Gustatory responses in macaque monkeys revealed with fMRI: Comments on taste, taste preference, and internal state

The Isle of Craig: Neuroanatomical and Functional Evidence for a Role of the Insular Cortex in Subjective Feelings

At the start of the twenty-first century, Arthur D. (Bud) Craig brought back to the fore the Island of Reil (insular cortex or insula). He did so by following, step by step, with rigor and tenacity, the afferent sensory pathway that informs the forebrain about the ongoing physiological status of the organs and tissues of the body. Along with his demonstration of the existence of a primate-specific ascending interoceptive pathway and his subsequent re-interpretation of Sherrington's concept of interoception, Bud Craig's seminal experiments and profound interpretations led him to make the groundbreaking proposals that the dorsal posterior insular cortex provides an ideal substrate for James's concept of emotional embodiment, that the insular cortex contextualizes interoception across a posterior-to-mid-to-anterior integration with multimodal activities, and that the anterior insular cortex has a crucial role in the evolutionary emergence of the awareness of subjective feelings in humans, for the purpose of optimizing metabolic energy usage. Bud Craig's unique work paves the path for further elucidation of the role of the insula and other brain regions in subjective feelings. His discoveries and proposals rest on implacable attention to neuroanatomical and neurophysiological details and a serendipitous quest for the fundamental evolutionary Logic of Life. This chapter provides a detailed description of the ascending interoceptive pathway and the functional and comparative neuroanatomy of the insular cortex in primates. Building on Bud Craig's work, our recent findings suggest that the primary interoceptive cortex serves as a representation of the spino-solitary-parabrachial neuraxis, merging with posterior-to-mid-anterior and dorsal-to-ventral processing streams that form a latticework integration pattern. At the ventral anterior tip of this integration, the von Economo neuron area closes the corticofugal interoceptive-autonomic loop of the sensory-motor homeostatic system through projections to all brainstem nuclei integrating interoceptive afferences.

Intrinsic functional clustering of the macaque insular cortex

The functional organization of the primate insula has been studied using a variety of techniques focussing on regional differences in either architecture, connectivity, or function. These complementary methods offered insights into the complex organization of the insula and proposed distinct parcellation schemes at varying levels of detail and complexity. The advent of imaging techniques that allow non-invasive assessment of structural and functional connectivity, has popularized data-driven connectivity-based parcellation methods to investigate the organization of the human insula. Yet, it remains unclear if the subdivisions derived from these data-driven clustering methods reflect meaningful descriptions of the functional specialization of the insula. In this study, we employed hierarchical clustering to examine the cluster parcellations of the macaque insula. As our aim was exploratory, we examined parcellations consisting of two up to ten clusters. Three different cluster validation methods (fingerprinting, silhouette, elbow) converged on a four-cluster solution as the most optimal representation of our data. Examining functional response properties of these clusters, in addition to their brain-wide functional connectivity suggested a functional specialization related to processing gustatory, somato-motor, vestibular and social visual cues. However, a more detailed functional differentiation aligning with previous functional investigations of insula subfields became evident at higher cluster numbers beyond the proposed optimal four clusters. Overall, our findings demonstrate that resting-state-based hierarchical clustering can provide a meaningful description of the insula's functional organization at some level of detail. Nonetheless, cluster parcellations derived from this method are best combined with data obtained through other modalities, to provide a more comprehensive and detailed account of the insula's complex functional organization.

Functional characterization of macaque insula using task-based and resting-state fMRI

Neurophysiological investigations over the past decades have demonstrated the involvement of the primate insula in a wide array of sensory, cognitive, affective and regulatory functions, yet the complex functional organization of the insula remains unclear. Here we examined to what extent non-invasive task-based and resting-state fMRI provides support for functional specialization and integration of sensory and motor information in the macaque insula. Task-based fMRI experiments suggested a functional specialization related to processing of ingestive/taste/distaste information in anterior insula, grasping-related sensorimotor responses in middle insula and vestibular information in posterior insula. Visual stimuli depicting social information involving conspecific`s lip-smacking gestures yielded responses in middle and anterior portions of dorsal and ventral insula, overlapping partially with the sensorimotor and ingestive/taste/distaste fields. Functional specialization/integration of the insula was further corroborated by seed-based whole brain resting-state analyses, showing distinct functional connectivity gradients across the anterio-posterior extent of both dorsal and ventral insula. Posterior insula showed functional correlations in particular with vestibular/optic flow network regions, mid-dorsal insula with vestibular/optic flow as well as parieto-frontal regions of the sensorimotor grasping network, mid-ventral insula with social/affiliative network regions in temporal, cingulate and prefrontal cortices and anterior insula with taste and mouth motor networks including premotor and frontal opercular regions.

A Matter of Taste: Roles of Taste Preference on Performance and Psychological Responses during Anaerobic Exercise

Various tastes including sweet, bitter, and sour have been shown to differentially influence physiological and psychological processes. Furthermore, ingestion of bitter and sweet solutions has been shown to acutely enhance exercise performance. However, the taste is highly individualized, and it is unclear if preference influences the ergogenic potential of taste. The purpose of this study was to investigate the effects of preferred and non-preferred drink tastes on anaerobic performance and psychological responses thereof. Physically active females participated in two counterbalanced sprint trials each with a different condition: (1) non-preferred taste (NPT), (2) Preferred taste (PT). Participants self-reported taste preferences (sweet, sour, bitter) with the highest-ranked taste being used for the PT condition and the lowest-ranked for NPT. For each visit, participants completed a 15 s Wingate Anaerobic Test (WAnT) prior to (PRE) ingestion of ~20 mL of their NP or PREF taste. Following ingestion, participants completed 2 min of active recovery, rated their taste preference of the solution, and completed another 15 s WAnT. The rate of perceived exertion (RPE), motivation, and enjoyment were measured through a visual analog scale following each WAnT. Anaerobic performance measures and heart rate (HR) were also obtained at the succession of each WAnT. Findings revealed no differences between taste conditions for mean power (p = 0.455), peak power (p = 0.824), or HR (p = 0.847). RPE was significantly lower with PT versus NPT (p = 0.006). Exercise enjoyment (p = 0.022) was higher with PT compared to NPT. NPT resulted in worse motivation compared to PRE (p = 0.001) while no changes were observed between PT and PRE (p = 0.197). These findings suggest that preferred drink taste may not enhance acute performance but improves psychological responses to maximal anaerobic exercise which may have implications for improving exercise training and adherence.

Association of increased abdominal adiposity at birth with altered ventral caudate microstructure

BACKGROUND

Neonatal adiposity is associated with a higher risk of obesity and cardiometabolic risk factors in later life. It is however unknown if central food intake regulating networks in the ventral striatum are altered with in-utero abdominal growth, indexed by neonatal adiposity in our current study. We aim to examine the relationship between striatal microstructure and abdominal adipose tissue compartments (AATCs) in Asian neonates from the Growing Up in Singapore Toward healthy Outcomes mother-offspring cohort.

STUDY DESIGN

About 109 neonates were included in this study. Magnetic resonance imaging (MRI) was performed for the brain and abdominal regions between 5 to 17 days of life. Diffusion-weighted imaging of the brain was performed for the derivation of caudate and putamen fractional anisotropy (FA). Abdominal imaging was performed to quantify AATCs namely superficial subcutaneous adipose tissue (sSAT), deep subcutaneous adipose tissue (dSAT), and internal adipose tissue (IAT). Absolute and percentage adipose tissue of total abdominal volume (TAV) were calculated.

RESULTS

We showed that AATCs at birth were significantly associated with increased FA in bilateral ventral caudate heads which are part of the ventral striatum (sSAT: β_left = 0.56, p < 0.001; β_right = 0.65, p < 0.001, dSAT: β_left = 0.43, p < 0.001; β_right = 0.52, p < 0.001, IAT: β_left = 0.30, p = 0.005; β_right = 0.32, p = 0.002) in neonates with low birth weights adjusted for gestational age.

CONCLUSIONS

Our study provides preliminary evidence of a potential relationship between neonatal adiposity and in-utero programming of the ventral striatum, a brain structure that governs feeding behavior.

Effective Connectivity in the Human Brain for Sour Taste, Retronasal Smell, and Combined Flavour

The anterior insula and rolandic operculum are key regions for flavour perception in the human brain; however, it is unclear how taste and congruent retronasal smell are perceived as flavours. The multisensory integration required for sour flavour perception has rarely been studied; therefore, we investigated the brain responses to taste and smell in the sour flavour-processing network in 35 young healthy adults. We aimed to characterise the brain response to three stimulations applied in the oral cavity—sour taste, retronasal smell of mango, and combined flavour of both—using functional magnetic resonance imaging. Effective connectivity of the flavour-processing network and modulatory effect from taste and smell were analysed. Flavour stimulation activated middle insula and olfactory tubercle (primary taste and olfactory cortices, respectively); anterior insula and rolandic operculum, which are associated with multisensory integration; and ventrolateral prefrontal cortex, a secondary cortex for flavour perception. Dynamic causal modelling demonstrated that neural taste and smell signals were integrated at anterior insula and rolandic operculum. These findings elucidated how neural signals triggered by sour taste and smell presented in liquid form interact in the brain, which may underpin the neurobiology of food appreciation. Our study thus demonstrated the integration and synergy of taste and smell.

Preferences for nutrients and sensory food qualities identify biological sources of economic values in monkeys

Value is a foundational concept in reinforcement learning and economic choice theory. In these frameworks, individuals choose by assigning values to objects and learn by updating values with experience. These theories have been instrumental for revealing influences of probability, risk, and delay on choices. However, they do not explain how values are shaped by intrinsic properties of the choice objects themselves. Here, we investigated how economic value derives from the biologically critical components of foods: their nutrients and sensory qualities. When monkeys chose nutrient-defined liquids, they consistently preferred fat and sugar to low-nutrient alternatives. Rather than maximizing energy indiscriminately, they seemed to assign subjective values to specific nutrients, flexibly trading them against offered reward amounts. Nutrient-value functions accurately modeled these preferences, predicted choices across contexts, and accounted for individual differences. The monkeys' preferences shifted their daily nutrient balance away from dietary reference points, contrary to ecological foraging models but resembling human suboptimal eating in free-choice situations. To identify the sensory basis of nutrient values, we developed engineering tools that measured food textures on biological surfaces, mimicking oral conditions. Subjective valuations of two key texture parameters-viscosity and sliding friction-explained the monkeys' fat preferences, suggesting a texture-sensing mechanism for nutrient values. Extended reinforcement learning and choice models identified candidate neuronal mechanisms for nutrient-sensitive decision-making. These findings indicate that nutrients and food textures constitute critical reward components that shape economic values. Our nutrient-choice paradigm represents a promising tool for studying food-reward mechanisms in primates to better understand human-like eating behavior and obesity.

Metaplasticity in the Ventral Pallidum as a Potential Marker for the Propensity to Gain Weight in Chronic High-Calorie Diet

A major driver of obesity is the increasing palatability of processed foods. Although reward circuits promote the consumption of palatable food, their involvement in obesity remains unclear. The ventral pallidum (VP) is a key hub in the reward system that encodes the hedonic aspects of palatable food consumption and participates in various proposed feeding circuits. However, there is still no evidence for its involvement in developing diet-induced obesity. Here we examine, using male C57BL6/J mice and patch-clamp electrophysiology, how chronic high-fat high-sugar (HFHS) diet changes the physiology of the VP and whether mice that gain the most weight differ in their VP physiology from others. We found that 10-12 weeks of HFHS diet hyperpolarized and decreased the firing rate of VP neurons without a major change in synaptic inhibitory input. Within the HFHS group, the top 33% weight gainers (WGs) had a more hyperpolarized VP with longer latency to fire action potentials on depolarization compared with bottom 33% of weight gainers (i.e., non-weight gainers). WGs also showed synaptic potentiation of inhibitory inputs both at the millisecond and minute ranges. Moreover, we found that the tendency to potentiate the inhibitory inputs to the VP might exist in overeating mice even before exposure to HFHS, thus making it a potential property of being an overeater. These data point to the VP as a critical player in obesity and suggest that hyperpolarized membrane potential of, and potentiated inhibitory inputs to, VP neurons may play a significant role in promoting the overeating of palatable food.SIGNIFICANCE STATEMENT In modern world, where highly palatable food is readily available, overeating is often driven by motivational, rather than metabolic, needs. It is thus conceivable that reward circuits differ between obese and normal-weight individuals. But is such difference, if it exists, innate or does it develop with overeating? Here we reveal synaptic properties in the ventral pallidum, a central hub of reward circuits, that differ between mice that gain the most and the least weight when given unlimited access to highly palatable food. We show that these synaptic differences also exist without exposure to palatable food, potentially making them innate properties that render some more susceptible than others to overeat. Thus, the propensity to overeat may have a strong innate component embedded in reward circuits.

Reward activity in ventral pallidum tracks satiety-sensitive preference and drives choice behavior

A key function of the nervous system is producing adaptive behavior across changing conditions, like physiological state. Although states like thirst and hunger are known to impact decision-making, the neurobiology of this phenomenon has been studied minimally. Here, we tracked evolving preference for sucrose and water as rats proceeded from a thirsty to sated state. As rats shifted from water choices to sucrose choices across the session, the activity of a majority of neurons in the ventral pallidum, a region crucial for reward-related behaviors, closely matched the evolving behavioral preference. The timing of this signal followed the pattern of a reward prediction error, occurring at the cue or the reward depending on when reward identity was revealed. Additionally, optogenetic stimulation of ventral pallidum neurons at the time of reward was able to reverse behavioral preference. Our results suggest that ventral pallidum neurons guide reward-related decisions across changing physiological states.

Statistical Learning Signals for Complex Visual Images in Macaque Early Visual Cortex

Animals of several species, including primates, learn the statistical regularities of their environment. In particular, they learn the temporal regularities that occur in streams of visual images. Previous human neuroimaging studies reported discrepant effects of such statistical learning, ranging from stronger occipito-temporal activations for sequences in which image order was fixed, compared with sequences of randomly ordered images, to weaker activations for fixed-order sequences compared with sequences that violated the learned order. Several single-unit studies in macaque monkeys reported that after statistical learning of temporal regularities, inferior temporal (IT) neurons show reduced responses to learned fixed-order sequences of visual images compared with random or mispredicted sequences. However, it is unknown how other macaque brain areas respond to such temporal statistical regularities. To address this gap, we exposed rhesus monkeys (Macaca mulatta) to two types of sequences of complex images. The “regular” sequences consisted of a continuous stream of quartets, and within each quartet, the image order was fixed. The quartets themselves were displayed, uninterrupted, in a random order. The same monkeys were exposed to sequences of other images having a pseudorandomized order (“random” sequence). After exposure, both monkeys were scanned with functional MRI (fMRI) using a block design with three conditions: regular sequence, random sequence, and fixation-only blocks. A whole-brain analysis showed a reduced activation in mainly the occipito-temporal cortex for the regular compared to the random sequences. Marked response reductions for the regular sequence were observed in early extrastriate visual cortical areas, including area V2, despite the use of rather complex images of animals. These data suggest that statistical learning signals are already present in early visual areas of monkeys, even for complex visual images. These monkey fMRI data are in line with recent human fMRI studies that showed a reduced activation in early visual areas for predicted compared with mispredicted complex images.

Global reward state affects learning and activity in raphe nucleus and anterior insula in monkeys

People and other animals learn the values of choices by observing the contingencies between them and their outcomes. However, decisions are not guided by choice-linked reward associations alone; macaques also maintain a memory of the general, average reward rate - the global reward state - in an environment. Remarkably, global reward state affects the way that each choice outcome is valued and influences future decisions so that the impact of both choice success and failure is different in rich and poor environments. Successful choices are more likely to be repeated but this is especially the case in rich environments. Unsuccessful choices are more likely to be abandoned but this is especially likely in poor environments. Functional magnetic resonance imaging (fMRI) revealed two distinct patterns of activity, one in anterior insula and one in the dorsal raphe nucleus, that track global reward state as well as specific outcome events.

Identification of an Amygdala-Thalamic Circuit That Acts as a Central Gain Mechanism in Taste Perceptions

Peripheral sources of individual variation in taste intensity perception have been well described. The existence of a central source has been proposed but remains unexplored. Here we used functional magnetic resonance imaging in healthy human participants (20 women, 8 men) to evaluate the hypothesis that the amygdala exerts an inhibitory influence that affects the "gain" of the gustatory system during tasting. Consistent with the existence of a central gain mechanism (CGM), we found that central amygdala response was correlated with mean intensity ratings across multiple tastants. In addition, psychophysiological and dynamic causal modeling analyses revealed that the connection strength between inhibitory outputs from amygdala to medial dorsal and ventral posterior medial thalamus predicted individual differences in responsiveness to taste stimulation. These results imply that inhibitory inputs from the amygdala to the thalamus act as a CGM that influences taste intensity perception.SIGNIFICANCE STATEMENT Whether central circuits contribute to individual variation in taste intensity perception is unknown. Here we used functional magnetic resonance imaging in healthy human participants to identify an amygdala-thalamic circuit where network dynamics and connectivity strengths during tasting predict individual variation in taste intensity ratings. This finding implies that individual differences in taste intensity perception do not arise solely from variation in peripheral gustatory factors.

Rapid event-related, BOLD fMRI, non-human primates (NHP): choose two out of three

Human functional magnetic resonance imaging (fMRI) typically employs the blood-oxygen-level-dependent (BOLD) contrast mechanism. In non-human primates (NHP), contrast enhancement is possible using monocrystalline iron-oxide nanoparticles (MION) contrast agent, which has a more temporally extended response function. However, using BOLD fMRI in NHP is desirable for interspecies comparison, and the BOLD signal’s faster response function promises to be beneficial for rapid event-related (rER) designs. Here, we used rER BOLD fMRI in macaque monkeys while viewing real-world images, and found visual responses and category selectivity consistent with previous studies. However, activity estimates were very noisy, suggesting that the lower contrast-to-noise ratio of BOLD, suboptimal behavioural performance, and motion artefacts, in combination, render rER BOLD fMRI challenging in NHP. Previous studies have shown that rER fMRI is possible in macaques with MION, despite MION’s prolonged response function. To understand this, we conducted simulations of the BOLD and MION response during rER, and found that no matter how fast the design, the greater amplitude of the MION response outweighs the contrast loss caused by greater temporal smoothing. We conclude that although any two of the three elements (rER, BOLD, NHP) have been shown to work well, the combination of all three is particularly challenging.

Open-Source Joystick Manipulandum for Decision-Making, Reaching, and Motor Control Studies in Mice

To make full use of optogenetic and molecular techniques in the study of motor control, rich behavioral paradigms for rodents must rise to the same level of sophistication and applicability. We describe the layout, construction, use and analysis of data from joystick-based reaching in a head-fixed mouse. The step-by-step guide is designed for both experienced rodent motor labs and new groups looking to enter into this research space. Using this platform, mice learn to consistently perform large, easily-quantified reaches, including during a two-armed bandit probabilistic learning task. The metrics of performance (reach trajectory, amplitude, speed, duration, and inter-reach interval) can be used to quantify behavior or administer stimulation in closed loop with behavior. We provide a highly customizable, low cost and reproducible open-source behavior training platform for studying motor control, decision-making, and reaching reaction time. The development of this software and hardware platform enables behavioral work to complement recent advances in rodents, while remaining accessible to smaller institutions and labs, thus providing a high-throughput method to study unexplored features of action selection, motivation, and value-based decisions.

Taste Quality Representation in the Human Brain

In the mammalian brain, the insula is the primary cortical substrate involved in the perception of taste. Recent imaging studies in rodents have identified a "gustotopic" organization in the insula, whereby distinct insula regions are selectively responsive to one of the five basic tastes. However, numerous studies in monkeys have reported that gustatory cortical neurons are broadly-tuned to multiple tastes, and tastes are not represented in discrete spatial locations. Neuroimaging studies in humans have thus far been unable to discern between these two models, though this may be because of the relatively low spatial resolution used in taste studies to date. In the present study, we examined the spatial representation of taste within the human brain using ultra-high resolution functional magnetic resonance imaging (MRI) at high magnetic field strength (7-tesla). During scanning, male and female participants tasted sweet, salty, sour, and tasteless liquids, delivered via a custom-built MRI-compatible tastant-delivery system. Our univariate analyses revealed that all tastes (vs tasteless) activated primary taste cortex within the bilateral dorsal mid-insula, but no brain region exhibited a consistent preference for any individual taste. However, our multivariate searchlight analyses were able to reliably decode the identity of distinct tastes within those mid-insula regions, as well as brain regions involved in affect and reward, such as the striatum, orbitofrontal cortex, and amygdala. These results suggest that taste quality is not represented topographically, but by a distributed population code, both within primary taste cortex as well as regions involved in processing the hedonic and aversive properties of taste.SIGNIFICANCE STATEMENT The insula is the primary cortical substrate involved in taste perception, yet some question remains as to whether this region represents distinct tastes topographically or via a population code. Using high field (7-tesla), high-resolution functional magnetic resonance imaging in humans, we examined the representation of different tastes delivered during scanning. All tastes activated primary taste cortex within the bilateral mid-insula, but no brain region exhibited any consistent taste preference. However, multivariate analyses reliably decoded taste quality within the bilateral mid-insula as well as the striatum, orbitofrontal cortex, and bilateral amygdala. This suggests that taste quality is represented by a spatial population code within regions involved in sensory and appetitive properties of taste.

Functional specialization of macaque premotor F5 subfields with respect to hand and mouth movements: A comparison of task and resting-state fMRI

Based on architectonic, tract-tracing or functional criteria, the rostral portion of ventral premotor cortex in the macaque monkey, also termed area F5, has been divided into several subfields. Cytoarchitectonical investigations suggest the existence of three subfields, F5c (convexity), F5p (posterior) and F5a (anterior). Electrophysiological investigations have suggested a gradual dorso-ventral transition from hand- to mouth-dominated motor fields, with F5p and ventral F5c strictly related to hand movements and mouth movements, respectively. The involvement of F5a in this respect, however, has received much less attention. Recently, data-driven resting-state fMRI approaches have also been used to examine the presence of distinct functional fields in macaque ventral premotor cortex. Although these studies have suggested several functional clusters in/near macaque F5, so far the parcellation schemes derived from these clustering methods do not completely retrieve the same level of F5 specialization as suggested by aforementioned invasive techniques. Here, using seed-based resting-state fMRI analyses, we examined the functional connectivity of different F5 seeds with key regions of the hand and face/mouth parieto-frontal-insular motor networks. In addition, we trained monkeys to perform either hand grasping or ingestive mouth movements in the scanner in order to compare resting-state with task-derived functional hand and mouth motor networks. In line with previous single-cell investigations, task-fMRI suggests involvement of F5p, dorsal F5c and F5a in the execution of hand grasping movements, while non-communicative mouth movements yielded particularly pronounced responses in ventral F5c. Corroborating with anatomical tracing data of macaque F5 subfields, seed-based resting-state fMRI suggests a transition from predominant functional correlations with the hand-motor network in F5p to mostly mouth-motor network functional correlations in ventral F5c. Dorsal F5c yielded robust functional correlations with both hand- and mouth-motor networks. In addition, the deepest part of the fundus of the inferior arcuate, corresponding to area 44, displayed a strikingly different functional connectivity profile compared to neighboring F5a, suggesting a different functional specialization for these two neighboring regions.