Cortical transcranial magnetic stimulation of the frontalis muscle: characteristic features and details on motor-evoked potentials of the forehead

Bilateral Changes in Cortical Motor Representation of the Tongue after Unilateral Peripheral Facial Paralysis: Evidence from Transcranial Magnetic Stimulation

Motor evoked potentials of the lingual muscles due to focal cortical transcranial magnetic stimulation were investigated in 5 patients with unilateral total facial paralysis with regard to amplitude as a function of the coil position on the interaural line. Maximum bilateral responses could be obtained at mean stimulus positions of about 6 to 8 cm lateral to the vertex. In comparison with healthy subjects, the patient group had significantly smaller mediolateral calculated centers for ipsilateral and contralateral responses. At the optimum stimulus positions, the patients' mean motor evoked potential amplitudes were significantly lower than those in healthy subjects. These alterations could be observed on both cortical hemispheres, but were more pronounced for the hemisphere contralateral to the side of facial paralysis. Thus, we provide strong evidence of bilateral changes in lingual cortical motor representation following facial paralysis with an invasion of the facial motor area by the tongue motor representation.

The facial motor system

Facial movements support a variety of functions in human behavior. They participate in automatic somatic and visceral motor programs, they are essential in producing communicative displays of affective states and they are also subject to voluntary control. The multiplicity of functions of facial muscles, compared to limb muscles, is reflected in the heterogeneity of their anatomical and histological characteristics that goes well beyond the conventional classification in single facial muscles. Such parcellation in different functional muscular units is maintained throughout the central representation of facial movements from the brainstem up to the neocortex. Facial movements peculiarly lack a conventional proprioceptive feedback system, which is only in part vicariated by cutaneous or auditory afferents. Facial motor activity is the main marker of endogenous affective states and of the affective valence of external stimuli. At the cortical level, a complex network of specialized motor areas supports voluntary facial movements and, differently from upper limb movements, in such network there does not seem to be a prime actor in the primary motor cortex.

Representation of facial muscles in human motor cortex

Whether there is a projection from the primary motor cortex (M1) to upper facial muscles and how the facial M1 area is modulated by intracortical inhibitory and facilitatory circuits remains controversial. To assess these issues, we applied transcranial magnetic stimulation (TMS) to the M1 and recorded from resting and active contralateral (C-OOc) and ipsilateral orbicularis oculi (I-OOc), and contralateral (C-Tr) and ipsilateral triangularis (I-Tr) muscles in 12 volunteers. In five subjects, the effects of stimulating at different scalp positions were assessed. Paired TMS at interstimulus intervals (ISIs) of 2 ms were used to elicit short interval intracortical inhibition (SICI) and ISI of 10 ms for intracortical facilitation (ICF). Long interval intracortical inhibition (LICI) was evaluated at ISIs between 50 and 200 ms, both at rest and during muscle activation. The silent period (SP) was also determined. C-OOc and I-OOc responses were recorded in all subjects. The optimal position for eliciting C-OOc responses was lateral to the hand representation in all subjects and MEP amplitude markedly diminished when the coil was placed 2 cm away from the optimal position. For the I-OOc, responses were present in more scalp sites and the latency decreased with more anterior placement of the coil. C-Tr response was recorded in 10 out of 12 subjects and the I-Tr muscle showed either no response or low amplitude response, probably due to volume conduction. SICI and ICF were present in the C-OOc and C-Tr, but not in the I-OOc muscle. Muscle activation attenuated SICI and ICF. LICI at rest showed facilitation at 50 ms ISI in all muscles, but there was no significant inhibition at other ISIs. There was no significant inhibition or facilitation with the LICI protocol during muscle contraction. The SP was present in the C-OOc, C-Tr and I-OOc muscles and the mean durations ranged from 92 to 104 ms. These findings suggest that the I-OOc muscle response is probably related to the first component (R1) of the blink reflex. There is M1 projection to the contralateral upper and lower facial muscles in humans and the facial M1 area is susceptible to cortical inhibition and facilitation, similar to limb muscles.

Human cortical motor representation of the larynx as assessed by transcranial magnetic stimulation (TMS)

OBJECTIVES

To analyze characteristic features and details on motor-evoked potentials (MEPs) of the cricothyroid and vocalis muscles from single-pulse cortical transcranial magnetic stimulation (TMS) in normal subjects to characterize cortical motor representation of laryngeal muscles.

STUDY DESIGN

Prospective, experimental investigation on healthy volunteers.

METHOD

MEPs of the cricothyroid and vocalis muscles elicited by cortical TMS with a figure-8-shaped coil were investigated in two groups of six healthy subjects each, with special regard to MEP amplitude as a function of the coil position on the head surface along the interaural line.

RESULTS

Bilateral reproducible responses of the cricothyroid and the vocalis muscles could be observed in all subjects. For the cricothyroid muscle, maximal responses were obtained at mean stimulus positions of 7.5 +/- 1.4 cm (contralateral) and of 7.3 +/- 1.3 cm (ipsilateral), respectively. For the vocalis muscle, we found maximal responses at mean stimulus positions of 10.3 +/- 1.9 cm (contralateral) and of 9.6 +/- 1.6 cm (ipsilateral), respectively. Despite a considerable overlap of these coil positions, from which reproducible MEPs could be elicited in both groups of the laryngeal muscles, statistically significant separation of the cricothyroid-and vocalis-associated cortical representation areas was possible.

CONCLUSIONS

Our observations point to two different cortical motor representation areas, with the cricothyroid muscle-related area being located more medially.

Tongue representation in the lateral cortical motor region of the human brain as assessed by transcranial magnetic stimulation

Motor evoked potentials (MEPs) of the lingual muscles elicited by focal cortical transcranial magnetic stimulation (TMS) with a figure 8-shaped coil were investigated in 17 healthy subjects with special regard to amplitude and onset latency as a function of the coil position on the head surface. Bilateral reproducible responses could be observed at coil positions mostly varying from approximately 6 to 13 cm lateral to the vertex. During moderate muscle activation, maximum responses were obtained at a mean stimulus position of about 11 cm lateral and 3 cm anterior to the vertex with similar onset latencies, but with significantly higher amplitudes on the contralateral side (3.5 +/- 1.9 mV, 9.5 +/- 1.1 ms contralateral; 2.6 +/- 1.5 mV, 9.7 +/-1.0 ms ipsilateral). Comparing our data on the orbicularis oculi muscle with the results obtained on lower lip muscles, we found a considerable overlap of those coil positions from which reproducible MEPs could be elicited in both groups of target muscles, but the lingual area was placed more laterally. Thus, a statistically significant separation of the cortical representation areas of lingual and lower lip mimetic muscles is possible by focal cortical TMS, reflecting somatotopic organization of the face-associated motor cortex.

Cortical representation of the orbicularis oculi muscle as assessed by transcranial magnetic stimulation (TMS)

OBJECTIVES

To analyze characteristic features and details on motor-evoked potentials (MEPs) of the orbicularis oculi muscle resulting from cortical transcranial magnetic stimulation (TMS) in normal subjects as a basis for further investigations on motorcortical representation in patients presenting with facial nerve diseases.

STUDY DESIGN

MEPs of the orbicularis oculi muscle resulting from focal cortical TMS with a figure-8-shaped coil were investigated in 17 healthy subjects with special regard to amplitude and onset latency as a function of the coil position on the head surface along the interaural line and in the anterior-posterior direction. The results were then compared with our data on lower-lip mimetic muscles and on the frontalis muscle obtained in previous studies.

RESULTS

Bilateral reproducible responses could be observed at coil positions varying from 1 to 13 cm lateral to the vertex. During moderate muscle activation, maximum responses (mean amplitude 0.75 +/- 0.44 mV contralateral, 0.74 +/- 0.36 mV ipsilateral) were obtained at a mean stimulus position of 8.6 +/- 1.6 cm lateral and 2.0 +/- 2.2 cm anterior to the vertex for contralateral responses, and of 8.6 +/- 2.0 cm lateral and 2.8 +/- 2.4 cm anterior to the vertex for ipsilateral responses, respectively. Voluntary muscle activation by forced eye-closure was associated with a further increase in mean amplitudes. At rest, bilateral responses could be elicited in 15 subjects (88.2%). During moderate muscle activation, the shortest mean onset latencies were obtained at the optimum stimulus position on the interaural line, both for contralateral (10.2 +/- 1.3 ms) and ipsilateral (10.6 +/- 1.5 ms) MEPs. Comparing our data on the orbicularis oculi muscle with those obtained on lower-lip muscles and on the frontalis muscle, there was a considerable overlap of coil positions from which reproducible MEPs could be elicited in all three groups of mimetic muscles, but with the orbicularis oculi area being placed between forehead and lower-lip motorcortical areas.

CONCLUSIONS

A statistically significant separation of the cortical representation areas of forehead, orbicularis oris, and lower-lip mimetic muscles is possible by focal cortical TMS reflecting a kind of somatotopic organization of the face-associated motorcortex. Compared with the results on lower-lip and forehead muscles, orbicularis oculi muscle responses show characteristics of both.