Beyond language: The unspoken sensory-motor representation of the tongue in non-primates, non-human and human primates

The English idiom “on the tip of my tongue” commonly acknowledges that something is known, but it cannot be immediately brought to mind. This phrase accurately describes sensorimotor functions of the tongue, which are fundamental for many tongue-related behaviors (e.g., speech), but often neglected by scientific research. Here, we review a wide range of studies conducted on non-primates, non-human and human primates with the aim of providing a comprehensive description of the cortical representation of the tongue's somatosensory inputs and motor outputs across different phylogenetic domains. First, we summarize how the properties of passive non-noxious mechanical stimuli are encoded in the putative somatosensory tongue area, which has a conserved location in the ventral portion of the somatosensory cortex across mammals. Second, we review how complex self-generated actions involving the tongue are represented in more anterior regions of the putative somato-motor tongue area. Finally, we describe multisensory response properties of the primate and non-primate tongue area by also defining how the cytoarchitecture of this area is affected by experience and deafferentation

“Is Butter a Carb?” Neural mechanisms of nutrient-sensing and food reward in the human brain

Sensing the nutrient composition of a food and the processing of this information by the brain’s reward system to regulate food consumption are crucial biological needs. However, dysfunction in neural reward pathways may also lead to overconsumption of certain nutrients, contributing to obesity and comorbid diseases. In the context of fat, the oral sensory mechanism of its detection is disputed, although there is substantial evidence for fat detection through oral textural properties. In this thesis, I investigate the neural correlates related to the specific textural properties of oral food stimuli with defined nutrient contents, as well as their formally measured economic reward values and psychophysical ratings during functional Magnetic Resonance Imaging (fMRI) in healthy human volunteers. These results are then correlated with an ad-libitum naturalistic eating test. The thesis contains the following chapters: Chapter I discusses the key background literature; Chapter II focuses on the optimisation of the design and stimuli; Chapter III provides a detailed analysis of behavioural data, through basic psychophysical ratings of food stimuli and modelling of subjective value data; Chapter IV describes the results of the neuroimaging component of the experiment, and Chapter V discusses the results of the project in the context of current literature. This project investigates the textural contributions to sensory fat detection and reward valuation. Crucially, it is the first time a formal fMRI investigation is done on the oral-lubricative nature of fat, demonstrating encoding of sliding friction in the midposterior insula and the oral somatosensory cortex, which supports the concept that fat detection occurs through texture. Furthermore, our results highlight the unique role of the orbitofrontal cortex in processing food texture parameters, their subjective perception, and integration to subjective value, before subsequent evaluation in the ventromedial prefrontal cortex.Wellcome Trust Henry Dale Fellowship (Fabian Grabenhorst

Cortical representation of illusory body perception in healthy persons and amputees: implications for the understanding and treatment of phantom limb pain

A disturbed body perception is characteristic for various neurological and mental disorders and becomes particularly evident in phantom phenomena after limb amputation. Body illusions, such as mirror visual feedback (MVF) illusions, have been shown to be efficient in treating chronic pain and to be further related to a reversal of cortical reorganization. The present thesis aimed at identifying the neural circuitry of illusory body perception in healthy subjects and unilateral upper-limb amputees using functional magnetic resonance imaging. Study 1 investigated the perceived mirror illusion capacity and the neural correlates of a novel MVF-device (the mirror glasses) in comparison to the well-established mirror box in healthy persons. Study 2 investigated the neural circuitry of stimulus-evoked non-painful phantom phenomena in unilateral upper-limb amputees. During mirror illusions, movements of the affected limb are visually recreated by movements of the contralateral limb. The visual recreation of the affected limb seems to be linked to a recruitment of the primary sensorimotor representation of the affected limb. In contrast to the mirror box, the mirror glasses limit the user’s view to the visual reflection of the moving hand as opposed to seeing both hands moving in synchrony. It has been proposed that seeing the actually moving limb in addition to the mirror reflection might have a distracting effect. Study 1 evaluated the utility of mirror glasses based on a comparison to the mirror box and tested the hypothesis that increased interhemispheric communication between motor hand representations might drive the activation in the non-mirrored limb representation. Mirror illusion capacity and brain circuitry were measured in a within-subject design during both MVF-conditions with 20 healthy subjects in counterbalanced order. The self-reported mirror illusion capacity and brain activation patterns did not significantly differ between both mirror tasks. The representation of the non-mirrored hand was recruited in both mirror tasks. A significant increase in interhemispheric connectivity between the hand areas, however, was only found in the mirror glasses condition, suggesting divergent mechanisms for the recruitment of the non-mirrored hand representation between both mirror tasks. Most amputees still perceive their amputated limb (phantom limb awareness). Phantom phenomena comprise a variety of non-painful and painful sensations allocated to the amputated limb. Some amputees experience non-painful phantom phenomena when the residual limb or other parts of the body are stimulated (evoked phantom sensations). The neural correlates of non-painful phantom phenomena remain unknown. Study 2 aimed to identify the neural circuitry of evoked nonpainful phantom sensations. Twelve upper-limb amputees who reliably perceived non-painful phantom sensations upon stimulation of distal body parts and 12 yoked controls (matched for sex and age) were investigated. Amputees were stimulated at a body site eliciting phantom sensation with a stimulus related on- and offset and a control site without illusory perception. Controls were stimulated at matched body sites. A conjunction analysis showed specificity of the left ventral premotor and inferior frontal cortices (BA44/45) for the perception of referred sensations. Generalized psychophysiological interaction analyses revealed a widespread network showing significant positive intra-parietal and fronto-parietal connectivity. Our study indicates a high convergence between the neural correlates of nonpainful phantom sensations and (other) body illusions. Both studies of the present thesis offer new insights into the understanding the neuronal basis of illusory body perception. Such illusory body perceptions are frequent in chronic pain and targeting these distortions of body perception has been shown to be fruitful for relieving pain and disability

Importancia del sistema neuromuscular y la sensibilidad táctil activa en prótesis implantosoportada vs. prótesis mucosportada

[ES] Se pretende estudiar las ventajas que presentan las rehabilitaciones implantosoportadas con respecto a otro tipo de prótesis convencionales basándome en el fenómeno de osteopercepción. Este fenómeno permite que haya un remplazo del ligamento periodontal ausente para conseguir una función del sistema estomatognático lo más parecida posible a la que existía previamente al edentulismo. Para ello, se va a llevar a cabo un estudio psicofísico; ya que este tipo de estudios son los que mejor representan la función masticatoria y además son mucho más reproducibles en el trabajo clínico diario. Este estudio psicofísico medirá el grosor mínimo interoclusal que es capaz de detectar un paciente portador de un determinado tipo de prótesis durante su oclusión normal. De esta manera, se busca demostrar que las rehabilitaciones implantosoportadas son superiores funcionalmente a aquellas rehabilitaciones mucosoportadas convencionales, y, además, que los valores de sensibilidad táctil que consiguen este tipo de rehabilitaciones son cercanos a los que presenta la dentición natural. Los objetivos que se pretenden alcanzar a través del estudio son: Revisar en la literatura los distintos estudios psicofísicos sobre sensibilidad táctil para poder comprender las diferencias de valores existentes en prótesis completas mucosoportadas e implantosoportadas. Y realizar un estudio psicofísico de medición de la sensibilidad táctil activa en distintas situaciones protésicas y comparar los valores entre sí; para poder estudiar la influencia que tiene el tipo de prótesis, el tiempo que cada paciente lleva portándola, la edad o el género en una sensibilidad táctil activa aumentada o disminuida

Exploring individual variation in oral perception

Diet plays a pivotal role in preventing, managing, and reducing the risk of weight gain, diabetes and heart disease. Increasing pressure is directed towards food manufacturers to offer healthier options. The challenge is to develop products which are both nutritious and accepted by the consumer. Oral sensitivity, and therefore product perception, varies greatly amongst individuals, and likely affects food choice. Taste phenotype and genotype are frequently proposed as markers for overall oral sensitivity. This thesis performs fundamental research to further current understanding of the impact of taste phenotype and genotype on the response to oral stimuli. The effect of 6-n-propylthiouracil (PROP) taster status (PTS), fungiform papillae density, TAS2R38 and gustin rs2274333 genotype on the perceived intensity of prototypical tastants and metallic stimuli is explored. Experiments were first conducted to develop oral stimuli for use in the subsequent fMRI studies, which interestingly identified that some metallic stimuli may have a gustatory component. Perceptually, few or no differences were identified across taste phenotypes or genotypes. Interestingly, functional Magnetic Resonance Imaging (fMRI) identified variation in cortical processing that was associated with PTS. PROP intensity ratings were found to correlate with cortical activation in the anterior insula, an area of the brain thought to be the primary gustatory cortex, in response to sweet and metallic stimuli, but not for sour, salt, bitter or umami stimuli. These limited differences observed may have been due to the occurrence of a concentration effect, where the increased gustatory sensitivity frequently associated with PROP tasters compared to PROP non-tasters was lost when administering strong supra-threshold stimuli used in the current study. These findings are of interest to food manufacturers and health professionals as they could indicate that taste phenotype and genotype has less impact on product perception, and therefore food choice, than previously proposed. Thermal taster status (TTS) refers to a new taste phenotype in which individuals perceive phantom tastes when the tongue is thermally stimulated, whilst thermal non-tasters (TnTs) only perceive temperature. In this thesis, variation in the phantom tastes reported by thermal tasters (TTs) are explored, and for the first time the temporal phantom taste response is measured. Different categories of temporal taste responses are identified, and interestingly it is shown that phantom tastes are perceived at variable temperature ranges across both TTs and taste qualities. Importantly, the onset of sweet taste was found to occur as the temperature increased between 22-35°C, supporting the hypothesis that the TRPM5 may be involved in sweet phantom taste responses. This is the first study to assess the brain’s response when thermally stimulating the tongue of TTs to elicit a phantom taste response. Interestingly, when using fMRI it is shown that at the time when TTs perceive a phantom taste, cortical activation is induced in the anterior insula, which is thought to be the primary gustatory cortex. This indicates that thermal stimulation may activate temperature sensitive gustatory nerve fibres in TTs, and supports the hypothesis of cross wiring between gustatory and trigeminal nerves. When comparing the cortical response to thermal stimulation of the tongue across TTs and TnTs, greater activation is observed in oral somatosensory areas of the brain in TTs compared to TnTs. These findings show cortical processing differs across thermal taste phenotype, and supports evidence that thermal taster status may be a marker for oral sensitivity. This original research provides a valuable contribution towards understanding the effect of taste phenotype and genotype on perception of prototypical taste, metallic, and thermal stimuli. The novel multidisciplinary approach of utilising sensory evaluation and fMRI techniques has provided valuable insights into the impact of taste phenotype on gustatory responses, and has suggested possible mechanisms that may be involved in thermal taste phenotype

Blinking and the Brain: Pathways and Pathology

A blink is a rapid bilateral eyelid closure and co-occurring eye movement. The eyes rotate down towards the tip of the nose and back up again. Seemingly, this generally unnoticed often repeated action is not very spectacular. However, if not for the occasional blink we would all be blind. A blink is an interesting phenomenon worth investigating. Through its role as a protective barrier for the eye and as a distributor of the eye’s tearfi lm, blinking is a necessity for our well-being but is also an important tool for neuroscience research and physicians. It is an extremely useful model to study motor performance, motor control, synaptic plasticity and is an excellent physiological instrument for the assessment of internal networks and nuclei [Nishimura, T. and Mori, K, 1996]. The blink rate, refl ex blink characteristics and learned blinks are the three main parameters that can be studied to this end. The blink rate is the frequency with which spontaneous blinks occur and can give information about the dopaminergic system [Karson 1988] and might even be usable as a measure of fatigue [Stern et al. 1994]. The refl ex blink is a rapid involuntary response evoked by external stimulation of the eye or eyelid. It has a protective function and is for instance used in research and physiological tests that use the blink to provide important information on the integrity of afferent and efferent pathways. However, important gaps remain in the knowledge of pathways underlying blinking, and aberrations in pathways or compensatory mechanisms are not fully understood. The learned blink is acquired during eyeblink or eyelid conditioning in which an involuntary blink-evoking stimulus is repeatedly combined with a neutral stimulus. After training the neutral stimulus can evoke the learned blink

Multimodal flavour perception: the impact of sweetness, bitterness, alcohol content and carbonation level on flavour perception

Flavour perception of food and beverages is a complex multisensory experience involving the gustatory, olfactory, trigeminal, auditory and visual senses. Thus, investigations into multimodal flavour perception require a multidisciplinary design of experiments approach. This research has focussed on beer flavour perception and the fundamental interactions between the main flavour components - sweetness, bitterness (from hop acids), alcohol content and carbonation level. A model beer was developed using representative ingredients which could be manipulated to systematically vary the concentration of the main flavour components in beer and was used in the following experiments. Using a full factorial design, the physical effect of ethanol, C02 and hop acid addition was determined by headspace analysis and in-nose expired breath (in-vivo) measurements. Results from headspace and in-vivo methods differed and highlighted the importance of in-vivo measures when correlating to sensory experience. Ethanol and C02 significantly increased volatile partitioning during model beverage consumption. The effects of ethanol and C02 appeared to be independent and therefore additive, which could account for up to 86% increase in volatile partitioning. This would increase volatile delivery to the olfactory bulb and thus potentially enhance aroma and flavour perception. This was investigated using quantitative descriptive analysis. Results showed that C02 significantly impacted all discriminating attributes, either directly or as a result of complex interactions with other design factors. C02 suppressed the sweetness of dextrose and interacted with hop acids to modify bitterness and tingly perception. Ethanol was the main driver of complexity of flavour and enhanced sweet perception. In a first study of its kind, the impact of C02 on gustatory perception was further investigated using functional magnetic resonance imaging (fMRI) to understand cortical response. In addition, classification of subjects into PROP taster status groups and thermal taster status groups was carried out. Groups were tested for their sensitivity to oral stimuli using sensory techniques and for the first time, cortical response to taste and C02 was investigated between groups using fMRI techniques and behavioural data. There was no correlation between PROP taster status and thermal taster status. PROP taster status groups varied in their cortical response to stimuli with PROP super-tasters showing significantly higher cortical activation to samples than PROP non-tasters. The mechanism for thermal taster status is not currently known but thermal tasters were found to have higher cortical activation in response to the samples. The difference in cortical activation between thermal taster groups was supported by behavioural data as thermal tasters least preferred, but were more able to discriminate the high C02 sample than thermal non-tasters. This research has provided in-depth study into the importance of flavour components in beer. It advances the limited data available on the effects of C02 on sensory perception in a carbonated beverage, providing sound data for the successful development of products with reduced ethanol or C02 levels. The use of functional magnetic resonance imaging has revealed for the first time that oral C02 significantly increases activation in the somatosensory cortex. However, C02 seemed to have a limited impact on activation strength in 'taste' areas, such as the anterior insula. Research comparing data from PROP taster status groups and thermal taster status groups has given insight into the possible mechanisms accounting for differences in oral intensity of stimuli