Localization of Palatal Area in Human Somatosensory Cortex

The Feel of Speech: Multisystem and Polymodal Somatosensation in Speech Production

PURPOSE

The oral structures such as the tongue and lips have remarkable somatosensory capacities, but understanding the roles of somatosensation in speech production requires a more comprehensive knowledge of somatosensation in the speech production system in its entirety, including the respiratory, laryngeal, and supralaryngeal subsystems. This review was conducted to summarize the system-wide somatosensory information available for speech production.

METHOD

The search was conducted with PubMed/Medline and Google Scholar for articles published until November 2023. Numerous search terms were used in conducting the review, which covered the topics of psychophysics, basic and clinical behavioral research, neuroanatomy, and neuroscience.

RESULTS AND CONCLUSIONS

The current understanding of speech somatosensation rests primarily on the two pillars of psychophysics and neuroscience. The confluence of polymodal afferent streams supports the development, maintenance, and refinement of speech production. Receptors are both canonical and noncanonical, with the latter occurring especially in the muscles innervated by the facial nerve. Somatosensory representation in the cortex is disproportionately large and provides for sensory interactions. Speech somatosensory function is robust over the lifespan, with possible declines in advanced aging. The understanding of somatosensation in speech disorders is largely disconnected from research and theory on speech production. A speech somatoscape is proposed as the generalized, system-wide sensation of speech production, with implications for speech development, speech motor control, and speech disorders.

Effects of Tongue Pressure on Cerebral Blood Volume Dynamics: A Functional Near-Infrared Spectroscopy Study

Tongue pressure measurement (TPM) is an indicator of oral function. However, the association between tongue pressure and cerebral activation remains unclear. We used near-infrared spectroscopy (NIRS) to examine the correlation between cerebral cortex activation and tongue pressure stimulation against the anterior palatal mucosa. We measured voluntary maximum tongue pressure (MTP) using a TPM device; a pressure value of approximately 60% of the MTP was used for the experimental tongue pressure (MTP_60%). We examined the effect of oral functional tongue pressure stimulation against the anterior palatal mucosa on cerebral activation using NIRS in 13 adults. Tongue pressure stimulation caused significant changes in cerebral blood flow in some areas compared with controls (p < 0.05). We performed a correlation analysis (p < 0.05) between MTP_60% and changes in oxygenated hemoglobin in all 47 NIRS channels. MTP_60% triggered activation of the right somatosensory motor area and right dorsolateral prefrontal cortex and deactivation of the anterior prefrontal cortex (APFC). TPM balloon-probe insertion in the oral cavity activated the bilateral somatosensory motor area and deactivated the wide area of the APFC. Moreover, MTP_60% via the TPM balloon probe activated the bilateral somatosensory and motor cortex areas. Tongue pressure stimulation changes cerebral blood flow, and NIRS is useful in investigating the relationship between oral stimulation and brain function.

The effect of flavor on the oral perception and palatability of viscosity in healthy human subjects

OBJECTIVES

Thickeners are frequently used in various foods, including ice cream and sauces, to impart viscosity. Generally, viscous foods have some flavor (smell and taste). In this study, we examined the effects of flavor on the oral perception and palatability of viscosity in humans.

METHODS

Viscous fluids were prepared by adding the commercial thickener Tsururinko® (0.5 and 3.0%) to water and apple juice, which were used as the control and flavor fluids, respectively. The viscosity and palatability perception of the test fluids were evaluated in nine healthy volunteers using a visual analog scale. In the other seven volunteers, fluid viscosities were measured before and after spitting following retention in the mouth for 5 s to investigate the dilution of viscous fluids by flavor-stimulated saliva.

RESULTS

With 1.5% Tsururinko®, there was no difference between the physical viscosity of water and apple juice, but the perceived viscosity of apple juice was significantly lower than that of water. With 3.0% Tsururinko®, the viscosity of apple juice was significantly higher than that of water, but the perceived viscosities did not differ significantly. The addition of Tsururinko® reduced palatability in water in a dose-dependent manner. Apple juice suppressed this Tsururinko®-induced reduction. The reduction in viscosity after spitting was significantly larger in apple juice than in water.

CONCLUSION

Our results suggest that a favorable flavor reduces the perception of oral viscosity, which is due to mixing with stimulated saliva, and suppresses the unpalatability of thickeners.

Somatosensory evoked magnetic fields of periodontal mechanoreceptors

To evaluate the localization of responses to stimulation of the periodontal mechanoreceptors in the primary somatosensory cortex, somatosensory evoked fields (SEFs) were measured for stimulation of the left mandibular canine and first molar using magnetoencephalography in 25 healthy subjects. Tactile stimulation used a handmade stimulus device which recorded the trigger at the moment of touching the teeth.SEFs for the canine and first molar were detected in 20 and 19 subjects, respectively. Both responses were detected in the bilateral hemispheres. The latency for the canine was 62.1 ± 12.9 ms in the ipsilateral hemisphere and 65.9 ± 14.8 ms in the contralateral hemisphere. The latency for the first molar was 47.4 ± 6.6 ms in the ipsilateral hemisphere and 47.8 ± 9.1 ms in the contralateral hemisphere. The latency for the first molar was significantly shorter than that for the canine. The equivalent current dipoles were estimated in the central sulcus and localized anteroinferiorly compared to the locations for the SEFs for the median nerve. No significant differences in three-dimensional coordinates were found between the canine and first molar. These findings demonstrate the precise location of the teeth within the orofacial representation area in the primary somatosensory cortex.

Effect of carrot puree in vegetable juice on linguapalatal swallowing pressure

This study aimed to ascertain the influence of various amounts (0-30%) of carrot puree (CP) in vegetable juice on linguapalatal swallowing pressure in healthy volunteers. Twenty healthy women (age range: 20-22 years) swallowed a 17-ml drink in a natural state, and linguapalatal swallowing pressure was measured using a special sensor sheet. Peak magnitude (maximum pressure of the tongue pushing on the hard palate), integrated values of linguapalatal swallowing pressure on the waveform, and duration of linguapalatal swallowing pressure were increased with increases in CP concentrations. The total integrated value for 30% CP vegetable juice was larger than that of vegetable juice with no CP. The apparent viscosity of the vegetable juice with a low CP concentration was smaller than that with a high CP concentration at the same shear rate. These results suggest that vegetable juice containing CP affects mechanoreceptor activity in the mouth and generates a neuromotor response.

PRACTICAL APPLICATIONS

This study aimed to ascertain the influence of various amounts of carrot puree (CP) in vegetable juice on linguapalatal swallowing pressure measured by using a special sensor sheet in healthy volunteers. Obtained results of this study clearly showed that vegetable juice containing CP affects the movement of the tongue in maneuvering the bolus. Moreover, the results demonstrated that this effect depended on the concentration of CP in the vegetable juice. These findings are expected to provide clinically valuable data on the effect of mechanical stimulation during the oral stage of swallowing.

Anatomical landmarks for the localization of the greater palatine foramen--a study of 1200 head CTs, 150 dry skulls, systematic review of literature and meta-analysis

Accurate knowledge of greater palatine foramen (GPF) anatomy is necessary when performing a variety of anaesthesiological, dental or surgical procedures. The first aim of this study was to localize the GPF in relation to multiple anatomical landmarks. The second aim was to perform a systematic review of literature, and to conduct a meta-analysis on the subject of GPF position to aid clinicians in their practice. One-hundred and fifty dry, adult, human skulls and 1200 archived head computed tomography scans were assessed and measured in terms of GPF relation to other anatomical reference points. A systematic literature search was performed using the PubMed, Embase and Web of Science databases, and a meta-analysis on the subject of GPF relation to the maxillary molars was conducted. On average, in the Polish population, the GPF was positioned 15.9 ± 1.5 mm from the midline maxillary suture (MMS), 3.0 ± 1.2 mm from the alveolar ridge (AR) and 17.0 ± 1.5 mm from the posterior nasal spine (PNS); 74.7% of GPF were positioned opposite the third maxillary molar (M3). Twenty-seven studies were included in the systematic review and 23 in the meta-analysis (n = 6927 GPF). The pooled prevalence of the GPF being positioned opposite the M3 was 63.9% (95% confidence interval = 56.6-70.9%). Concluding, the GPF is most often located opposite the M3 in the majority of the world's populations. The maxillary molars are the best landmarks for locating the GPF. In edentulous patients the most useful points for approximating the position of the GPF are the AR, MMS and PNS. This study introduces an easy and repeatable classification to reference the GPF to the maxillary molars.

Somatosensory processing of the tongue in humans

We review research on somatosensory (tactile) processing of the tongue based on data obtained using non-invasive neurophysiological and neuroimaging methods. Technical difficulties in stimulating the tongue, due to the noise elicited by the stimulator, the fixation of the stimulator, and the vomiting reflex, have necessitated the development of specialized devices. In this article, we show the brain activity relating to somatosensory processing of the tongue evoked by such devices. More recently, the postero-lateral part of the tongue has been stimulated, and the brain response compared with that on stimulation of the antero-lateral part of the tongue. It is likely that a difference existed in somatosensory processing of the tongue, particularly around primary somatosensory cortex, Brodmann area 40, and the anterior cingulate cortex.

Cortical representation area of human dental pulp

To elucidate the dental pulp-representing area in the human primary somatosensory cortex and the presence of A-beta fibers in dental pulp, we recorded somatosensory-evoked magnetic fields from the cortex in seven healthy persons using magnetoencephalography. Following non-painful electrical stimulation of the right maxillary first premolar dental pulp, short latency (27 ms) cortical responses on the magnetic waveforms were observed. However, no response was seen when stimulation was applied to pulpless teeth, such as devitalized teeth. The current source generating the early component of the magnetic fields was located anterior-inferiorly compared with the locations for the hand area in the primary somatosensory cortex. These results demonstrate the dental pulp representation area in the primary somatosensory cortex, and that it receives input from intradental A-beta neurons, providing a detailed organizational map of the orofacial area, by adding dental pulp to the classic "sensory homunculus".