Hemispheric dominance of cortical activity evoked by focal electrogustatory stimuli

Effect of Regular Consumption of a Miraculin-Based Food Supplement on Taste Perception and Nutritional Status in Malnourished Cancer Patients: A Triple-Blind, Randomized, Placebo-Controlled Clinical Trial-CLINMIR Pilot Protocol

Taste disorders are common among cancer patients undergoing chemotherapy, with a prevalence ranging from 20% to 86%, persisting throughout treatment. This condition leads to reduced food consumption, increasing the risk of malnutrition. Malnutrition is associated not only with worse treatment efficacy and poor disease prognosis but also with reduced functional status and quality of life. The fruit of Synsepalum dulcificum (Daniell), commonly known as miracle berry or miracle fruit, contains miraculin, a taste-modifying protein with profound effects on taste perception. The CLINMIR Protocol is a triple-blind, randomized, placebo-controlled clinical trial designed to evaluate the regular consumption of a food supplement containing a miraculin-based novel food, dried miracle berry (DMB), on the taste perception (measured through electrogustometry) and nutritional status (evaluated through the GLIM Criteria) of malnourished cancer patients under active antineoplastic treatment. To this end, a pilot study was designed with 30 randomized patients divided into three study arms (150 mg DMB + 150 mg freeze-dried strawberries, 300 mg DMB, or placebo) for three months. Throughout the five main visits, an exhaustive assessment of different parameters susceptible to improvement through regular consumption of the miraculin-based food supplement will be conducted, including electrical and chemical taste perception, smell perception, nutritional and morphofunctional assessment, diet, quality of life, the fatty acid profile of erythrocytes, levels of inflammatory and cancer-associated cytokines, oxidative stress, antioxidant defense system, plasma metabolomics, and saliva and stool microbiota. The primary anticipated result is that malnourished cancer patients with taste distortion who consume the miraculin-based food supplement will report an improvement in food taste perception. This improvement translates into increased food intake, thereby ameliorating their nutritional status and mitigating associated risks. Additionally, the study aims to pinpoint the optimal dosage that provides maximal benefits. The protocol adheres to the SPIRIT 2013 Statement, which provides evidence-based recommendations and is widely endorsed as an international standard for trial protocols. The clinical trial protocol has been registered at the platform for Clinical Trials (NCT05486260).

Eating-induced seizures: A semiological sign of the right temporal pole

OBJECTIVE

Eating-induced seizures (EIS) are a rare form of reflex seizures. The objective of this study was to report a series of cases of EIS involving patients admitted to our epilepsy unit, and to analyze the clinical characteristics, etiology, and treatment response of this type of infrequent seizure.

METHODS

We performed a single-center retrospective analysis of all consecutive patients diagnosed with epilepsy with eating-induced seizures between 2008 and 2020.

RESULTS

We included eight patients (six women) with mean age 54.75 years (range: 40-79), and mean age at epilepsy onset 30.75 years (range: 9-58 years). EIS were triggered during a meal in 5/8 (at dinner 1/8, at breakfast in 1/8, and without time preference in 3/8), by a certain flavor in 1/8, by eating different textures or drinking soft drinks in 1/8, and by slicing food in 1/8. All patients suffered nonreflex seizures and 3/8 other types of reflex seizures. In 6/8 of patients, EIS originated in the right hemisphere. In 5/8, the EIS progressed to impaired awareness with oromandibular automatisms. In 6/8, the epilepsy was drug-resistant. Temporopolar encephalocele was the most frequent etiology, in 4/8. Three of the eight underwent surgical treatment, with Engel IA 1 year in 3/3. Three of the eight were treated with vagal stimulation therapy, with McHugh A 1 year in 2/3.

SIGNIFICANCE

In our series, eating-induced seizures were observed in patients with focal epilepsy. It was frequently drug-resistant and started predominantly in the right hemisphere, due to temporal pole involvement in half of the patients.

Processing of Sweet, Astringent and Pungent Oral Stimuli in the Human Brain

Taste and oral somatosensation are intimately related to each other from peripheral receptors to the central nervous system. Oral astringent sensation is thought to contain both gustatory and somatosensory components. In the present study, we compared the cerebral response to an astringent stimulus (tannin), with the response to one typical taste stimulus (sweet - sucrose) and one typical somatosensory stimulus (pungent - capsaicin) using functional magnetic resonance imaging (fMRI) of 24 healthy subjects. Three distributed brain sub-regions responded significantly different to the three types of oral stimulations: lobule IX of the cerebellar hemisphere, right dorsolateral superior frontal gyrus, and left middle temporal gyrus. This suggests that these regions play a major role in the discrimination of astringency, taste, and pungency.

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.

A low-cost open-architecture taste delivery system for gustatory fMRI and BCI experiments

BACKGROUND

Tasting is a complex process involving chemosensory perception and cognitive evaluation. Different experimental designs and solution delivery approaches may in part explain the variability reported in literature. These technical aspects certainly limit the development of taste-related brain computer interface devices.

NEW METHOD

We propose a novel modular, scalable and low-cost device for rapid injection of small volumes of taste solutions during fMRI experiments that gathers the possibility to flexibly increase the number of channels, allowing complex multi-dimensional taste experiments. We provide the full description of the hardware and software architecture and illustrate the application of the working prototype in single-subject event-related fMRI experiments by showing the BOLD responses to basic taste qualities and to five intensities of tastes during the course of perception.

RESULTS

The device is shown to be effective in activating multiple clusters within the gustatory pathway and a precise time-resolved event-related analysis is shown to be possible by the impulsive nature of the induced perception.

COMPARISON WITH EXISTING METHOD(S)

This gustometer represents the first implementation of a low-cost, easily replicable and portable device that is suitable for all kinds of fMRI taste experiments. Its scalability will boost the experimental design of more complex multi-dimensional fMRI studies of the human taste pathway.

CONCLUSIONS

The gustometer represents a valid open-architecture alternative to other available devices and its spread and development may contribute to an increased standardization of experimental designs in human fMRI studies of taste perception and pave the way to the development of novel taste-related BCIs.

Hungry brains: A meta-analytical review of brain activation imaging studies on food perception and appetite in obese individuals

The dysregulation of food intake in chronic obesity has been explained by different theories. To assess their explanatory power, we meta-analyzed 22 brain-activation imaging studies. We found that obese individuals exhibit hyper-responsivity of the brain regions involved in taste and reward for food-related stimuli. Consistent with a Reward Surfeit Hypothesis, obese individuals exhibit a ventral striatum hyper-responsivity in response to pure tastes, particularly when fasting. Furthermore, we found that obese subjects display more frequent ventral striatal activation for visual food cues when satiated: this continued processing within the reward system, together with the aforementioned evidence, is compatible with the Incentive Sensitization Theory. On the other hand, we did not find univocal evidence in favor of a Reward Deficit Hypothesis nor for a systematic deficit of inhibitory cognitive control. We conclude that the available brain activation data on the dysregulated food intake and food-related behavior in chronic obesity can be best framed within an Incentive Sensitization Theory. Implications of these findings for a brain-based therapy of obesity are briefly discussed.

Cortical representation of different taste modalities on the gustatory cortex: A pilot study

Background

Right insular cortex is involved in taste discrimination, but its functional organization is still poorly known. In general, sensory cortices represent the spatial prevalence of relevant features for each sensory modality (visual, auditory, somatosensory) in an ordered way across the cortical space. Following this analogy, we hypothesized that primary taste cortex is organized in similar ordered way in response to six tastes with known receptorial mechanisms (sweet, bitter, sour, salt, umami, CO₂)_.

Design

Ten normal subjects were enrolled in a pilot study. We used functional magnetic resonance imaging (fMRI), a high resolution cortical registration method, and specialized procedures of feature prevalence localization, to map fMRI responses within the right insular cortex, to water solutions of quinine hydrochloride (bitter), Acesulfamate K (sweet), sodium chloride (salt), mono potassium glutamate + inosine 5' mono phosphate (Umami), citric acid (sour) and carbonated water (CO₂). During an fMRI scan delivery of the solutions was applied in pseudo-random order interleaved with cleaning water.

Results

Two subjects were discarded due to excessive head movements. In the remaining subjects, statistically significant activations were detected in the fMRI responses to all tastes in the right insular cortex (p<0.05, family-wise corrected for multiple comparisons). Cortical representation of taste prevalence highlighted two spatially segregated clusters, processing two and three tastes coupled together (sweet-bitter and salt-umami-sour), with CO₂ in between.

Conclusions

Cortical representation of taste prevalence within the right primary taste cortex appears to follow the ecological purpose of enhancing the discrimination between safe nutrients and harmful substances.

Cortical and Subcortical Substrates of Cranial Nerve Function

The pivotal role of cranial nerves in a wholesome life experience cannot be overemphasized. Research has opened new avenues to understand cranial nerve function. Classical concept of strict bilateral cortical control of cranial nerves has given way to concepts of hemispheric dominance and hemispheric lateralization. An astute Neuroradiologist should keep abreast of these concepts and help patients and referring physicians by applying this knowledge in reading images. This chapter provides an overview of cranial nerve function and latest concepts pertaining to their cortical and subcortical control.

Time for Taste-A Review of the Early Cerebral Processing of Gustatory Perception

The first successfully recorded event-related potential (ERP) for taste, one of our basic senses, was published nearly half a century ago. Despite this large time span, surprisingly little is known about the early neural processing of taste perception. Here, we are providing a comprehensive and critical overview of over four decades of research, with a focus on the temporal dimension of cerebral taste processing in healthy humans. For this purpose, we review studies using techniques that permit a high temporal resolution, namely, electroencephalography and magnetoencephalography, ERP, and event-related magnetic fields (ERF). Our current knowledge of taste ERP is interpreted in the context of our understanding of other, nonchemical senses. Gaps in the existing literature are identified and discussed. Finally, we suggest directions for future investigations using gustatory ERP/ERF.

Reward abnormalities among women with full and subthreshold bulimia nervosa: a functional magnetic resonance imaging study

OBJECTIVE

To test the hypothesis that women with full and subthreshold bulimia nervosa show abnormal neural activation in response to food intake and anticipated food intake relative to healthy control women.

METHOD

Females with and without full/subthreshold bulimia nervosa recruited from the community (N = 26) underwent functional magnetic resonance imaging (fMRI) during receipt and anticipated receipt of chocolate milkshake and a tasteless control solution.

RESULTS

Women with bulimia nervosa showed trends for less activation than healthy controls in the right anterior insula in response to anticipated receipt of chocolate milkshake (vs. tasteless solution) and in the left middle frontal gyrus, right posterior insula, right precentral gyrus, and right mid dorsal insula in response to consumptions of milkshake (vs. tasteless solution).

DISCUSSION

Bulimia nervosa may be related to potential hypofunctioning of the brain reward system, which may lead these individuals to binge eat to compensate for this reward deficit, though the hypo-responsivity might be a result of a history of binge eating highly palatable foods.

Gustatory imagery reveals functional connectivity from the prefrontal to insular cortices traced with magnetoencephalography

Our experience and prejudice concerning food play an important role in modulating gustatory information processing; gustatory memory stored in the central nervous system influences gustatory information arising from the peripheral nervous system. We have elucidated the mechanism of the “top-down” modulation of taste perception in humans using functional magnetic resonance imaging (fMRI) and demonstrated that gustatory imagery is mediated by the prefrontal (PFC) and insular cortices (IC). However, the temporal order of activation of these brain regions during gustatory imagery is still an open issue. To explore the source of “top-down” signals during gustatory imagery tasks, we analyzed the temporal activation patterns of activated regions in the cerebral cortex using another non-invasive brain imaging technique, magnetoencephalography (MEG). Gustatory imagery tasks were presented by words (Letter G-V) or pictures (Picture G-V) of foods/beverages, and participants were requested to recall their taste. In the Letter G-V session, 7/9 (77.8%) participants showed activation in the IC with a latency of 401.7±34.7 ms (n = 7) from the onset of word exhibition. In 5/7 (71.4%) participants who exhibited IC activation, the PFC was activated prior to the IC at a latency of 315.2±56.5 ms (n = 5), which was significantly shorter than the latency to the IC activation. In the Picture G-V session, the IC was activated in 6/9 (66.7%) participants, and only 1/9 (11.1%) participants showed activation in the PFC. There was no significant dominance between the right and left IC or PFC during gustatory imagery. These results support those from our previous fMRI study in that the Letter G-V session rather than the Picture G-V session effectively activates the PFC and IC and strengthen the hypothesis that the PFC mediates “top-down” control of retrieving gustatory information from the storage of long-term memories and in turn activates the IC.

Cortical hemodynamic responses to intravenous thiamine propyldisulphide administration detected by multichannel near infrared spectroscopy (NIRS) system

Intravenous injection of thiamine propyldisulphide (TPD), which induces sensation of a garlic-like odor, has been used as a representative subjective olfactory test in Japan. However, cortical loci activated by TPD still remain unclear. We recorded cerebral hemodynamic responses (changes in Oxy-Hb concentrations) induced by TPD administration using whole-head multi-channel near infrared spectroscopy (NIRS) system based on 3D-MRIs. TPD as an odorant and saline as a control were injected from the cephalic vein in the left forearm in ten male normosmic (five young and five elderly) subjects and five dysosmic elderly patients. The all normosmic, but not dysosmic, subjects felt the garlic-like odor in the all TPD trials. There was no significant difference in hemodynamic responses between the young and elderly normosmic subjects. However, TPD injection induced significantly larger hemodynamic responses in the bilateral operculums, bilateral dorsolateral prefrontal cortices (PFC) and anteromedial PFC in the normosmic subjects, compared with saline injection. Onset latencies of these hemodynamic responses were significantly correlated with onset latencies of subjective odor sensation in the normosmic subjects. Comparison of hemodynamic responses between the normosmic and dysosmic subjects indicated a significant difference in the bilateral operculums. The results demonstrated that Oxy-Hb increases in the bilateral operculums reflected olfactory sensation induced by TPD injection. Consideration of a route for intravenous TPD to reach the olfactory mucosa suggests that these hemodynamic responses might be attributed to food-related retronasal olfactory responses to TPD.

Identification of human gustatory cortex by activation likelihood estimation

Over the last two decades, neuroimaging methods have identified a variety of taste-responsive brain regions. Their precise location, however, remains in dispute. For example, taste stimulation activates areas throughout the insula and overlying operculum, but identification of subregions has been inconsistent. Furthermore, literature reviews and summaries of gustatory brain activations tend to reiterate rather than resolve this ambiguity. Here, we used a new meta-analytic method [activation likelihood estimation (ALE)] to obtain a probability map of the location of gustatory brain activation across 15 studies. The map of activation likelihood values can also serve as a source of independent coordinates for future region-of-interest analyses. We observed significant cortical activation probabilities in: bilateral anterior insula and overlying frontal operculum, bilateral mid dorsal insula and overlying Rolandic operculum, and bilateral posterior insula/parietal operculum/postcentral gyrus, left lateral orbitofrontal cortex (OFC), right medial OFC, pregenual anterior cingulate cortex (prACC) and right mediodorsal thalamus. This analysis confirms the involvement of multiple cortical areas within insula and overlying operculum in gustatory processing and provides a functional "taste map" which can be used as an inclusive mask in the data analyses of future studies. In light of this new analysis, we discuss human central processing of gustatory stimuli and identify topics where increased research effort is warranted.

Model-free fMRI group analysis using FENICA

Exploratory analysis of functional MRI data allows activation to be detected even if the time course differs from that which is expected. Independent Component Analysis (ICA) has emerged as a powerful approach, but current extensions to the analysis of group studies suffer from a number of drawbacks: they can be computationally demanding, results are dominated by technical and motion artefacts, and some methods require that time courses be the same for all subjects or that templates be defined to identify common components. We have developed a group ICA (gICA) method which is based on single-subject ICA decompositions and the assumption that the spatial distribution of signal changes in components which reflect activation is similar between subjects. This approach, which we have called Fully Exploratory Network Independent Component Analysis (FENICA), identifies group activation in two stages. ICA is performed on the single-subject level, then consistent components are identified via spatial correlation. Group activation maps are generated in a second-level GLM analysis. FENICA is applied to data from three studies employing a wide range of stimulus and presentation designs. These are an event-related motor task, a block-design cognition task and an event-related chemosensory experiment. In all cases, the group maps identified by FENICA as being the most consistent over subjects correspond to task activation. There is good agreement between FENICA results and regions identified in prior GLM-based studies. In the chemosensory task, additional regions are identified by FENICA and temporal concatenation ICA that we show is related to the stimulus, but exhibit a delayed response. FENICA is a fully exploratory method that allows activation to be identified without assumptions about temporal evolution, and isolates activation from other sources of signal fluctuation in fMRI. It has the advantage over other gICA methods that it is computationally undemanding, spotlights components relating to activation rather than artefacts, allows the use of familiar statistical thresholding through deployment of a higher level GLM analysis and can be applied to studies where the paradigm is different for all subjects.

Neural correlates of evaluative compared with passive tasting

We used functional magnetic resonance imaging to test the hypothesis that the nature of the neural response to taste varies as a function of the task the subject is asked to perform. Subjects received sweet, sour, salty and tasteless solutions passively and while evaluating stimulus presence, pleasantness and identity. Within the insula and overlying operculum the location of maximal response to taste vs. tasteless varied as a function of task; however, the primary taste cortex (anterior dorsal insula/frontal operculum--AIFO), as well as a more ventral region of anterior insula, responded to taste vs. tasteless irrespective of task. Although the response here did not depend upon task, preferential connectivity between AIFO and the amygdala (bilaterally) was observed when subjects tasted passively compared with when they performed a task. This suggests that information transfer between AIFO and the amygdala is maximal during implicit processing of taste. In contrast, a region of the left lateral orbitofrontal cortex (OFC) responded preferentially to taste and to tasteless when subjects evaluated pleasantness, and was preferentially connected to earlier gustatory relays (caudomedial OFC and AIFO) when a taste was present. This suggests that processing in the lateral OFC organizes the retrieval of gustatory information from earlier relays in the service of computing perceived pleasantness. These findings show that neural encoding of taste varies as a function of task beyond that of the initial cortical representation.

Leaving a bad taste in your mouth but not in my insula

Previous research has implicated regions of anterior insula/frontal operculum in processing conspecific facial expressions of disgust. It has been suggested however that there are a variety of disgust facial expression components which relate to the disgust-eliciting stimulus. The nose wrinkle is predominantly associated with irritating or offensive smells, the mouth gape and tongue extrusion with distaste and oral irritation, while a broader range of disgust elicitors including aversive interpersonal contacts and certain moral offenses are associated primarily with the upper lip curl. Using functional magnetic resonance imaging, we show that activity in the anterior insula/frontal operculum is seen only in response to canonical disgust faces, exhibiting the nose wrinkle and upper lip curl, and not in response to distaste facial expressions, exhibiting a mouth gape and tongue protrusion. Canonical disgust expressions also result in activity in brain regions linked to social cognition more broadly, including dorsal medial prefrontal cortex, posterior cingulate cortex, temporo-parietal junction and superior temporal sulcus. We interpret these differences in relation to the relative functional and communicative roles of the different disgust expressions and suggest a significant role for appraisal processes in the insula activation to facial expressions of disgust.

The cortical chronometry of electrogustatory event-related potentials

Electrogustometry (EGM) is the standard tool to assess gustatory functions in clinical environments. The stimulation elicits a percept often described as metallic, sour or salty, also referred to as electric taste. To date, the neuronal mechanisms that underlie electric taste perception are not yet fully understood. Electroencephalographic (EEG) approaches will certainly complement behavioral procedures and, furthermore, extend the understanding of gustatory processing in general and disturbances of gustatory functions in particular. We used anodal pulses applied to the tip of the participants' tongue while EEG was recorded. The major disadvantage of combining EEG and EGM, namely the electrical stimulation artifact, was overcome by means of Independent Component Analysis (ICA), which separated the EGM artifact from the neural portion of the EEG. After artifact correction, we found a largely uncontaminated electrogustatory event-related potential (eGERP) at both individual and group level. Furthermore, source analysis revealed an early involvement of bilateral insular cortices and the adjacent operculi, the areas comprising the primary taste cortex. The procedures, described in detail, pave the way for the eGERP to become an affordable and objective tool for the assessment of taste function, and thus to complement behavioral measures (i.e. EGM detection thresholds). Furthermore, they render the access to different levels of the electrogustatory processing pathway possible and by doing so they may aid the identification and localisation of lesions that cause taste disturbances.

Cortical representation of taste-modifying action of miracle fruit in humans

Red berries of a tropical plant called miracle fruit, Richadella dulcifica, reduce the sour and aversive taste of acids and add sweet and palatable taste. To elucidate the brain mechanism of this unique action of miracle fruit, we recorded taste-elicited magnetic fields of the human cerebral cortex. The initial taste responses were localized in the fronto-parietal opercular/insular cortex reported as the primary taste area. The mean latency of the response to citric acid after chewing miracle fruit was essentially the same as that for sucrose and was 250-300 ms longer than that for citric acid. Since it is known that stimulation with acids after the action of miracle fruit induces both sweetness and sourness responses in the primate taste nerves, the present results suggest that the sourness component of citric acid is greatly diminished at the level of subcortical relays, and mostly sweetness information reaches the cortical primary taste area. We propose the idea that the qualitative aspect of taste is processed in the primary taste area and the affective aspect is represented by the pattern of activation among the different cortical areas.

Prefrontal activity during taste encoding: an fNIRS study

To elucidate the function of the lateral prefrontal cortex (LPFC) in taste encoding, it is worth applying to taste, the psychological paradigms of intentional memorization that have been used with other extensively studied senses, and thus updating current models for LPFC functions to include a taste modality. Using multichannel functional near-infrared spectroscopy (fNIRS), we examined the LPFC's of healthy volunteers (N = 18) during the intentional memorization of a basic taste. In order to minimize the confounding effects of verbal processes that are known to employ the left LPFC, we used quaternary taste mixtures that were difficult to verbalize, and confined analysis to those who did not use a verbal strategy during memorization (N = 10). In order to examine the results in association with data in the literature, the location of activity was probabilistically estimated and anatomically labeled in the Montreal Neurological Institute (MNI) standard brain space. By contrasting the cortical activation under encoding conditions with that under control conditions without memory requirement, we found activation in the bilateral ventro-LPFC and the right posterior portion of the LPFC. The activation pattern was consistent with previous studies on the encoding of nonverbal materials using other senses. This suggests that models for LPFC functions that derive from previous studies can be generalized to intentional encoding processes of taste information, at least at a macro-structural level. The current study also demonstrates that, by using fNIRS, LPFC functions on taste can be examined with experimental paradigms relevant to those used for other senses.

The neurocognitive bases of human multimodal food perception: sensory integration

This review addresses a fundamental neuroscientific question in food perception: how multimodal features of food are integrated. Much research and conceptualization has emerged related to multisensory integration in vision, audition and somatosensation, while it remains poorly understood and researched within the chemical and mouth feel senses. This review aims to bridge this gap. We discuss the main concepts in the fields of auditory, visual and somatosensory multisensory integration and relate them to oral-sensory (gustatory and somatosensory) and olfactory (orolfactory) interactions. We systematically review the psychophysical literature pertaining to intra- and intermodal interactions related to food perception, while making explicit distinctions between peripheral and central interactions. As the neural bases of crossmodal orolfaction currently are poorly understood, we introduce several plausible neuroscientific models, which provide a framework for further neuroscientific exploration in this area. We are guided by a new meta-analysis of the odor-taste neuroimaging literature, as well as by single-unit, anatomical and psychophysical studies. Finally, we propose strong involvement of recurrent neural networks in multisensory integration and make suggestions for future research.

Functional MRI Detection of Activation in the Primary Gustatory Cortices in Humans

Magnetoencephalography (MEG) has recently revealed that the transitions between the parietal operculum (Pop) and the insula (area G) and the ventral end of the central sulcus (cs) were activated with the shortest latency by instrumental gustatory stimulation, which suggests that the location of the primary gustatory area is in these two regions. However, studies using other noninvasive brain-imaging methods such as positron-emission tomography or functional magnetic resonance imaging (fMRI) with manual application of tastants into the mouth have been unable to confirm this. The present study examined cortical activation by repetitive stimulation of the tongue tip with 1 M NaCl with a computer-controlled stimulator and used fMRI to detect it. In individual brains, activations were detected with multiple comparisons (false discovery rate) across the whole brain corrected (threshold at P < 0.05) at both area G and frontal operculum (Fop) in 8 of 11 subjects and at the rolandic operculum (Rop) in 7 subjects. Activations were also found at the ventral end of the cs (n = 3). Group analysis with random-effect models (multiple comparison using familywise error in regions of interest, P < 0.02) revealed activation at area G in both hemispheres and in the Fop, Rop, and ventral end of the cs on the left side. The present study revealed no activation on the gyrus of the external cerebral surface except for the Rop. Taking MEG findings into consideration, the present findings strongly indicate that the primary gustatory area is present at both the transition between the Pop and insula and the Rop including the gray matter within a ventral part of the cs.

Postnatal maturation of cortical serotonin lateral asymmetry in gerbils is vulnerable to both environmental and pharmacological epigenetic challenges

Long-term effects of postnatal differential rearing conditions and/or early methamphetamine (MA) application on serotonin (5-HT) fibre density were investigated in several cortical areas of both hemispheres of gerbils. The aim of this study was twofold: (1) Is the 5-HT fibre innervation of the cerebral cortex lateralised, and (2) if so, do postnatal environmental conditions and/or an early drug challenge interfere with development of 5-HT cerebral asymmetries? For that purpose, male gerbils were reared either under semi-natural or restricted environmental and social conditions, under both conditions once (on postnatal day 14) being treated with either a single dose of MA (50 mg/kg, i.p.) or saline. On postnatal day 110, 5-HT fibres were immunohistochemically stained and innervation densities quantified in prefrontal cortex, insular cortex, frontal cortex, parietal cortex, and entorhinal cortex. It was found that (1) 5-HT innervation in the cerebral cortex was clearly lateralised; (2) direction and extent of this asymmetry were not uniformly distributed over the different areas investigated; (3) both early methamphetamine challenge and rearing condition differentially interfered with adult 5-HT cerebral asymmetry; (4) combining MA challenge with subsequent restricted rearing tended to reverse the effects of MA on 5-HT cerebral asymmetry in some of the cortical areas investigated; and (5) significant responses in 5-HT cerebral asymmetry only occurred in prefrontal and entorhinal association cortices. The present findings suggest that the ontogenesis of cortical laterality is influenced by epigenetic factors and that disturbances of the postnatal maturation of lateralised functions may be associated with certain psychopathological behaviours.

Functional imaging of gustatory perception and imagery: “top-down” processing of gustatory signals

By recalling gustatory memories, it is possible to generate vivid gustatory perceptions in the absence of gustatory inputs. This gustatory image influences our gustatory processing. However, the mechanism of the "top-down" modulation of gustatory perception in the human is still unclear. Our findings propose a new perspective on the neural basis of gustatory processing. Although gustatory imagery and gustatory perception shared common parts of neural substrates, there was an asymmetrical topography of activation in the insula: the left insula was predominantly activated by gustatory imagery tasks. In addition, the middle and superior frontal gyri were not activated by gustatory perception but they participated in the generation of gustatory hallucinations. These regions in the frontal cortex may mediate the "top-down" control of retrieving gustatory information from the storage of long-term memories.

New imaging techniques: integrating structural and functional imaging in the head and neck

Traditionally, the mainstay of head and neck MR imaging has been the identification of structural alterations resulting from pathology. Now, the advent of fast MR imaging techniques provides the opportunity for radiologists to integrate structural and functional imaging in the head and neck. This article highlights functional imaging techniques that provide a means toward a complete evaluation of structural integrity and function in various systems of the head and neck.

Experience-Dependent Neural Integration of Taste and Smell in the Human Brain

Flavor perception arises from the central integration of peripherally distinct sensory inputs (taste, smell, texture, temperature, sight, and even sound of foods). The results from psychophysical and neuroimaging studies in humans are converging with electrophysiological findings in animals and a picture of the neural correlates of flavor processing is beginning to emerge. Here we used event-related fMRI to evaluate brain response during perception of flavors (i.e., taste/odor liquid mixtures not differing in temperature or texture) compared with the sum of the independent presentation of their constituents (taste and/or odor). All stimuli were presented in liquid form so that olfactory stimulation was by the retronasal route. Mode of olfactory delivery is important because neural suppression has been observed in chemosensory regions during congruent taste–odor pairs when the odors are delivered by the orthonasal route and require subjects to sniff. There were 2 flavors. One contained a familiar/congruent taste–odor pair (vanilla/sweet) and the other an unfamiliar/incongruent taste–odor pair (vanilla/salty). Three unimodal stimuli, including 2 tastes (sweet and salty) and one odor (vanilla), as well as a tasteless/odorless liquid (baseline) were presented. Superadditive responses during the perception of the congruent flavor compared with the sum of its constituents were observed in the anterior cingulate cortex (ACC), dorsal insula, anterior ventral insula extending into the caudal orbitofrontal cortex (OFC), frontal operculum, ventral lateral prefrontal cortex, and posterior parietal cortex. These regions were not present in a similar analysis of the incongruent flavor compared with the sum of its constituents. All of these regions except the ventrolateral prefrontal cortex were also isolated in a direct contrast of congruent − incongruent. Additionally, the anterior cingulate, posterior parietal cortex, frontal operculum, and ventral insula/caudal OFC were also more active in vanilla + salty minus incongruent, suggesting that delivery of an unfamiliar taste–odor combination may lead to suppressed neural responses. Taken together with previous findings in the literature, these results suggest that the insula, OFC, and ACC are key components of the network underlying flavor perception and that taste–smell integration within these and other regions is dependent on 1) mode of olfactory delivery and 2) previous experience with taste/smell combinations.

Taste-related activity in the human dorsolateral prefrontal cortex

Taste remains one of the least-explored human senses. Cortical taste responses were investigated using neuroimaging in 40 subjects tasting a range of different taste stimuli compared to a neutral tasteless control. Activation was found in the anterior insula/frontal opercular taste cortex and caudal orbitofrontal cortex, both areas established as taste cortical areas by neuronal recordings in primates. A novel finding in this study was a highly significant response to taste in the dorsolateral prefrontal cortex. This may reflect an effect of taste on cognitive processing to help optimise or modify behavioural strategies involved in executive control; or it could reflect the engagement of this region in attentional processing by a taste input.

The evaluation of brain activity in response to taste stimuli--a pilot study and method for central taste activation as assessed by event-related fMRI

BACKGROUND

Brain pathways contribute to the regulation of appetite behaviors, and advancements in brain imaging offer new opportunities in determining whether disturbances of these pathways play a role in pathological feeding behaviors in humans. We developed a standardized method for the assessment of brain activation in response to taste stimuli.

METHODS

Five healthy control women were positioned in a 1.5 T GE magnet resonance (MR) scanner for functional MR imaging (fMRI). They received 1.0 cm3 samples of 1 M glucose solution or artificial saliva (25 mM KCl, 2 mM NaHCO3). Fluid challenges were delivered by a programmable syringe pump (J-Kem Scientific, St. Louis, MO). E-Prime software (Psychology Software Tools Inc., Pittsburgh, PA) coordinated taste stimulation with MR scanning. Data were analyzed using NeuroImaging software (NIS).

RESULTS

Healthy women showed increased orbitofrontal cortex activation when glucose was compared to artificial saliva. In addition, mesial and lateral temporal cortical regions contrasted glucose from artificial saliva.

CONCLUSIONS

This study demonstrates a design for the systematic study of brain activation after taste stimulation using fMRI and computer controlled stimulus delivery. The results are consistent with previous studies, showing activation in higher order brain centers that are involved in emotional coding of taste experience.

Dissociation of neural representation of intensity and affective valuation in human gustation

We used a 2 x 2 factorial design to dissociate regions responding to taste intensity and taste affective valence. Two intensities each of a pleasant and unpleasant taste were presented to subjects during event-related fMRI scanning. The cerebellum, pons, middle insula, and amygdala responded to intensity irrespective of valence. In contrast, valence-specific responses were observed in anterior insula/operculum extending into the orbitofrontal cortex (OFC). The right caudolateral OFC responded preferentially to pleasant compared to unpleasant taste, irrespective of intensity, and the left dorsal anterior insula/operculuar region responded preferentially to unpleasant compared to pleasant tastes equated for intensity. Responses best characterized as an interaction between intensity and pleasantness were also observed in several limbic regions. These findings demonstrate a functional segregation within the human gustatory system. They also show that amygdala activity may be driven by stimulus intensity irrespective of valence, casting doubt upon the notion that the amygdala responds preferentially to negative stimuli.

New imaging techniques: integrating structural and functional imaging in the head and neck

The application of fast MRI techniques provides the opportunity to image function in various systems of the head and neck. Incorporating fMRI techniques into head and neck imaging protocols provides the potential for the head and neck radiologist to investigate structural integrity and function and thus play a central role in the diagnostic and prognostic work-up of the patient.