The aim of this study was to evaluate the sensation of each tooth type at the cortical level. The tactical sensation from teeth plays an important role in controlling the masticatory system. However, the role of each tooth type has not been determined. Functional near-infrared spectroscopy (fNIRS) was used to detect changes in cerebral blood flow in the somatosensory cortex of 12 healthy volunteers. Painless vibrotactile stimuli were applied to 8 teeth (left maxillary and mandibular incisors, canines, 1(st) premolars, or 1(st) molars). The somatosensory cortex was activated during stimulation of all teeth. A comparison of cortical activation revealed significantly greater activation during stimulation of the maxillary and mandibular first molars. However, no significant differences were seen between any other teeth. These results indicate that the first molar is the most sensitive tooth type at the cortical level, and provide basic data on the relationship between input from individual tooth type and brain activation. These data could be useful for understanding the neural mechanisms of individual tooth types.

The objective of this study was to evaluate occlusal condition by assessing brain activity in the prefrontal cortex, which is associated with emotion. Functional near-infrared spectroscopy (fNIRS) was used to detect changes in cerebral blood flow in the prefrontal cortex of 12 healthy volunteers. The malocclusion model was a custom-made splint that forced the mandible into retrusion. A splint with no modification was used as a control. The cortical activation during clenching was compared between the retrusive position condition and the control condition. A visual analog scale score for discomfort was also obtained during clenching and used to evaluate the interaction between fNIRS data and psychiatric changes. Activation of the prefrontal cortex was significantly greater during clenching in the mandibular retrusive condition than during clenching in the control condition. Furthermore, Spearman rank-correlation coefficient revealed a parallel relation between prefrontal cortex activation and visual analog scale score for discomfort. These results indicate that fNIRS can be used to objectively evaluate the occlusal condition by evaluating activity in the prefrontal cortex.

ObjectiveThe aim of this study was to examine, after setting several restorations, the influence of adjusted occlusal interference during gum chewing on blood flow in the prefrontal area as determined using near-infrared spectroscopy.Material and methodsThe physiological rate was assessed using a visual analog scale (VAS) questionnaire. We selected 16 patients who desired prosthetic restorative treatment on the lateral dentition, and eight healthy volunteers. Subjects were divided into three eight-person groups. One group received restorations on the premolar area (PA), another group received restorations on the molar area (MA), and the control group (CT) received no prosthetic restorations. The spectroscope was fastened to the frontal region of the head after placement of the final restoration, but before adjustment.ResultsPre-adjustment (first gum chewing for CT) blood flow in the prefrontal cortex was measured during gum chewing. Blood flow was again measured during gum chewing after the restoration (second gum chewing for CT) had been adjusted in accordance with the subjective assessment of the patient while wearing the device. The VAS provided quantification of comfort during gum chewing before and after restoration adjustment. For the PA and MA groups, adjusting restorations decreased discomfort significantly during gum chewing. Moreover, in the MA group, prefrontal blood flow was significantly reduced, and blood flow correlated with discomfort.ConclusionsActivation of the prefrontal area may provide an objective criterion for judging the functionality of occlusion after prosthetic occlusal reconstruction and/or orthodontics.

Background The purpose of this study was to assess the cortical-level sensory differences between natural teeth with a periodontal membrane and dental implants. Material and Methods We used functional near-infrared spectroscopy (fNIRS) to measure brain activity in the cerebral cortex of 12 patients who had both natural teeth and dental implants in the lower molar region. Painless vibratory tactile stimulation was performed on both the natural teeth and the dental implants. Results Activation was seen in the somatosensory cortex during stimulation of both natural teeth and dental implants. A comparison of cortical activation showed no significant differences between natural teeth and dental implants. Conclusions These results indicate the possible existence of sensory input to the cerebral cortex via dental implants as well as natural teeth, and thus suggest that may not only the periodontal membrane be involved in the signaling pathway. The data from this experiment may help us for understanding the neural mechanisms underlying natural teeth and dental implants. Key words: fNIRS, natural teeth, implants, brain activity, somatosensory cortex.

Objective: Taste is an important sense for humans, but research on taste sensation is currently limited. Clinical evaluations of taste rely on subjective reports, and there are currently no objective evaluations available. In this study, we consider the possibility of objectively evaluating taste by measuring prefrontal cortex activity with functional near-infrared spectroscopy during taste stimulation with sweet, salty, sour, and bitter taste stimuli. Methods: We recruited 6 male and 3 female healthy adult volunteers to participate in this study. Taste solutions or distilled water (as a control) were dropped onto the tongue using a dropper. The degree of discomfort in response to the taste solution was evaluated on a visual analog scale. Results: We observed no significant differences between stimuli on any spectroscopy channel (P>0.05). We observed no reductions in oxyhemoglobin levels with the sweet stimuli, but progressively greater reductions with salty, sour, and bitter stimuli. We observed that responses to sour (P<0.05) and bitter (P<0.01) stimuli were significantly different from those to control at channel 1. We found a negative correlation between visual analog scale scores and minimum oxyhemoglobin levels (r=-0.641, P<0.01). Conclusions: Our measurements of prefrontal cortex activity with near-infrared spectroscopy suggest it may be a useful objective evaluation of taste stimulation.