate conclusions about the organization of taste information in gustatory cortex (GC). In this study, we used neuroanatomical and imaging approaches to delineate the likely area of insular cortex given to gustatory function and to characterize taste responses within this delineated area in female and male C57BL/6J mice. Anterograde tracers were injected into the taste thalamus (the medial parvicellular portion of the ventral posterior medial division, VPMpc) of mice and the thalamic terminal field was investigated across the cortex. Working within the delineated area, we used two-photon imaging to measure basic taste responses in >780 neurons in layer 2/3 located just posterior to the middle cerebral artery. A nonbiased, hierarchical cluster analysis revealed multiple clusters of cells responding best to either individual or combinations of taste stimuli. Taste quality was represented in the activity of taste-responsive cells; however, there was no apparent spatial organization of primary taste qualities in this region.SIGNIFICANCE STATEMENT Recent studies investigating taste coding within the gustatory cortex have reported highly segregated, taste-specific regions containing only narrowly tuned cells responding to a single taste separated by large non-taste-coding areas. However, focusing on the center of this area, we found a large number of taste responsive cells ranging from narrowly to broadly responsive with no apparent local spatial organization. Further, population analysis reveals that activity in the neuronal population in this area appears to be related to measures of taste quality or hedonics.

Overlapping Representation of Primary Tastes in a Defined Region of the Gustatory Cortex

Both physiological and imaging approaches have led to often-disparate conclusions about the organization of taste information in gustatory cortex (GC). In this study, we used neuroanatomical and imaging approaches to delineate the likely area of insular cortex given to gustatory function and to characterize taste responses within this delineated area in female and male C57BL/6J mice. Anterograde tracers were injected into the taste thalamus (the medial parvicellular portion of the ventral posterior medial division, VPMpc) of mice and the thalamic terminal field was investigated across the cortex. Working within the delineated area, we used two-photon imaging to measure basic taste responses in >780 neurons in layer 2/3 located just posterior to the middle cerebral artery. A nonbiased, hierarchical cluster analysis revealed multiple clusters of cells responding best to either individual or combinations of taste stimuli. Taste quality was represented in the activity of taste-responsive cells; however, there was no apparent spatial organization of primary taste qualities in this region.SIGNIFICANCE STATEMENT Recent studies investigating taste coding within the gustatory cortex have reported highly segregated, taste-specific regions containing only narrowly tuned cells responding to a single taste separated by large non-taste-coding areas. However, focusing on the center of this area, we found a large number of taste responsive cells ranging from narrowly to broadly responsive with no apparent local spatial organization. Further, population analysis reveals that activity in the neuronal population in this area appears to be related to measures of taste quality or hedonics.

Sensory Cortical Activity Is Related to the Selection of a Rhythmic Motor Action Pattern

Rats produce robust, highly distinctive orofacial rhythms in response to taste stimuli-responses that aid in the consumption of palatable tastes and the ejection of aversive tastes, and that are sourced in a multifunctional brainstem central pattern generator. Several pieces of indirect evidence suggest that primary gustatory cortex (GC) may be a part of a distributed forebrain circuit involved in the selection of particular consumption-related rhythms, although not in the production of individual mouth movements per se. Here, we performed a series of tests of this hypothesis. We first examined the temporal relationship between GC activity and orofacial behaviors by performing paired single-neuron and electromyographic recordings in awake rats. Using a trial-by-trial analysis, we found that a subset of GC neurons shows a burst of activity beginning before the transition between nondistinct and taste-specific (i.e., consumption-related) orofacial rhythms. We further showed that shifting the latency of consumption-related behavior by selective cueing has an analogous impact on the timing of GC activity. Finally, we showed the complementary result, demonstrating that optogenetic perturbation of GC activity has a modest but significant impact on the probability that a specific rhythm will be produced in response to a strongly aversive taste. GC appears to be a part of a distributed circuit that governs the selection of taste-induced orofacial rhythms.

SIGNIFICANCE STATEMENT

In many well studied (typically invertebrate) sensorimotor systems, top-down modulation helps motor-control regions "select" movement patterns. Here, we provide evidence that gustatory cortex (GC) may be part of the forebrain circuit that performs this function in relation to oral behaviors ("gapes") whereby a substance in the mouth is rejected as unpalatable. We show that GC palatability coding is well timed to play this role, and that the latency of these codes changes as the latency of gaping shifts with learning. We go on to show that by silencing these neurons, we can change the likelihood of gaping. These data help to break down the sensory/motor divide by showing a role for sensory cortex in the selection of motor behavior.

Sweetener Intake by Rats Selectively Bred for Differential Saccharin Intake: Sucralose, Stevia, and Acesulfame Potassium

Behavioral responses to sweeteners have been used to study the evolution, mechanisms, and functions of taste. Occidental low and high saccharin consuming rats (respectively, LoS and HiS) have been selectively outbred on the basis of saccharin intake and are a valuable tool for studying variation among individuals in sweetener intake and its correlates. Relative to HiS rats, LoS rats consume smaller amounts of all nutritive and nonnutritive sweeteners tested to date, except aspartame. The lines also differ in intake of the commercial product Splenda; the roles of sucralose and saccharides in the difference are unclear. The present study extends prior work by examining intake of custom mixtures of sucralose, maltodextrin, and sugars and Splenda by LoS and HiS rats (Experiment 1A-1D), stevia and a constituent compound (rebaudioside A; Experiment 2A-2E), and acesulfame potassium tested at several concentrations or with 4 other sweeteners at one concentration each (Experiment 3A-3B). Results indicate that aversive side tastes limit intake of Splenda, stevia, and acesulfame potassium, more so among LoS rats than among HiS rats. In addition, regression analyses involving 5 sweeteners support the idea that both sweetness and bitterness are needed to account for intake of nonnutritive sweeteners, more so among LoS rats. These findings contribute to well developed and emerging literatures on sweetness and domain-general processes related to gustation.

Single-neuron responses to intraoral delivery of odor solutions in primary olfactory and gustatory cortex

Smell plays a major role in our perception of food. Odorants released inside the mouth during consumption are combined with taste and texture qualities of a food to guide flavor preference learning and food choice behavior. Here, we built on recent physiological findings that implicated primary sensory cortex in multisensory flavor processing. Specifically, we used extracellular recordings in awake rats to characterize responses of single neurons in primary olfactory (OC) and gustatory cortex (GC) to intraoral delivery of odor solutions and compare odor responses to taste and plain water responses. The data reveal responses to olfactory, oral somatosensory, and gustatory qualities of intraoral stimuli in both OC and GC. Moreover, modality-specific responses overlap in time, indicating temporal convergence of multisensory, flavor-related inputs. The results extend previous work suggesting a role for primary OC in mediating influences of taste on smell that characterize flavor perception and point to an integral role for GC in olfactory processing.NEW & NOTEWORTHY Food perception is inherently multisensory, taking into account taste, smell, and texture qualities. However, the neural mechanisms underlying flavor perception remain unknown. Recording neural activity directly from the rat brain while animals consume multisensory flavor stimuli, we demonstrate that information about odor, taste, and mouthfeel of food converges on primary taste and smell cortex. The results suggest that processing of naturalistic, multisensory information involves an interacting network of primary sensory areas.

Orosensory contributions to dysphagia: a link between perception of sweet and sour taste and pharyngeal delay time

Pharyngeal delay is a significant swallowing disorder often resulting in aspiration. It is suspected that pharyngeal delay originates from sensory impairment, but a direct demonstration of a link between oral sensation and pharyngeal delay is lacking. In this study involving six patients with complaints of dysphagia, taste sensation of the oral tongue was measured and subsequently related to swallowing kinematics. It was found that a response bias for sour taste was significantly correlated with pharyngeal delay time on paste, highlighting oral sensory contributions to swallow motor dysfunctions. Investigating the precise nature of such a link between oral sensation and dysphagia would constitute a basis for understanding the disorder. The results of this study highlight oral sensory contributions to pharyngeal swallow events and provide impetus to examine this link in larger samples of dysphagic patients.

The Behavioral Relevance of Cortical Neural Ensemble Responses Emerges Suddenly

Whereas many laboratory-studied decisions involve a highly trained animal identifying an ambiguous stimulus, many naturalistic decisions do not. Consumption decisions, for instance, involve determining whether to eject or consume an already identified stimulus in the mouth and are decisions that can be made without training. By standard analyses, rodent cortical single-neuron taste responses come to predict such consumption decisions across the 500 ms preceding the consumption or rejection itself; decision-related firing emerges well after stimulus identification. Analyzing single-trial ensemble activity using hidden Markov models, we show these decision-related cortical responses to be part of a reliable sequence of states (each defined by the firing rates within the ensemble) separated by brief state-to-state transitions, the latencies of which vary widely between trials. When we aligned data to the onset of the (late-appearing) state that dominates during the time period in which single-neuron firing is correlated to taste palatability, the apparent ramp in stimulus-aligned choice-related firing was shown to be a much more precipitous coherent jump. This jump in choice-related firing resembled a step function more than it did the output of a standard (ramping) decision-making model, and provided a robust prediction of decision latency in single trials. Together, these results demonstrate that activity related to naturalistic consumption decisions emerges nearly instantaneously in cortical ensembles. Significance statement: This paper provides a description of how the brain makes evaluative decisions. The majority of work on the neurobiology of decision making deals with "what is it?" decisions; out of this work has emerged a model whereby neurons accumulate information about the stimulus in the form of slowly increasing firing rates and reach a decision when those firing rates reach a threshold. Here, we study a different kind of more naturalistic decision--a decision to evaluate "what shall I do with it?" after the identity of a taste in the mouth has been identified--and show that this decision is not made through the gradual increasing of stimulus-related firing, but rather that this decision appears to be made in a sudden moment of "insight."

Dynamic taste responses of parabrachial pontine neurons in awake rats

The parabrachial nuclei of the pons (PbN) receive almost direct input from taste buds on the tongue and control basic taste-driven behaviors. Thus it is reasonable to hypothesize that PbN neurons might respond to tastes in a manner similar to that of peripheral receptors, i.e., that these responses might be narrow and relatively "dynamics free." On the other hand, the majority of the input to PbN descends from forebrain regions such as gustatory cortex (GC), which processes tastes with "temporal codes" in which firing reflects first the presence, then the identity, and finally the desirability of the stimulus. Therefore a reasonable alternative hypothesis is that PbN responses might be dominated by dynamics similar to those observed in GC. Here we examined simultaneously recorded single-neuron PbN (and GC) responses in awake rats receiving exposure to basic taste stimuli. We found that pontine taste responses were almost entirely confined to canonically identified taste-PbN (t-PbN). Taste-specificity was found, furthermore, to be time varying in a larger percentage of these t-PbN responses than in responses recorded from the tissue around PbN (including non-taste-PbN). Finally, these time-varying properties were a good match for those observed in simultaneously recorded GC neurons-taste-specificity appeared after an initial nonspecific burst of action potentials, and palatability emerged several hundred milliseconds later. These results suggest that the pontine taste relay is closely allied with the dynamic taste processing performed in forebrain.

Evolutionary conserved brainstem circuits encode category, concentration and mixtures of taste

Evolutionary conserved brainstem circuits are the first relay for gustatory information in the vertebrate brain. While the brainstem circuits act as our life support system and they mediate vital taste related behaviors, the principles of gustatory computations in these circuits are poorly understood. By a combination of two-photon calcium imaging and quantitative animal behavior in juvenile zebrafish, we showed that taste categories are represented by dissimilar brainstem responses and generate different behaviors. We also showed that the concentration of sour and bitter tastes are encoded by different principles and with different levels of sensitivity. Moreover, we observed that the taste mixtures lead to synergistic and suppressive interactions. Our results suggest that these interactions in early brainstem circuits can result in non-linear computations, such as dynamic gain modulation and discrete representation of taste mixtures, which can be utilized for detecting food items at broad range of concentrations of tastes and rejecting inedible substances.

Extensive Gustatory Cortex Lesions Significantly Impair Taste Sensitivity to KCl and Quinine but Not to Sucrose in Rats

Recently, we reported that large bilateral gustatory cortex (GC) lesions significantly impair taste sensitivity to salts in rats. Here we extended the tastants examined to include sucrose and quinine in rats with ibotenic acid-induced lesions in GC (GCX) and in sham-operated controls (SHAM). Presurgically, immediately after drinking NaCl, rats received a LiCl or saline injection (i.p.), but postsurgical tests indicated a weak conditioned taste aversion (CTA) even in controls. The rats were then trained and tested in gustometers to discriminate a tastant from water in a two-response operant taste detection task. Psychometric functions were derived for sucrose, KCl, and quinine. Our mapping system was used to determine placement, size, and symmetry of the lesions (~91% GC damage on average). For KCl, there was a significant rightward shift (ΔEC50 = 0.57 log10 units; p<0.001) in the GCX psychometric function relative to SHAM, replicating our prior work. There was also a significant lesion-induced impairment (ΔEC50 = 0.41 log10 units; p = 0.006) in quinine sensitivity. Surprisingly, taste sensitivity to sucrose was unaffected by the extensive lesions and was comparable between GCX and SHAM rats. The fact that such large bilateral GC lesions did not shift sucrose psychometric functions relative to SHAM, but did significantly compromise quinine and KCl sensitivity suggests that the neural circuits responsible for the detection of specific taste stimuli are partially dissociable. Lesion-induced impairments were observed in expression of a postsurgical CTA to a maltodextrin solution as assessed in a taste-oriented brief-access test, but were not reflected in a longer term 46-h two-bottle test. Thus, deficits observed in rats after extensive damage to the GC are also dependent on the test used to assess taste function. In conclusion, the degree to which the GC is necessary for the maintenance of normal taste detectability apparently depends on the chemical and/or perceptual features of the stimulus.

A Multisensory Network for Olfactory Processing

Primary gustatory cortex (GC) is connected (both mono- and polysynaptically) to primary olfactory (piriform) cortex (PC)-connections that might be hypothesized to underlie the construction of a "flavor" percept when both gustatory and olfactory stimuli are present. Here we use multisite electrophysiology and optical inhibition of GC neurons (GCx, produced via infection with ArchT) to demonstrate that, indeed, during gustatory stimulation, taste-selective information is transmitted from GC to PC. We go on to show that these connections impact olfactory processing even in the absence of gustatory stimulation: GCx alters PC responses to olfactory stimuli presented alone, enhancing some and eliminating others, despite leaving the path from nasal epithelium to PC intact. Finally, we show the functional importance of this latter phenomenon, demonstrating that GCx renders rats unable to properly recognize odor stimuli. This sequence of findings suggests that sensory processing may be more intrinsically integrative than previously thought.

Extensive lesions in rat insular cortex significantly disrupt taste sensitivity to NaCl and KCl and slow salt discrimination learning

While studies of the gustatory cortex (GC) mostly focus on its role in taste aversion learning and memory, the necessity of GC for other fundamental taste-guided behaviors remains largely untested. Here, rats with either excitotoxic lesions targeting GC (n = 26) or sham lesions (n = 14) were assessed for postsurgical retention of a presurgically LiCl-induced conditioned taste aversion (CTA) to 0.1M sucrose using a brief-access taste generalization test in a gustometer. The same animals were then trained in a two-response operant taste detection task and psychophysically tested for their salt (NaCl or KCl) sensitivity. Next, the rats were trained and tested in a NaCl vs. KCl taste discrimination task with concentrations varied. Rats meeting our histological inclusion criterion had large lesions (resulting in a group averaging 80% damage to GC and involving surrounding regions) and showed impaired postsurgical expression of the presurgical CTA (LiCl-injected, n = 9), demonstrated rightward shifts in the NaCl (0.54 log10 shift) and KCl (0.35 log10 shift) psychometric functions, and displayed retarded salt discrimination acquisition (n = 18), but eventually learned and performed the discrimination comparable to sham-operated animals. Interestingly, the degree of deficit between tasks correlated only modestly, if at all, suggesting that idiosyncratic differences in insular cortex lesion topography were the root of the individual differences in the behavioral effects demonstrated here. This latter finding hints at some degree of interanimal variation in the functional topography of insular cortex. Overall, GC appears to be necessary to maintain normal taste sensitivity to NaCl and KCl and for salt discrimination learning. However, higher salt concentrations can be detected and discriminated by rats with extensive damage to GC suggesting that the other resources of the gustatory system are sufficient to maintain partial competence in these tasks, supporting the view that such basic sensory-discriminative taste functions involve distributed processes among central gustatory structures.

Central role for the insular cortex in mediating conditioned responses to anticipatory cues

Reward-related circuits are fundamental for initiating feeding on the basis of food-predicting cues, whereas gustatory circuits are believed to be involved in the evaluation of food during consumption. However, accumulating evidence challenges such a rigid separation. The insular cortex (IC), an area largely studied in rodents for its role in taste processing, is involved in representing anticipatory cues. Although IC responses to anticipatory cues are well established, the role of IC cue-related activity in mediating feeding behaviors is poorly understood. Here, we examined the involvement of the IC in the expression of cue-triggered food approach in mice trained with a Pavlovian conditioning paradigm. We observed a significant change in neuronal firing during presentation of the cue. Pharmacological silencing of the IC inhibited food port approach. Such a behavior could be recapitulated by temporally selective inactivation during the cue. These findings represent the first evidence, to our knowledge, that cue-evoked neuronal activity in the mouse IC modulates behavioral output, and demonstrate a causal link between cue responses and feeding behaviors.

Unconditioned oromotor taste reactivity elicited by sucrose and quinine is unaffected by extensive bilateral damage to the gustatory zone of the insular cortex in rats

Rats display stereotypical oromotor and somatic responses to small volumes of intraorally infused taste solutions. These behaviors, known as taste reactivity, are categorized by their association with ingestion or rejection and are thought to reflect the palatability of the stimulus. Because supracollicular decerebrate rats display normal taste reactivity responses, it would appear that forebrain structures are not necessary for generating them. However, because moving the plane of transection rostrally, or damaging or manipulating specific ventral forebrain sites disrupts normal taste reactivity behavior, lesions of the gustatory cortex, a region that has been suggested to be involved with palatability processing, may do the same. In the current study, rats received two injections of either ibotenic acid (N=12) or vehicle (N=8), targeting the conventionally defined gustatory cortex in each hemisphere, and were implanted with intraoral cannulae. Following recovery, their responses to intraoral infusions (0.23ml in 1min) of dH2O, sucrose (1.0M and 0.1M), and quinine hydrochloride (3mM and 0.3mM) were video recorded. Analysis of brains with sufficient bilateral lesions (N=10) revealed that, on average, approximately 94% of the gustatory cortex was destroyed. These extensive bilateral lesions had no significant effect on taste reactivity; the numbers of ingestive and aversive responses to sucrose and quinine were similar between groups. Though these findings do not rule out involvement of the gustatory cortex in palatability processing, they make evident that the region of insular cortex destroyed is not necessary for the normal expression of unconditioned affective behavioral responses to taste stimuli.

Sensory cortical population dynamics uniquely track behavior across learning and extinction

Neural responses in many cortical regions encode information relevant to behavior: information that necessarily changes as that behavior changes with learning. Although such responses are reasonably theorized to be related to behavior causation, the true nature of that relationship cannot be clarified by simple learning studies, which show primarily that responses change with experience. Neural activity that truly tracks behavior (as opposed to simply changing with experience) will not only change with learning but also change back when that learning is extinguished. Here, we directly probed for this pattern, recording the activity of ensembles of gustatory cortical single neurons as rats that normally consumed sucrose avidly were trained first to reject it (i.e., conditioned taste aversion learning) and then to enjoy it again (i.e., extinction), all within 49 h. Both learning and extinction altered cortical responses, consistent with the suggestion (based on indirect evidence) that extinction is a novel form of learning. But despite the fact that, as expected, postextinction single-neuron responses did not resemble "naive responses," ensemble response dynamics changed with learning and reverted with extinction: both the speed of stimulus processing and the relationships among ensemble responses to the different stimuli tracked behavioral relevance. These data suggest that population coding is linked to behavior with a fidelity that single-neuron coding is not.

Processing of hedonic and chemosensory features of taste in medial prefrontal and insular networks

Most of the research on cortical processing of taste has focused on either the primary gustatory cortex (GC) or the orbitofrontal cortex (OFC). However, these are not the only areas involved in taste processing. Gustatory information can also reach another frontal region, the medial prefrontal cortex (mPFC), via direct projections from GC. mPFC has been studied extensively in relation to its role in controlling goal-directed action and reward-guided behaviors, yet very little is known about its involvement in taste coding. The experiments presented here address this important point and test whether neurons in mPFC can significantly process the physiochemical and hedonic dimensions of taste. Spiking responses to intraorally delivered tastants were recorded from rats implanted with bundles of electrodes in mPFC and GC. Analysis of single-neuron and ensemble activity revealed similarities and differences between the two areas. Neurons in mPFC can encode the chemosensory identity of gustatory stimuli. However, responses in mPFC are sparser, more narrowly tuned, and have a later onset than in GC. Although taste quality is more robustly represented in GC, taste palatability is coded equally well in the two areas. Additional analysis of responses in neurons processing the hedonic value of taste revealed differences between the two areas in temporal dynamics and sensitivities to palatability. These results add mPFC to the network of areas involved in processing gustatory stimuli and demonstrate significant differences in taste-coding between GC and mPFC.

Processing of hedonic and chemosensory features of taste in medial prefrontal and insular networks

Most of the research on cortical processing of taste has focused on either the primary gustatory cortex (GC) or the orbitofrontal cortex (OFC). However, these are not the only areas involved in taste processing. Gustatory information can also reach another frontal region, the medial prefrontal cortex (mPFC), via direct projections from GC. mPFC has been studied extensively in relation to its role in controlling goal-directed action and reward-guided behaviors, yet very little is known about its involvement in taste coding. The experiments presented here address this important point and test whether neurons in mPFC can significantly process the physiochemical and hedonic dimensions of taste. Spiking responses to intraorally delivered tastants were recorded from rats implanted with bundles of electrodes in mPFC and GC. Analysis of single-neuron and ensemble activity revealed similarities and differences between the two areas. Neurons in mPFC can encode the chemosensory identity of gustatory stimuli. However, responses in mPFC are sparser, more narrowly tuned, and have a later onset than in GC. Although taste quality is more robustly represented in GC, taste palatability is coded equally well in the two areas. Additional analysis of responses in neurons processing the hedonic value of taste revealed differences between the two areas in temporal dynamics and sensitivities to palatability. These results add mPFC to the network of areas involved in processing gustatory stimuli and demonstrate significant differences in taste-coding between GC and mPFC.

Extensive lesions in the gustatory cortex in the rat do not disrupt the retention of a presurgically conditioned taste aversion and do not impair unconditioned concentration-dependent licking of sucrose and quinine

Although damage to gustatory cortex (GC) in the rat has been reported to severely impair, if not eliminate, retention of a presurgically conditioned taste aversion (CTA), it has equivocal effects on taste preference as measured by intake tests. Because intake tests can be influenced by nongustatory (e.g., postingestive) factors, we employed the brief-access taste test to assess the effects of ibotenic acid-induced lesions targeting the GC on unconditioned licking to a sucrose and then a quinine concentration series in a specialized lickometer. As a functional lesion assessment, a presurgical CTA to 0.1M NaCl was established in thirsty rats by following 15-min intake with intraperitoneal administration of either LiCl (or NaCl for control) on 2 occasions. Both conditioned sham-operated (SHAM) rats and rats with histologically confirmed extensive damage to the GC (GCX) avoided a NaCl concentration series relative to unconditioned controls in a postsurgical brief-access CTA test, with no difference between the surgical groups in their responses to NaCl or similar concentrations of KCl. GCX rats also did not differ from SHAM rats in the EC50 of concentration-response functions for sucrose or quinine. Clearly, the critical cortical area required for the retention of a presurgical CTA falls outside of the extensive area of damage, which was well centered within the conventionally defined gustatory zone of the insular cortex. The absence of an effect on unconditioned responsiveness to sucrose and quinine suggests that the damaged region is also unnecessary for the normal expression of affective licking responses to tastants.

Lateral hypothalamus contains two types of palatability-related taste responses with distinct dynamics

The taste of foods, in particular the palatability of these tastes, exerts a powerful influence on our feeding choices. Although the lateral hypothalamus (LH) has long been known to regulate feeding behavior, taste processing in LH remains relatively understudied. Here, we examined single-unit LH responses in rats subjected to a battery of taste stimuli that differed in both chemical composition and palatability. Like neurons in cortex and amygdala, LH neurons produced a brief epoch of nonspecific responses followed by a protracted period of taste-specific firing. Unlike in cortex, however, where palatability-related information only appears 500 ms after the onset of taste-specific firing, taste specificity in LH was dominated by palatability-related firing, consistent with LH's role as a feeding center. Upon closer inspection, taste-specific LH neurons fell reliably into one of two subtypes: the first type showed a reliable affinity for palatable tastes, low spontaneous firing rates, phasic responses, and relatively narrow tuning; the second type showed strongest modulation to aversive tastes, high spontaneous firing rates, protracted responses, and broader tuning. Although neurons producing both types of responses were found within the same regions of LH, cross-correlation analyses suggest that they may participate in distinct functional networks. Our data shed light on the implementation of palatability processing both within LH and throughout the taste circuit, and may ultimately have implications for LH's role in the formation and maintenance of taste preferences and aversions.

Lateral hypothalamus contains two types of palatability-related taste responses with distinct dynamics

The taste of foods, in particular the palatability of these tastes, exerts a powerful influence on our feeding choices. Although the lateral hypothalamus (LH) has long been known to regulate feeding behavior, taste processing in LH remains relatively understudied. Here, we examined single-unit LH responses in rats subjected to a battery of taste stimuli that differed in both chemical composition and palatability. Like neurons in cortex and amygdala, LH neurons produced a brief epoch of nonspecific responses followed by a protracted period of taste-specific firing. Unlike in cortex, however, where palatability-related information only appears 500 ms after the onset of taste-specific firing, taste specificity in LH was dominated by palatability-related firing, consistent with LH's role as a feeding center. Upon closer inspection, taste-specific LH neurons fell reliably into one of two subtypes: the first type showed a reliable affinity for palatable tastes, low spontaneous firing rates, phasic responses, and relatively narrow tuning; the second type showed strongest modulation to aversive tastes, high spontaneous firing rates, protracted responses, and broader tuning. Although neurons producing both types of responses were found within the same regions of LH, cross-correlation analyses suggest that they may participate in distinct functional networks. Our data shed light on the implementation of palatability processing both within LH and throughout the taste circuit, and may ultimately have implications for LH's role in the formation and maintenance of taste preferences and aversions.

Speed and accuracy of taste identification and palatability: impact of learning, reward expectancy, and consummatory licking

Despite decades of study, it remains a matter of controversy as to whether in rats taste identification is a rapid process that occurs in about 250-600 ms (one to three licks) or a slow process that evolves over seconds. To address this issue, we trained rats to perform a taste-cued two-response discrimination task (2-RDT). It was found that, after learning, regardless of intensity, the delivery of 10 μl of a tastant (e.g., NaCl or monopotassium glutamate, MPG) was sufficient to identify its taste with maximal accuracy within 400 ms. However, despite overtraining, rats rarely stopped licking in one lick. Thus, a one-drop lick reaction task was developed in which subjects had to rapidly stop licking after release of a stop signal (tastants including water) to obtain rewards. The faster they stopped licking, the greater the reward. Rats did not stop licking after receiving either hedonically positive or negative stop signals, and thus failed to maximize rewards even when reinforced with even larger rewards. In fact, the higher the sucrose concentration given as a stop signal, the greater the number of consummatory licks elicited. However, with a stop signal of 2 mM quinine HCl, they stopped licking in ~370 ms, a time faster than that for sucrose or water, thus showing that in this rapid period, quinine HCl evoked an unpalatable response. Indeed, only when rats licked an empty sipper tube would they usually elicit a single lick to obtain a reward (operant licking). In summary, these data indicate that within 400 ms, taste identification and palatability, must either occur simultaneously or with marked overlap.