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FOOLING THE SENSE OF TOUCH: NEW EVIDENCE ON THE ROLE OF BODY REPRESENTATIONS IN SHAPING SOMATIC PERCEPTION
No full textNella vita di tutti i giorni, capita numerose volte di toccare oggetti di diverso tipo con le mani. Il nostro cervello non è un ricevitore passivo di tali informazioni. Infatti, l'elaborazione degli input sensoriali coinvolge diverse rappresentazioni corporee che influiscono sul modo in cui percepiamo gli oggetti. Numerosi studi hanno dimostrato che l’elaborazione degli stimoli tattili avviene a diversi livelli. Un primo livello coinvolge le mappe somatotopiche, rappresentazioni stabili del corpo che non tengono conto dei cambiamenti posturali. Un secondo livello, coinvolge lo schema corporeo, una rappresentazione dinamica che si riadatta in base ai cambiamenti posturali. Questa tesi indaga il ruolo delle diverse rappresentazioni corporee nel modulare la percezione di stimoli innocui e dolorosi applicati sulle dita. Tali aspetti sono stati indagati attraverso due illusioni somatosensoriali: l’illusione di Aristotele e l’illusione della griglia termica. Nello specifico, il primo studio indaga i correlati percettivi delle alterazioni a carico della rappresentazioni somatotopiche delle dita e il modo in cui tali alterazioni possono influenzare la relazione funzionale tra più dita nella percezione tattile. Per tale scopo, abbiamo utilizzato l' illusione di Aristotele quale paradigma sperimentale e la distonia focale della mano come modello di rappresentazioni somatotopiche delle dita alterate. Tali alterazioni potrebbero riflettersi sulla relazione funzionale tra più dita nella percezione tattile. La distonia focale della mano, tuttavia, presenta disordini somatosensoriali comuni ad altre forme di distonia focale e alla malattia di Parkinson. Questo potrebbe essere dovuto ad una comune alterazione a carico dei gangli della base. Al fine di escludere ogni possibile ruolo dei gangli della base nel modulare l’illusione di Aristotele in pazienti con distonia focale della mano abbiamo condotto due esperimenti in cui i tali pazienti e quelli con altre forme di distonia focale e malattia di Parkinson sono stati confrontati rispetto a quanto percepissero o meno l’illusione di Aristotele. I risultati dei due esperimenti 1) confermano che la rappresentazione somatotopica delle dita è specificamente alterata nella distonia focale della mano, e 2) dimostrano che tale alterazione si associa ad un cambiamento nella relazione funzionale tra le dita nella percezione tattile specifica per la distonia focale della mano. Il secondo studio ha analizzato il ruolo dello schema corporeo nell’elaborazione di stimoli termo-tattili. L’obiettivo in questo caso era quello di studiare come l’organizzazione spaziale di diverse parti del corpo oggetto di stimolazione moduli l’illusione della griglia termica. Tale illusione è frequentemente utilizzata per studiare i fattori che influenzano la percezione del dolore. L’illusione della griglia termica deriva dall’applicare temperature calde e fredde innocue secondo uno specifico pattern caldo-freddo-caldo. Tale pattern di stimolazione determina che la temperatura fredda sia percepita come bollente. Nel nostro studio abbiamo utilizzato lo stesso pattern di stimolazione ma applicando ciascuna temperatura sull’indice, il medio e l’anulare. Incrociando o meno l’indice e il medio dei partecipanti, è emerso che la sensazione di caldo bruciante si riduceva quando il pattern spaziale delle temperature era freddo-caldo-caldo, indipendentemente che le dita fossero o meno incrociate. Tali risultati dimostrano come il cervello tenga conto della posizione relativa che ciascuno stimolo occupa rispetto agli altri nel definire la sensazione di calore doloroso evocato dall’illusione della griglia termica.In everyday life, we use our hands to explore or manipulate objects. The processing of somatosensory inputs involves different representations of the body, and the resulting subjective experience of touch is strongly influenced by the type of body representation used for sensory processing. In the first instance tactile stimuli are processed within somatotopic maps that preserve the topographical organization of the physical body. Successively, the somatosensory inputs are further processed within the body schema which takes track of the postural changes of the body. In this framework, the current thesis is aimed at investigating the specific role of somatotopic maps and body schema in modulating the multi-digit tactile and thermo-tactile perception. This purpose has been achieved by “fooling” the sense of touch through two somatosensory illusions: Aristotle’s illusion and the Thermal Grill Illusion. In the first study we investigated the tactile processing occurring within the somatotopic maps. We approached to this issue by studying the multi-digit tactile perception as influenced by alterations in somatosensory maps. To address this specific aim, focal hand dystonia was chosen as a model of altered somatotopic maps. We used the Aristotle’s illusion to specifically investigate the functional meaning of the interplay between different fingers in tactile perception. Since, behavioural and neurophysiological studies indicate that the mechanisms underlying this illusion involve primary somatosensory cortex, and since a number of FHD neuroimaging studies have widely demonstrated anatomical alterations in the primary somatosensory cortex of FHD patients, we would expect that the Aristotle’s illusion is compromised in focal hand dystonia. This result would demonstrate a case of interdigit functional somatosensory alterations as specific counterpart of the widely reported anatomical alteration of somatosensory maps. However, FHD shares common somatosensory alterations features with other forms of focal dystonia and with PD. This is probably due to a common underlying factor: the abnormal basal ganglia activity. In order to exclude any possible role of basal ganglia in potentially modulation of Aristotle’s illusion in FHD we performed two experiments in which we compared the Aristotle’s illusion in FHD with other forms of focal dystonia and PD. Altogether, the two experiments: 1) confirm that fingers representation is specifically altered in FHD, and 2) demonstrate that abnormal alterations in fingers representation determine distorted tactile perception of objects simultaneously touched by different digits. The second study investigated the role the of body schema in processing thermotactile stimuli. More precisely, we studied how multiple thermotactile innocuous stimuli are combined in an unusual paradoxical painful sensation, termed Thermal Grill Illusion (TGI). In the classical TGI, innocuous warm and cold stimuli are arranged in a warm-cold-warm fashion. Intriguingly, this type of stimulation evokes a feeling of burning heat. In the second study, we applied the warm-cold-warm pattern on the first three digits (index, middle, and ring) and we observed whether the TGI was modulated by changing the relative spatial position of the fingers, that is by crossing or uncrossing the middle over the index. Using this simple method we found that the perceptual experience of paradoxical pain might arise from the integration of multiple thermal inputs based on the relative position of the stimulated fingers according to external frame of reference. Indeed, in the crossed position, the paradoxical heat sensation was reduced when the cooled finger was the middle, and increased when the cooled finger was the index. This interesting result indicates that the brain takes into account the relative spatial position of each stimulus to produce the paradoxical heat sensation termed TGI
Proprioceptive drift is affected by the intermanual distance rather than the distance from the body's midline in the rubber hand illusion
Full text linkIn the rubber hand illusion (RHI), simultaneous brush stroking of a subject's hidden hand and a visible rubber hand induces a transient illusion of the latter to "feel like it's my hand" and a proprioceptive drift of the hidden own hand toward the rubber hand. Recent accounts of the RHI have suggested that the illusion would only occur if weighting of conflicting sensory information and their subsequent integration results in a statistically plausible compromise. In three different experiments, we investigated the role of distance between the two hands as well as their proximity to the body's midline in influencing the occurrence of the illusion. Overall, the results suggest that the illusion is abolished when placing the two hands apart, therefore increasing the mismatch between the visual and proprioceptive modality, whereas the proximity of the two hands to the body's midline plays only a minor role on the subjective report of the illusion. This might be driven by the response properties of visuotactile bimodal cells encoding the peripersonal space around the hand
Trait-related neural basis of attentional bias to emotions: a tDCS study
Full text linkNegative emotional stimuli can strongly bias attention, particularly in individuals with high levels of dispositional negative affect (NA). The current study investigated whether the prefrontal cortex (PFC), a brain region involved in the top-down regulation of emotional processing, plays a different role in controlling attention to emotions, depending on the individual NA. Sham and anodal transcranial direct current stimulation (tDCS) was delivered over the right or left PFC while assessing attentional bias (AB) to emotions (happy, angry, sad faces) in individuals with higher and lower trait NA. When tDCS was inactive (sham), individuals with higher trait NA showed AB toward angry and away from sad faces, while individuals with lower trait NA presented with no AB. Right anodal-tDCS abolished the AB toward angry faces and induced an AB toward sad faces in individuals with higher trait NA, while no effect was found in individuals with lower trait NA. Left anodal-tDCS abolished any AB in individuals with higher trait NA and induced an AB away from happy faces in individuals with lower trait NA. These findings confirm a critical role of trait NA in AB to emotions and demonstrate a different involvement of PFC in emotional processing based on dispositional affect
Aristotle’s Illusion in Parkinson’s Disease: Evidence for Normal Interdigit Tactile Perception
Full text linkSensory alterations, a common feature of such movement disorders as Parkinson's disease (PD) and dystonia, could emerge as epiphenomena of basal ganglia dysfunction. Recently, we found a selective reduction of tactile perception (Aristotle's illusion, the illusory doubling sensation of one object when touched with crossed fingers) in the affected hand of patients with focal hand dystonia. This suggests that reduced tactile illusion might be a specific feature of this type of dystonia and could be due to abnormal somatosensory cortical activation. The aim of the current study was to investigate whether Aristotle's illusion is reduced in the affected hand of patients with PD. We tested 15 PD patients, in whom motor symptoms were mainly localised to one side of the body, and 15 healthy controls. Three pairs of fingers were tested in crossed (evoking the illusion) or parallel position (not evoking the illusion). A sphere was placed in the contact point between the two fingers and the blindfolded participants had to say whether they felt one or two stimuli. Stimuli were applied on the affected and less or unaffected side of the PD patients. We found no difference in illusory perception between the PD patients and the controls, nor between the more affected and less/unaffected side, suggesting that Aristotle's illusion is preserved in PD. The retained tactile illusion in PD and its reduction in focal hand dystonia suggest that the basal ganglia, which are dysfunctional in both PD and dystonia, may not be causally involved in this function. Instead, the level of activation between digits in the somatosensory cortex may be more directly involved. Finally, the similar percentage of illusion in the more affected and less or unaffected body sides indicates that the illusory perception is not influenced by the presence or amount of motor symptoms
Attentional bias to emotions after prolonged endurance exercise is modulated by age
No full textPhysical exercise has an impact in biasing attention to positive or negative emotional stimuli. While attentional shift to emotions varies with age, evidence is lacking on the effect of prolonged endurance exercise on age-related attentional bias to emotions. This study aims at filling this knowledge gap, by applying a dot-probe task to measure attentional bias to emotions before and after a half-marathon in healthy participants of different ages (age range 21-65 years). State anxiety, positive and negative affect were also assessed. Younger adults showed attentional bias towards anger and away from sadness after the race, supporting the hypothesis of the congruency between the high-arousing task and the associated emotion (anger) in the modulation of attention. Conversely, older adults showed a bias away from anger, likely representing an attempt to maintain an optimal emotional level after the competition. This study sheds new light on how age impacts on emotional mechanisms involved in prolonged endurance exercise and suggests that regulatory processes in response to stress may be involved differently, depending on age
Bodily self-perception during voluntary actions: The causal contribution of premotor cortex and cerebellum
No full textVoluntary actions are accompanied by the experience of controlling one's own movements (sense of agency) and the feeling that the moving body part belongs to one's self (sense of body ownership). So far, agency and body ownership have been investigated separately, leaving the neural underpinnings of the relation between the two largely unexplored. The aim of this study was to explore the causal role of two multisensory brain regions, that is the premotor cortex (PMc) and the cerebellum, in agency and body ownership concurrently on the same behavioral task, i.e., the moving Rubber Hand Illusion (mRHI). Participants watched a rubber hand while moving their hidden hand. The type of movement (active or passive) and posture of the rubber hand (congruent or incongruent) differed in three conditions (active congruent, passive congruent, active incongruent), so that agency and ownership could be elicited either separately or concurrently. Agency and ownership were measured by subjective report and proprioceptive drift. Sham and anodal transcranial direct current stimulation (tDCS) were delivered to the PMc (Experiment 1) or the cerebellum (Experiment 2) prior to the mRHI task. Independent of the site or type of tDCS, subjective reports revealed that both agency and ownership were evoked in the active congruent condition, ownership but not agency in the passive congruent condition, and agency but not ownership in the active incongruent condition. The proprioceptive drift was evoked in the active congruent and the passive congruent condition. Anodal tDCS over the PMc reduced the feeling of agency in the active congruent condition, while it enhanced proprioceptive drift when applied over the cerebellum. These findings suggest a specific causal contribution of the PMc and the cerebellum to bodily self-perception during voluntary movement, with the PMc mainly involved in awareness of action and the cerebellum in proprioceptive adaptation of body position in space
Bridge-to-drain: How nanoparticles can promote coalescence in model polymer blends
Full text linkHypothesis
Multiphase liquids with a droplet-in-matrix morphology are ubiquitous in many industries, from food to cosmetics and pharmaceuticals to plastics. The challenge is to control the average droplet size, which is a key parameter for the performance of the material. Nanoparticles at the droplet-matrix interface make it possible to stabilize polymer blends against coalescence. However, it has been shown that very low amounts of nanoparticles can have the opposite effect and surprisingly promote coalescence. Regardless of whether this phenomenon is desirable or not, it is important to understand it and potentially utilize it for rational design of multiphase fluids.
Experiments
We use microfluidics to unveil the mechanism of nanoparticle-induced coalescence in a model blend of polydimethylsiloxane in poly(iso)butylene (PDMS/PB 4/96 vol/vol) containing tiny amounts (up to about 0.2 wt%) of zinc oxide (ZnO) nanoparticles driven at the droplet-matrix interface via a two-step mixing protocol.
Results
Despite negligible effects on rheology and interfacial energy, the nanoparticles significantly promote coalescence. Analysis of hundreds of coalescence events revealed that the nanoparticles bridge colliding droplets and keep them in contact long enough to allow drainage of the matrix film even when the collisions occur at unfavorable angles where bare droplets do not coalesce. This novel “bridge-to-drain” mechanism requires that (i) the droplets are only partially covered by the particles and (ii) the latter have the ability to bridge droplets. A dimensionless critical surface coverage fraction was defined, above which the nanoparticles stop promoting coalescence and start stabilizing the microstructure
Transforming the Thermal Grill Effect by Crossing the Fingers
Full text linkSummaryThe relation between pain perception and spatial representation of the body is poorly understood. In the thermal grill illusion (TGI), alternating non-noxious warm and cold temperatures cause a paradoxical, sometimes painful, sensation of burning heat [1]. We combined thermal grill stimulation with crossing the fingers to investigate whether nociceptively mediated sensation depends on the somatotopic or spatiotopic configuration of thermal inputs. We stimulated the index, middle, and ring fingers when the middle finger either was or was not crossed over the index to generate “warm-cold-warm” patterns in either somatotopic or spatiotopic coordinates. Participants adjusted a temperature delivered to the other hand until it matched their perception of the cold target finger (index or middle). We found significant temperature overestimation when the target was central within the spatial configuration (warm-cold-warm) compared to when it was peripheral (cold-warm-warm). Crucially, this effect depended on the spatiotopic configuration of thermal inputs, but it was independent of the finger posture and present for both index and middle target fingers—the thermal grill effect for the middle finger was abolished when it was crossed over the index to adopt a spatiotopically peripheral position, while the same effect was newly generated for the index finger by the same postural change. Our results suggest that the locations of multiple stimuli are remapped into external space as a group; nociceptively mediated sensations depended not on the body posture, but rather on the external spatial configuration formed by the pattern of thermal stimuli in each posture
Curing Kinetics of Bioderived Furan-Based Epoxy Resins: Study on the Effect of the Epoxy Monomer/Hardener Ratio
No full textThe potential of furan-based epoxy thermosets as a greener alternative to diglycidyl ether of Bisphenol A (DGEBA)-based resins has been demonstrated in recent literature. Therefore, a deep investigation of the curing behaviour of these systems may allow their use for industrial applications. In this work, the curing mechanism of 2,5-bis[(oxiran-2-ylmethoxy)methyl]furan (BOMF) with methyl nadic anhydride (MNA) in the presence of 2-methylimidazole as a catalyst is analyzed. In particular, three systems characterized by different epoxy/anhydride molar ratios are investigated. The curing kinetics are studied through differential scanning calorimetry, both in isothermal and non-isothermal modes. The total heat of reaction of the epoxy resin as well as its activation energy are estimated by the non-isothermal measurements, while the fitting of isothermal data with Kamal’s autocatalytic model provides the kinetic parameters. The results are discussed as a function of the resin composition. The global activation energy for the curing process of BOMF/MNA resins is in the range 72–79 kJ/mol, depending on both the model used and the sample composition; higher values are experienced by the system with balanced stoichiometry. By the fitting of the isothermal analysis, it emerged that the order of reaction is not only dependent on the temperature, but also on the composition, even though the values range between 0.31 and 1.24
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