Somatosensory Processing of the Tongue in Humans

Abstract

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.

Functional MRI cortical activations from unilateral tactile-taste stimulations of the tongue

“…Since the classic study by Penfield and Boldrey [42] it is known that contrary to the limbs, but similar to other orofacial structures, the tongue is bilaterally represented in the primary somatosensory cortex [4,10,43,50,61]. Correspondingly, the activations found by us in this cortical region were bilateral, independent of whether stimulation was performed on the right or left or with salt or water.…”

“…There is a consensus that both sides of the tongue in both the somatosensory and the gustatory modalities are represented in the cortex of both hemispheres [1,2,10,22,[38][39][40]59], most likely with a predominance of contralateral over ipsilateral representations in the somatosensory modality and an opposite predominance of ipsilateral over contralateral representation in the taste modality [3,10,22]. There are also reports of a comparatively greater reactivity of the right hemisphere to both somatosensory [50] and gustatory lingual stimuli [52]. These interhemispheric differences may be subject to a large interindividual variability, including a possible correlation with manual dominance [6,15,16].…”

“…Unlike other mammals, in primates Physiology & Behavior 151 (2015) [221][222][223][224][225][226][227][228][229] (including humans) taste information from the latter nucleus is conveyed to the gustatory portion of the thalamic ventroposteromedial nucleus not through a pontine relay, but directly through the central tegmental tract [46,48]. Projections from the thalamus to the cortex establish multiple representations of the tongue, partly somatosensory, partly gustatory, partly integrating both modalities, in regions of the parietal, frontal and insular cortex [10,46,48,50,61]. The most important primary somatosensory and motor representations of the tongue lie in the inner and inferior part of the perirolandic region of the frontal and parietal opercula [4,10,30,48,50,61], whereas the most important primary taste representation of the cortex is in the middle-anterior insula with extensions in the frontal and parietal opercula [17,22,25,26,34,35,53,56].…”

“…Projections from the thalamus to the cortex establish multiple representations of the tongue, partly somatosensory, partly gustatory, partly integrating both modalities, in regions of the parietal, frontal and insular cortex [10,46,48,50,61]. The most important primary somatosensory and motor representations of the tongue lie in the inner and inferior part of the perirolandic region of the frontal and parietal opercula [4,10,30,48,50,61], whereas the most important primary taste representation of the cortex is in the middle-anterior insula with extensions in the frontal and parietal opercula [17,22,25,26,34,35,53,56]. On the basis of brain responses to astringent and pungent taste stimulations in humans, the Rolandic part of the operculum has been suggested as a cortical region specifically devoted to the integration of gustatory and somatosensory inputs from the tongue [7,16].…”

The Ventral Primary Somatosensory Cortex of the Primate Brain: Innate Neural Interface for Dexterous Orofacial Motor Control

“…The ipsilateral representation is a distinctive feature of oral structures, such as the teeth and tongue and each side of the oral structure is represented in area 3b of both hemispheres. This was confirmed in a wide range of primates: prosimian primates, such as the African galago [10], New World monkeys, such as the squirrel monkey and owl monkey [16,18], Old World monkeys, such as macaques [15,19,20], and humans (see review [21]). Such corepresentation of the contralateral and ipsilateral sides of oral structures may facilitate the convergence of input from functionally related portions of both sides (bilateral integration described in the next section).…”