Functional magnetic resonance imaging evidence for a representation of the ear in human primary somatosensory cortex: comparison with magnetoencephalography study

Neurophysiologic Effects of Transcutaneous Auricular Vagus Nerve Stimulation (taVNS) via Electrical Stimulation of the Tragus: A Concurrent taVNS/fMRI Study and Review

(Appeared originally in Brain Stimulation 2018; 11:492-500) Reprinted with permission from Elsevier.

First application of 7-T ultra-high field diffusion tensor imaging to detect altered microstructure of thalamic-somatosensory anatomy in trigeminal neuralgia

OBJECTIVE

Trigeminal neuralgia (TN) is a debilitating neurological disease that commonly results from neurovascular compression of the trigeminal nerve (CN V). Although the CN V has been extensively studied at the site of neurovascular compression, many pathophysiological factors remain obscure. For example, thalamic-somatosensory function is thought to be altered in TN, but the abnormalities are inadequately characterized. Furthermore, there are few studies using 7-T MRI to examine patients with TN. The purpose of the present study was to use 7-T MRI to assess microstructural alteration in the thalamic-somatosensory tracts of patients with TN by using ultra-high field MRI.

METHODS

Ten patients with TN and 10 age- and sex-matched healthy controls underwent scanning using 7-T MRI with diffusion tensor imaging. Structural images were segmented with an automated algorithm to obtain thalamus and primary somatosensory cortex (S1). Probabilistic tractography was performed between the thalamus and S1, and the microstructure of the thalamic-somatosensory tracts was compared between patients with TN and controls.

RESULTS

Fractional anisotropy of the thalamic-somatosensory tract ipsilateral to the site of neurovascular compression was reduced in patients (mean 0.43) compared with side-matched controls (mean 0.47, p = 0.01). The mean diffusivity was increased ipsilaterally in patients (mean 6.58 × 10-4 mm2/second) compared with controls (mean 6.15 × 10-4 mm2/second, p = 0.02). Radial diffusivity was increased ipsilaterally in patients (mean 4.91 × 10-4 mm2/second) compared with controls (mean 4.44 × 10-4 mm2/second, p = 0.01). Topographical analysis revealed fractional anisotropy reduction and diffusivity elevation along the entire anatomical S1 arc in patients with TN.

CONCLUSIONS

The present study is the first to examine microstructural properties of the thalamic-somatosensory anatomy in patients with TN and to evaluate quantitative differences compared with healthy controls. The finding of reduced integrity of these white matter fibers provides evidence of microstructural alteration at the level of the thalamus and S1, and furthers the understanding of TN neurobiology.

Cortical processing of irrelevant somatosensory information from the leg is altered by attention during early movement preparation

The ability to actively suppress, or gate, irrelevant sensory information is needed for safe and efficient walking in sensory-rich environments. Both attention and the late phase of motor preparation alter somatosensory evoked potentials (SEPs) in healthy adults. The aim of this study was to examine the effect of attention on the processing of irrelevant somatosensory information during the early phase of preparation of plantarflexion movements. Young healthy individuals received tibial nerve stimulation while electroencephalography (EEG) recorded SEPs over the Cz electrode. Three conditions were tested in both legs: 1) Rest, 2) Attend To the stimulated limb, and 3) Attend Away from the stimulated limb. In conditions 2 and 3, vibration (80 Hz) was applied over the medial soleus muscle to cue voluntary plantarflexion movements of the stimulated (Attend To) or non-stimulated leg (Attend Away). Only SEPs delivered during early preparation were averaged for statistical analysis. Results demonstrated a main effect of condition for the N40 and N70 indicating that SEP amplitudes in the Attend To condition were smaller than rest (p ≤ 0.02). For the P50, no interaction effects or main effects were found (p ≥ 0.08). There was no main effect of leg for any component measured. The results indicate that gating of irrelevant sensory information during early preparation occurs in the leg when attention is directed within the same limb. If attention alters the somatosensory stimuli from a leg movement, then directing attention may affect safe community walking.

The Homuncular Jigsaw: Investigations of Phantom Limb and Body Awareness Following Brachial Plexus Block or Avulsion

Many neuropsychological theories agree that the brain maintains a relatively persistent representation of one’s own body, as indicated by vivid “phantom” experiences. It remains unclear how the loss of sensory and motor information contributes to the presence of this representation. Here, we focus on new empirical and theoretical evidence of phantom sensations following damage to or an anesthetic block of the brachial plexus. We suggest a crucial role of this structure in understanding the interaction between peripheral and central mechanisms in health and in pathology. Studies of brachial plexus function have shed new light on how neuroplasticity enables “somatotopic interferences”, including pain and body awareness. Understanding the relations among clinical disorders, their neural substrate, and behavioral outcomes may enhance methods of sensory rehabilitation for phantom limbs.

Phantom limb sensations in the ear of a patient with a brachial plexus lesion

Referred phantom sensations are frequently reported following a peripheral injury. However, very few cases describe such sensations of the ear, and it remains unclear how the aural nerve territory can be remapped to one specific peripheral nerve region. We report on a patient with brachial plexus avulsion who underwent sensory testing and was asked to report the location of the stimulated site and any other sensations experienced. The patient spontaneously described the sensation of his arm being separate from his body. Despite visual input, he felt that his fist was closed, with his thumb pointing inward. Importantly, he felt clear and reproducible sensations from the affected arm when the ipsilateral ear was touched. These referred sensations were noted just 15 days after sustaining the injury. The arm nerve territory was systematically remapped to a specific aural nerve territory by applying both manual and electrical stimulation. Stimulation of the external ear, which is innervated by the vagus nerve, showed high spatial specificity for the dorsal and volar skin surfaces of the limb, and clearly delineated digits. Somatosensory-evoked potentials indicated that cortical adaptation in the somatosensory stream transferred a spatially organized map of the limb to the skin of the outer ear. This referral of sensations to the ear, as distinct from the face, provides evidence of highly specific topographical reorganization of the central nervous system following peripheral injury. Rapid map changes in the phantom sensation to the ear as a function of stimulation of vagus nerve suggest that the reorganization process can occur in cortex rather than in the brainstem.

Neurophysiologic effects of transcutaneous auricular vagus nerve stimulation (taVNS) via electrical stimulation of the tragus: A concurrent taVNS/fMRI study and review

BACKGROUND

Electrical stimulation of the auricular branch of the vagus nerve (ABVN) via transcutaneous auricular vagus nerve stimulation (taVNS) may influence afferent vagal networks. There have been 5 prior taVNS/fMRI studies, with inconsistent findings due to variability in stimulation targets and parameters.

OBJECTIVE

We developed a taVNS/fMRI system to enable concurrent electrical stimulation and fMRI acquisition to compare the effects of taVNS in relation to control stimulation.

METHODS

We enrolled 17 healthy adults in this single-blind, crossover taVNS/fMRI trial. Based on parameters shown to affect heart rate in healthy volunteers, participants received either left tragus (active) or earlobe (control) stimulation at 500 μs 25 HZ for 60 s (repeated 3 times over 6 min). Whole brain fMRI analysis was performed exploring the effect of: active stimulation, control stimulation, and the comparison. Region of interest analysis of the midbrain and brainstem was also conducted.

RESULTS

Active stimulation produced significant increased BOLD signal in the contralateral postcentral gyrus, bilateral insula, frontal cortex, right operculum, and left cerebellum. Control stimulation produced BOLD signal activation in the contralateral postcentral gyrus. In the active vs. control contrast, tragus stimulation produced significantly greater BOLD increases in the right caudate, bilateral anterior cingulate, cerebellum, left prefrontal cortex, and mid-cingulate.

CONCLUSION

Stimulation of the tragus activates the cerebral afferents of the vagal pathway and combined with our review of the literature suggest that taVNS is a promising form of VNS. Future taVNS/fMRI studies should systematically explore various parameters and alternative stimulation targets aimed to optimize this novel form of neuromodulation.

A Trio of Concerns

This essay assesses the two most significant changes in psychology over the past century: the attempt to localize psychological phenomena in restricted brain sites and the search for genetic contributions to behavior and psychopathology. Although there are advantages to these new developments, they are accompanied by some questionable assumptions. Because the investigators in these domains often relate variation in their biological measures to variation in personality traits evaluated with questionnaires, an analysis of the unique properties of the verbalreport questionnaires is presented. It is suggested that future research on human personality should try to combine semantic reports with behaviors and biological data in order to arrive at more fruitful constructs.

Vestibular and visual responses in human posterior insular cortex

The central hub of the cortical vestibular network in humans is likely localized in the region of posterior lateral sulcus. An area characterized by responsiveness to visual motion has previously been described at a similar location and named posterior insular cortex (PIC). Currently it is not known whether PIC processes vestibular information as well. We localized PIC using visual motion stimulation in functional magnetic resonance imaging (fMRI) and investigated whether PIC also responds to vestibular stimuli. To this end, we designed an MRI-compatible caloric stimulation device that allowed us to stimulate bithermally with hot temperature in one ear and simultaneously cold temperature in the other or with warm temperatures in both ears for baseline. During each trial, participants indicated the presence or absence of self-motion sensations. We found activation in PIC during periods of self motion when vestibular stimulation was carried out with minimal visual input. In combined visual-vestibular stimulation area PIC was activated in a similar fashion during congruent and incongruent stimulation conditions. Our results show that PIC not only responds to visual motion but also to vestibular stimuli related to the sensation of self motion. We suggest that PIC is part of the cortical vestibular network and plays a role in the integration of visual and vestibular stimuli for the perception of self motion.

Reappraisal of the somatosensory homunculus and its discontinuities

Neuroscience folklore has it that somatotopy in human primary somatosensory cortex (SI) has two significant discontinuities: the hands and face map onto adjacent regions in SI, as do the feet and genitalia. It has been proposed that these conjunctions in SI result from coincident sources of stimulation in the fetal position, where the hands frequently touch the face, and the feet the genitalia. Computer modeling using a Hebbian variant of the self-organizing Kohonen net is consistent with this proposal. However, recent work reveals that the genital representation in SI for cutaneous sensations (as opposed to tumescence) is continuous with that of the lower trunk and thigh. This result, in conjunction with reports of separate face innervation and its earlier onset of sensory function, compared to that of the rest of the body, allows a reappraisal of homuncular organization. It is proposed that the somatosensory homunculus comprises two distinct somatotopic regions: the face representation and that of the rest of the body. Principles of self-organization do not account satisfactorily for the overall homuncular map. These results may serve to alert computational modelers that intrinsic developmental factors can override simple rules of plasticity.

Cortical representation of tympanic membrane movements due to pressure variation: an fMRI study

Middle ear sensory information has never been localized in the homunculus of the somatosensory cortex (S1). We investigated the somatosensory representation of the middle ear in 15 normal hearing subjects. We applied small air pressure variations to the tympanic membrane while performing a 3T-fMRI study. Unilateral stimulations of the right ear triggered bilateral activations in the caudal part of the postcentral gyrus in Brodmann area 43 (BA 43) and in the auditory associative areas 42 (BA 42) and 22 (BA 22). BA 43 has been found to be involved in activities accompanying oral intake and could be more largely involved in pressure activities in the oropharynx area. The tympanic membrane is indirectly related to the pharynx area through the action of tensor tympani, which is a Eustachian tube muscle. The Eustachian tube muscles have a role in pressure equalization in the middle ear and also have a role in the pharyngeal phase of swallowing. Activation of BA 42 and BA 22 could reflect activations associated with the bilateral acoustic reflex triggered prior to self-vocalization to adjust air pressure in the oropharynx during speech. We propose that BA 43, 42, and 22 are the cortical areas associated with middle ear function. We did not find representation of tympanic membrane movements due to pressure in S1, but its representation in the postcentral gyrus in BA 43 seems to suggest that at least part of this area conveys pure somatosensory information.

Ear movement induced by electrical cortical stimulation

Cortical areas that control ear movement have not been reported in humans. We describe a rare case in which ear auricle movement was induced by extraoperative electrical cortical stimulation. A 21-year-old man with intractable localization-related epilepsy was admitted for presurgical evaluation. Subdural electrodes were implanted over the right temporal and frontal regions. Tonic upward contraction of the left ear auricle was elicited by stimulating the subdural electrode on the posterior portion of the right superior temporal gyrus close to the end of the Sylvian fissure. No other body movements or auditory symptoms were elicited. A possible mechanism underlying this rare phenomenon is discussed.

Dodecapus: An MR-compatible system for somatosensory stimulation

Somatotopic mapping of human body surface using fMRI is challenging. First, it is difficult to deliver tactile stimuli in the scanner. Second, multiple stimulators are often required to cover enough area of the complex-shaped body surface, such as the face. In this study, a computer-controlled pneumatic system was constructed to automatically deliver air puffs to 12 locations on the body surface through an MR-compatible manifold (Dodecapus) mounted on a head coil inside the scanner bore. The timing of each air-puff channel is completely programmable and this allows systematic and precise stimulation on multiple locations on the body surface during functional scans. Three two-condition block-design "Localizer" paradigms were employed to localize the cortical representations of the face, lips, and fingers, respectively. Three "Phase-encoded" paradigms were employed to map the detailed somatotopic organizations of the face, lips, and fingers following each "Localizer" paradigm. Multiple somatotopic representations of the face, lips, and fingers were localized and mapped in primary motor cortex (MI), ventral premotor cortex (PMv), polysensory zone (PZ), primary (SI) and secondary (SII) somatosensory cortex, parietal ventral area (PV) and 7b, as well as anterior and ventral intraparietal areas (AIP and VIP). The Dodecapus system is portable, easy to setup, generates no radio frequency interference, and can also be used for EEG and MEG experiments. This system could be useful for non-invasive somatotopic mapping in both basic and clinical studies.

Contralateral and ipsilateral responses in primary somatosensory cortex following electrical median nerve stimulation—an fMRI study

OBJECTIVE

Ten healthy adult subjects were examined using functional magnetic resonance imaging (fMRI) to investigate responses in the contralateral and ipsilateral primary somatosensory cortex (SI) following electrical stimulation of the median nerve.

METHODS

The right and left median nerves were stimulated alternately at the wrist in the different sessions. First, the location of the response in contralateral SI was identified following median nerve stimulation, and then, a spherical search volume with a 10mm radius centered on the region of the contralateral response was determined. Whether or not fMRI activation occurred within this sphere following ipsilateral stimulation was examined using a 3T MR imager.

RESULTS

A response in contralateral SI was observed in 8 of the 10 subjects in right and left hemisphere. Responses in ipsilateral SI were observed in 6 of 8 subjects in right hemisphere, and the region of the response tended to be posterior to the contralateral region. On the other hand, in left hemisphere, the ipsilateral responses were found in three.

CONCLUSIONS

In the present study, not only contralateral SI but also ipsilateral SI was activated following median nerve. The location of the ipsilateral activation was significantly more posterior than the contralateral one in right hemisphere.

SIGNIFICANCE

The region of activation in ipsilateral SI was located in the posterior portion of post central gyrus, corresponding to around BA2 and 5 in human.

Face representation in the human primary somatosensory cortex

To investigate the representation of facial skin areas in the primary somatosensory cortex (SI), we recorded magnetic fields evoked by air pressure-induced tactile stimulation applied to six points on the face, lower lip and thumb. The thumb area in the SI was located more medial and superior to the lip area, which was consistent with Penfield's homunculus. However, the representations of all skin-covered areas including forehead, cheek, nose and chin in the SI were located between the thumb and lower lip area. There was no significant difference in location among the six facial points. Our results imply that lips occupy a large area of the face representation in the SI, whereas only a small area located between the thumb and lip areas is devoted to skin-covered surfaces. This is the first study showing that the facial skin areas in the human SI are located between the thumb and lower lip areas and close together.

Effect of tactile interference stimulation of the ear in human primary somatosensory cortex: a magnetoencephalographic study

OBJECTIVE

To confirm the somatotopic representation of the ear in the primary somatosensory cortex (SI), we studied the tactile interference effects on somatosensory evoked magnetic fields (SEFs) following stimulation of the ear (Helix, Lobulus and Tragus).

METHODS

We applied tactile interference stimulation to the neck or face area continuously and concurrently while a time-locked electrical stimulation was applied to the ear. If the amplitude would be reduced by the interference, this would indicate that the cortical representation for both the time-locked electrical stimulation and the continuous interference stimulation overlapped. A two or 3-source model, Source 1 in the neck area of SI, Source 2 in the face area of SI, and Source 3 in the secondary somatosensory cortex (SII), was found to be the most appropriate by brain electric source analysis (BESA).

RESULTS

Amplitudes of Sources 1 and 2 in most interference conditions were decreased. Source 1 following stimulation of all 3 sites was significantly reduced when the interference was applied to the neck area. Source 2 following stimulation of all 3 sites was significantly reduced when the interference was applied to the face area.

CONCLUSIONS

These findings showed that the interference effect was found in both the neck and face areas of SI following the ear stimulation.

SIGNIFICANCE

The representation of the ear in SI might be located in both the neck and face areas.