Effect of mastication on regional cerebral blood flow in humans examined by positron-emission tomography with ¹⁵O-labelled water and magnetic resonance imaging

Sensorimotor cortical activation in patients with sleep bruxism

Sleep bruxism is assumed to be triggered by a dysfunctional subcortical and cortical network. This study investigates sensorimotor cortical activation in patients with sleep bruxism during clenching and chewing. Nine polysomnographically diagnosed patients and nine healthy control subjects underwent magnetoencephalography (MEG). During clenching and chewing, patients with bruxism revealed significantly larger event-related desynchronization in the somatomotor area (Brodmann area 4) than healthy subjects. Group differences in the muscle activity were ruled out by electromyography (EMG) assessments during MEG. This result might be regarded as a consequence of increased sensorimotor cortical representation of the tongue and chewing musculature due to an enhanced parafunctional muscle activity in bruxers potentially triggered by occlusal factors. Alternatively, a secondary activation of cortical structures during sleep bruxism in the context of an activated network of subcortical and cortical structures might lead to increased cortical representation of the chewing musculature via use dependent plasticity.

Effect of dental status on changes in mastication in patients with obesity following bariatric surgery

Background

Patients scheduled for bariatric surgery (BS) are encouraged to chew slowly in order to optimise the digestion process. The influence of dental status on patients' ability to comply with advice on chewing behaviour is poorly documented. This study aims to compare modifications of chewing function before and after BS in three groups of obese patients differing in dental status.

Method and Findings

A cohort of 46 obese women provided three groups: FD group: fully dentate (7–10 functional dental units [FU]); PD group: partially dentate (4–6 FU) without partial dentures; DW group: partial and complete denture wearers. Chewing time (CT), number of chewing cycles (CC), and chewing frequency (CF) were measured before and after surgery during mastication of standardised samples of raw carrot, peanuts, banana, apple and jelly. The median particle-size distribution (D50) of the pre-swallowed bolus was also evaluated for peanut and carrot. Before surgery, the PD and DW groups exhibited greater mean CCs and CTs than the FD group (SNK p<0.05) and produced a bolus with higher granulometry (SNK, p<0.05) than the FD group. After surgery, CT and CC increased for all groups and for all foods, but not statistically significant for jelly. The resulting changes in bolus granulometry observed depended on both food and dental status. The granulometry of carrot bolus remained as fine or as coarse in FD and DW groups respectively as it was before surgery while it was significantly decreased in the PD group (Student's test, p<0.001).

Conclusions

After bariatric surgery, all the obese patients, regardless of dental status modified their chewing kinematics. The effects of this chewing behaviour on bolus granulometry depended on dental status and type of food. Further studies are needed to understand better the impact of dental status on feeding behaviour and nutrition in patients with obesity.

Associations between self-assessed masticatory ability and higher brain function among the elderly

Among the elderly, the quality of higher brain function is a contributing factor in performing activities of daily living. The aim of the study is to elucidate, epidemiologically, associations between mastication and higher brain function. A total of 208 community-dwelling elderly persons, aged 70-74 years, were enrolled. Self-assessed masticatory ability (masticatory ability) was classified into one of three categories: ability to chew all kinds of food, ability to chew only slightly hard food, or ability to chew only soft or pureed food. Brain function was assessed by four neuropsychological tests: Raven's Colored Progressive Matrices (RCPM) test, the Verbal Paired Associates 1 (VerPA) task and the Visual Paired Associates 1 task (from the Wechsler Memory Scale Revised Edition), and the Block Design subtest (from the Wechsler Adult Intelligence Scales-Third Edition). Correlations between masticatory ability and each test were examined using Spearman rank correlation coefficients. Multinominal logistic regression models were conducted with the neuropsychological tests as the dependent variables and masticatory ability as the principal independent variable to adjust for age, gender, educational background, social activity, drinking/smoking habits, chronic medical conditions and dental status. Significant correlations were found between the RCPM test, the VerPA task, the Block Design test and masticatory ability. In multinominal logistic regression models, poor masticatory ability was significantly and independently related to the categories under the mean-s.d. points compared with those of the mean ± s.d. ranges for RCPM test and the VerPA task. Significant associations may exist between mastication and higher brain function among the elderly.

Masticatory function and cognitive function

Recent studies have suggest that masticatory (chewing) function is useful for maintaining neurocognitive function in the elderly. For example, a reduced ability to masticate, such as that resulting from toothlessness or soft-diet feeding, causes learning and memory deficits in aged animals and pathologic changes in the hippocampus. In addition, occlusal disharmony impairs hippocampal memory processes via chronic stress, and induces similar hippocampal pathology. Chewing, however, rescues stress-induced suppression of long-term potentiation in the hippocampus and the stress-induced impairment of hippocampal-dependent learning. These findings strongly suggest a link between mastication and neurocognitive function.

Transfer Function Analysis of Cerebral Autoregulation Dynamics during Jaw Movements

Mastication as a mild but life-long exercise has been examined as a possibility for maintaining brain function in the elderly. Little is known, however, about the cerebral circulatory response during masticatory movement. The aim of this study was to develop a monitoring system for circulation dynamics during masticatory movement and to apply this system to the study of cerebral autoregulation. Cerebral blood flow, heart rate, and arterial blood pressure were simultaneously recorded, and changes in these circulatory systems were quantitatively evaluated in 38 young healthy volunteers. Transfer function analysis was also performed on blood pressure and cerebral blood flow for investigation of cerebral autoregulation during gum chewing. Although increases in cerebral blood flow, mean blood pressure, and heart rate suggested the activation of cerebral and systemic circulation during gum chewing, increased cerebral circulation was independent of systemic circulation. Our results suggest that cerebral autoregulation is well maintained during jaw movements.

fMRI study of brain activity elicited by oral parafunctional movements

Parafunctional masticatory activity, such as the tooth clenching and grinding that is associated with bruxism, is encountered by clinicians in many disciplines, including dentistry, neurology and psychiatry. Despite this, little is known about the neurological basis for these activities. To identify the brain network engaged in such complex oromotor activity, functional magnetic resonance imaging (fMRI) was used to elucidate the brain activation patterns of 20 individuals (10 males and 10 females, mean s.d. age of 26.3+/-4.1 years) with (parafunctional, PFx group, 5M/5F) and without (normal functional, NFx group, 5 M/5F) self-reported parafunctional grinding and clenching habits during clenching and grinding tasks. Subject group classification was based on: (i) self-reported history, (ii) clinical examination, (iii) evaluation of dental casts and (iv) positive responses to the temporomandibular disorder (TMD) History Questionnaire [Dworkinand LeResche, Journal of Craniomandibular Disorders, (1992) 6:301]. While subjects performed these oromotor tasks, each wore a custom-designed oral appliance minimizing head motion during imaging. Mean per cent signal changes showed significant between group differences in motor cortical (supplementary motor area, sensorimotor cortex and rolandic operculum) and subcortical (caudate) regions. Supplementary motor area data suggest that motor planning and initiation, particularly during the act of clenching, are less prominent in individuals with oromotor parafunctional behaviours. The overall extent of activated areas was reduced in subjects with self-reported parafunctional masticatory activity compared with the controls. This study's methodology and findings provide an initial step in understanding the neurological basis of parafunctional masticatory activities that are relevant for therapeutic research applications of temporomandibular joint and muscle disorders and associated comorbidities.

Occlusion and brain function: mastication as a prevention of cognitive dysfunction

Research in animals and humans has shown that mastication maintains cognitive function in the hippocampus, a brain area important for learning and memory. Reduced mastication, an epidemiological risk factor for the development of dementia in humans, attenuates spatial memory and causes hippocampal neurons to deteriorate morphologically and functionally, especially in aged animals. Active mastication rescues the stress-attenuated hippocampal memory process in animals and attenuates the perception of stress in humans by suppressing endocrinological and autonomic stress responses. Active mastication further improves the performance of sustained cognitive tasks by increasing the activation of the hippocampus and the prefrontal cortex, the brain regions that are essential for cognitive processing. Abnormal mastication caused by experimental occlusal disharmony in animals produces chronic stress, which in turn suppresses spatial learning ability. The negative correlation between mastication and corticosteroids has raised the hypothesis that the suppression of the hypothalamic-pituitary-adrenal (HPA) axis by masticatory stimulation contributes, in part, to preserving cognitive functions associated with mastication. In the present review, we examine research pertaining to the mastication-induced amelioration of deficits in cognitive function, its possible relationship with the HPA axis, and the neuronal mechanisms that may be involved in this process in the hippocampus.

Effects of chewing gum on cognitive function, mood and physiology in stressed and non-stressed volunteers

RATIONALE

Recent research suggests that chewing gum may improve aspects of cognitive function and mood. There is also evidence suggesting that chewing gum reduces stress. It is important, therefore, to examine these two areas and to determine whether contextual factors (chewing habit, type of gum, and personality) modify such effects.

OBJECTIVES

The aims of the present study were: (i) to determine whether chewing gum improved mood and mental performance; (ii) to determine whether chewing gum had benefits in stressed individuals; and (iii) to determine whether chewing habit, type of gum and level of anxiety modified the effects of gum.

SUBJECTS AND METHODS

A cross-over study involving 133 volunteers was carried out. Each volunteer carried out a test session when they were chewing gum and without gum, with order of gum conditions counterbalanced across subjects. Baseline sessions were conducted prior to each test session. Approximately half of the volunteers were tested in 75 dBA noise (the stress condition) and the rest in quiet. Volunteers were stratified on chewing habit and anxiety level. Approximately, half of the volunteers were given mint gum and half fruit gum. The volunteers rated their mood at the start and end of each session and had their heart rate monitored over the session. Saliva samples were taken to allow cortisol levels (good indicator of alertness and stress) to be assayed. During the session, volunteers carried out tasks measuring a range of cognitive functions (aspects of memory, selective and sustained attention, psychomotor speed and accuracy).

RESULTS

Chewing gum was associated with greater alertness and a more positive mood. Reaction times were quicker in the gum condition, and this effect became bigger as the task became more difficult. Chewing gum also improved selective and sustained attention. Heart rate and cortisol levels were higher when chewing which confirms the alerting effect of chewing gum.

CONCLUSIONS

Overall, the results suggest that chewing gum produces a number of benefits that are generally observed and not context-dependent. In contrast to some previous research, chewing gum failed to improve memory. Further research is now required to increase our knowledge of the behavioral effects of chewing gum and to identify the underlying mechanisms.

Steigert Kaugummikauen das kognitive Leistungsvermögen?

Aufgrund weniger und methodisch verbesserungsbedürftiger Studien mit kleinen Stichproben wird behauptet, Kaugummikauen wirke sich positiv auf Lernleistungen, Konzentration und Gedächtnis aus. Mit zwei randomisierten Experimenten (N1 = 548 bzw. N2 = 504 Kinder; 5. und 6. Jahrgangsstufen) wurde überprüft, ob «Kauer» in ausgewählten Intelligenzsubtests (Experiment I) und bei Aufmerksamkeits- bzw. Konzentrations- sowie Gedächtnisaufgaben (Experiment II) bessere Leistungen erbringen als «Nichtkauer». Statistisch signifikante (aber nur kleine) Unterschiede resultierten lediglich bei zwei Konzentrationskennwerten sowie bei Aufgaben zur Erfassung von kurzfristigem Behalten zugunsten der «Nichtkauer». Die vorliegenden Ergebnisse verdeutlichen den Stellenwert einer methodisch adäquaten Replikation von an kleinen Stichproben gewonnenen und vorschnell popularisierten Befunden.

Influence of voluntary control of masticatory side and rhythm on cerebral hemodynamics

The aim of this study was to investigate the influence on cerebral hemodynamics of voluntary control of masticatory side and rhythm during gum chewing. Blood flow velocity in the middle cerebral artery was measured using transcranial Doppler ultrasonography to evaluate cerebral circulation in healthy volunteers. Heart rate and masseter muscle activity were recorded simultaneously. Volunteers performed three tasks: (1) free gum chewing, (2) gum chewing in which mastication was limited to the right side, and (3) gum chewing in which mastication was limited to the right side and rhythm was set at 1.0 Hz. Changes in cerebral circulation during pre-task, on-task, and post-task periods were analyzed using random effects model, and differences in cerebral circulation and muscle activity between tasks were analyzed using the Friedman test. In all tasks, on-task cerebral circulation was greater than pre-task. Muscle activity and masticatory rhythm varied between tasks, whereas the rate of increase in cerebral circulation did not differ significantly among tasks. These results suggest that cerebral circulation is activated during gum chewing, irrespective of voluntary control of masticatory side and rhythm.

The effect of mastication on human motor preparation processing: a study with CNV and MRCP

To clarify the effect of mastication on motor preparation processing using electroencephalography (EEG), we investigated the effect of mastication on contingent negative variation (CNV) and reaction time (RT) in Experiment 1, and movement-related cortical potentials (MRCPs) in Experiment 2. The twelve subjects performed four CNV or MRCP sessions, and in the Mastication condition chewed a gum base during the resting period between sessions, Pre (before chewing) and Post 1, 2, and 3 (after chewing). In the Control condition, the subjects performed the same sessions without chewing gum during the intervals between sessions on another day. In Experiment 1, the mean amplitudes of the early- and late-CNV were significantly larger in Mastication than Control at Post 2 and Post 3. RT also differed significantly between Mastication and Control at Post 3. By contrast, in Experiment 2, there were no significant differences between Mastication and Control for the mean amplitudes of MRCPs including Bereitschaftspotential (BP) and negative slope (NS') in any session. These results suggest that mastication influences cognitive processing reflected by CNV with stimulus-triggered movement, rather than motor-related processing reflected by MRCPs relating to self-initiated movement, and provide evidence concerning the mechanisms for the effect of mastication on the human brain.

Cortical lateralization of bilateral symmetric chin movements and clinical relevance in tumor patients—A high field BOLD–FMRI study

Although unilateral lesion studies concerning the opercular part of primary motor cortex report clinically severe motor deficits (e.g. anarthria, masticatory paralysis), functional lateralization of this area has not yet been addressed in neuroimaging studies. Using BOLD-FMRI, this study provides the first quantitative evaluation of a possible cortical lateralization of symmetric chin movements (rhythmic contraction of masticatory muscles) in right-handed healthy subjects and presurgical patients suffering tumorous lesions in the opercular primary motor cortex. Data were analyzed according to "activation volume" and "activation intensity". At group level, results showed a strong left-hemispheric dominance for chin movements in the group of healthy subjects. In contrast, patients indicated dominance of the healthy hemisphere. Here, a clinically relevant dissociation was found between "activation volume" and "activation intensity": Although "activation volume" may be clearly lateralized to the healthy hemisphere, "activation intensity" may indicate residual functionally important tissue close to the pathological tissue. In these cases, consideration of BOLD-FMRI maps with the exclusive focus on "activation volume" may lead to erroneous presurgical conclusions. We conclude that comprehensive analyses of presurgical fMRI data may help to avoid sustained postoperative motor deficits and dysarthria in patients with lesions in the opercular part of primary motor cortex.

Functional magnetic resonance imaging of brain activity during chewing and occlusion by natural teeth and occlusal splints

Brain imaging based on functional magnetic resonance (fMRI) is a useful tool for examination of neuronal networks and cerebral structures subserving visiospatial function. The purpose of this study was to compare the brain activity during chewing and occlusal function in centric occlusion on natural teeth or on occlusal splints. Four tasks were performed by 13 healthy, fully dentate subjects (21-32 years old, 6 female and 7 male): occlusal tap-tap movements in centric occlusion by natural teeth, after application of a maxillary occlusal splint and chewing movements on left and right sided rubberdam strips. In order to reveal which areas of the brain were more strongly activated, conjunction analyses between the different tasks were performed for each subject and for the average values of brain signal activity of all subjects. Whilst several known foci of activity were subtracted, differences of significant activity rested in areas of the sensorimotor cortex. Mainly ipsitaterality of hemispheres concerned the left and right sided chewing, whereas the conjunction between tap-tap movements on natural teeth and splint occlusion indicated only one weak, but significant activation foci. The study confirms fMRT as one of the most useful developing methods to clear up neuro-cortical effectiveness of occlusion and occlusal therapy.

Analysis of brain activity immediately before conscious teeth clenching using magnetoencephalographic method

The reasons for unconscious teeth clenching have not been clarified. The long-term goal of our project was the elucidation of processing in the brain immediately before unconscious teeth clenching, in order to clarify its significance in humans. The objective of the present study was to establish a magnetoencephalographic (MEG) method of measuring brain activity immediately before clenching, and to clarify the time-course of brain activity immediately before conscious clenching. We measured the MEG signal in six subjects before, during and after clenching in a protocol that restricted head movement <5 mm. We derived tomographic estimates of brain activity for each time slice of data, as well as time courses for regional brain activations. Analysis of the tomographic images and time courses yielded statistical maps of activity in the motor, pre-motor and somatosensory cortices immediately before clenching in all subjects. Activations were found bilaterally, but with a strong unilateral bias in most subjects. Our results demonstrate that the MEG procedures, we have introduced are capable of measuring brain activity immediately before clenching, and indicate that analysis should begin from at least 200 ms before electromyogram onset.

The effects of chewing versus caffeine on alertness, cognitive performance and cardiac autonomic activity during sleep deprivation

Chewing has been shown to alleviate feelings of sleepiness and improve cognitive performance during the day. This study investigated the effect of chewing on alertness and cognitive performance across one night without sleep as well as the possible mediating role of cardiac autonomic activity. Fourteen adults participated in a randomized, counterbalanced protocol employing a chewing, placebo and caffeine condition. Participants completed tasks assessing psychomotor vigilance, tracking, grammatical reasoning, alertness and sleepiness each hour across the night. All participants received either placebo or caffeine (200 mg), while the chewing condition also chewed on a tasteless and odorless substance for 15 min each hour. Heart rate (HR), root mean square of the successive differences in R-R intervals on the ECG (RMSSD), and preejection period (PEP) were simultaneously recorded. Alertness and cognitive performance amongst the chewing condition did not differ or were in fact worse when compared with placebo. Similarly, measures of HR and RMSSD remained the same between these two conditions; however, PEP was reduced in the later part of the night in the chewing condition compared with a relative increase for placebo. Caffeine led to improved speed and accuracy on cognitive tasks and increased alertness when compared with chewing. Relative increases in RMSSD and reductions in HR were demonstrated following caffeine; however, no change in PEP was seen. Strong associations between cardiac parasympathetic activity and complex cognitive tasks, as well as between subjective alertness and simpler cognitive tasks, suggest a differential process mediating complex versus simple cognitive performance during sleep deprivation.

Muscle contraction accelerates IL-6 mRNA expression in the rat masseter muscle

OBJECTIVE

This study was conducted to determine if interleukin-6 (IL-6) and interleukin-1beta (IL-1beta) mRNA expression increase in response to muscle contraction caused by repetitive electrical stimulation of the rat masseter muscle.

METHODS

Male Wistar rats weighing 140-160 g were divided randomly into the following three groups: electrical stimulation (ES) group (n=21), carrageenan injection (CI) group (n=24), and ES under dantrolene sodium (muscle relaxant) injection (ESDI) group (n=7). ES or CI was done to the left masseter; and mock ES or mock CI to the right. Muscle tissues on both sides were sampled for total RNA isolation. Real-time RT-PCR was performed, with the cyclophilin A (CypA) mRNA level in each sample as an internal control. Mean relative IL-6 (il-6/cypA) and IL-1beta (il-1beta/cypA) mRNA levels were compared between the experimental and mock-treated sides within each group.

RESULTS

Mean IL-6/CypA levels in the ES- or CI-treated muscle significantly increased, without any significant incremental change observed in either mock-treated muscle. Interestingly, the increase in the il-6/cypA level caused by the ES was suppressed by the injection of dantrolene sodium in the ESDI group. Furthermore, the mean il-1beta/cypA level in the CI-treated masseter also significantly increased without any significant incremental change observed in the mock-treated muscle. However, there was no significant difference in the mean il-1beta/cypA levels in the masseter between the ES- and the mock-treated sides.

CONCLUSIONS

These results show that IL-6 mRNA expression in the rat masseter muscle was accelerated by the CI or by repetitive muscle contraction induced by ES. Since the mRNA level of IL-1beta, a well-known proinflammatory cytokine, was not altered by the contraction, the accelerated IL-6 mRNA expression elicited by the muscle contraction does not seem to be related to local inflammation.

Strategies for block-design fMRI experiments during task-related motion of structures of the oral cavity

Functional MRI (fMRI) studies of jaw motion, speech, and swallowing disorders have been hampered by motion artifacts. Tissue motion perturbs the static magnetic field, creating geometric distortions in echo-planar images that lead to many false positives in activation maps. These problems have restricted blood oxygenation level-dependent (BOLD) fMRI studies involving orofacial muscles to event-related designs, which offer weak contrast-to-noise ratios when compared to block designs. Two new approaches are described that greatly reduce false positives in the activation maps created by the distortions in block-design fMRI studies involving jaw and tongue motion during chewing. First, an appropriate task duration of 10-14 s was found to maximize functional contrast while minimizing motion artifacts. Second, three motion-sensitive postprocessing methods were applied successively to examine the temporal and spatial characteristics of responses and identify and remove false positives caused by motion artifacts. These techniques are shown to allow the use of block design in an fMRI study of a jaw motion task. Extension to speech and swallowing tasks is discussed.

Evaluation of the effects of mastication and swallowing on gastric motility using electrogastrography

OBJECTIVES

The influence of mastication and swallowing on gastric motor function was evaluated by electrogastrography (EGG) and abdominal ultrasonography.

METHODS

The subjects were 30 elderly patients with tubal feeding without mastication and swallowing (T group) and 30 elderly controls who processed food by mastication and swallowing (C group). Gastric motor function was percutaneously examined before and after the ingestion of 250 ml of a liquid diet using an electrogastrograph (NIPRO EGG, A and D, Tokyo, Japan). The cross-sectional area of the gastric antrum was measured at 1 and 30 min after the start of ingestion of the liquid diet by external ultrasonography of the abdomen, and the gastric excretion function was evaluated. Furthermore, the spectral analysis of heart rate variability was performed using Holter electrocardiograms before and after ingestion. The low frequency power (LF power, 0.04-0.15 Hz), high frequency power (HF power, 0.15-0.40 Hz), and the LF/HF ratio were determined.

RESULTS

The peak amplitude at 3 cycles per minute (cpm) was significantly increased after ingestion in the C and T groups (p<0.05), and the ratio of increase was significantly lower in the T group (p<0.05). The mean amplitude for the brady-gastria and tachy-gastria was significantly higher in the T group than in the C group (p<0.05). The gastric excretion function, as evaluated by external ultrasonography of the abdomen, was significantly lower in the T group than in the C group (p<0.05). An analysis of heart rate variability demonstrated that the HF power, a parameter of parasympathetic activity, after ingestion was significantly higher in the C group than in the T group (p<0.05). No changes in LF power or LF/HF ratio, parameters of sympathetic activity, were induced by ingestion in either the C or T groups.

CONCLUSIONS

The parasympathetic nerve dominantly controls gastric motor function, but autonomic nervous activity is reduced in patients who are unable to masticate and swallow food, resulting in adverse effects on gastric motor function and excretion function. Mastication and swallowing not only prepare food for passage from the oral cavity to the esophagus but are also important in terms of subsequent events that occur in stomach. It has been proposed that autonomic nervous activity might be involved in mastication and swallowing.

The functional neuroanatomy of coordinated orofacial movements: sparse sampling fMRI of whistling

Whistling serves as a model for a skilful coordinated orofacial movement with sensorimotor integration of auditory and proprioceptive input. The neural substrate of whistling was investigated by sparse sampling functional MRI (fMRI) where the motor task occurred during a silent interval between successive image acquisitions to minimize task-related imaging artefacts. Whistling recruited a symmetrically represented neural network including primary motor and ventral premotor cortex (PMv), SMA, cingulate gyrus, basal ganglia, primary and secondary somatosensory cortex, amygdala, thalamus and cerebellum. A temporal analysis revealed higher activity of left sensory cortex, right PMv and cerebellum during late execution compared to initiation of whistling. Task-related signal changes in right PMv and right paravermal cerebellum were found to correlate with the amplitude of the whistle sound in a separate correlation analysis. The findings emphasize the role of ventral premotor cortex, cerebellum and somatosensory areas as integrators of afferent input within a distributed orofacial sensorimotor network.

Mastication influences the survival of newly generated cells in mouse dentate gyrus

We examined the effects of soft food and tooth loss on neurogenesis in the adult dentate gyrus. Four-week-old mice were subjected to a powder diet for 10 weeks with or without removal of molars. They received a daily injection of bromodeoxyuridine (BrdU) at 14 weeks of age for 12 consecutive days. The number of BrdU-positive cells in the dentate gyrus of these mice did not differ from that of control at 1 day after the last BrdU injection. However, the BrdU-positive cells in these mice showed a larger reduction in number than in control at 5 weeks after the BrdU injection and the ratio of neurons to BrdU-positive cells decreased in the molarless mice. These results suggest that mastication influences the survival of newly generated cells in the adult dentate gyrus.

Role of glucose in chewing gum-related facilitation of cognitive function

This study tests the hypothesis that chewing gum leads to cognitive benefits through improved delivery of glucose to the brain, by comparing the cognitive performance effects of gum and glucose administered separately and together. Participants completed a battery of cognitive tests in a fully related 2 x 2 design, where one factor was Chewing Gum (gum vs. mint sweet) and the other factor was Glucose Co-administration (consuming a 25 g glucose drink vs. consuming water). For four tests (AVLT Immediate Recall, Digit Span, Spatial Span and Grammatical Transformation), beneficial effects of chewing and glucose were found, supporting the study hypothesis. However, on AVLT Delayed Recall, enhancement due to chewing gum was not paralleled by glucose enhancement, suggesting an alternative mechanism. The glucose delivery model is supported with respect to the cognitive domains: working memory, immediate episodic long-term memory and language-based attention and processing speed. However, some other mechanism is more likely to underlie the facilitatory effect of chewing gum on delayed episodic long-term memory.