Download Neurophysiologic markers in laryngeal muscles indicate functional

Clinical Neurophysiology 125 (2014) 1912–1922
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Clinical Neurophysiology
journal homepage: www.elsevier.com/locate/clinph
Neurophysiologic markers in laryngeal muscles indicate functional
anatomy of laryngeal primary motor cortex and premotor cortex
in the caudal opercular part of inferior frontal gyrus
Vedran Deletis a,b,⇑,1, Maja Rogić a,1, Isabel Fernández-Conejero c,1, Andreu Gabarrós c, Ana Jerončić a,d
a
Laboratory for Human and Experimental Neurophysiology (LAHEN), School of Medicine, University of Split, Split, Croatia
Department for Intraoperative Neurophysiology, Roosevelt Hospital, New York, NY, USA
University Hospital Bellvitge, Barcelona, Spain
d
Department for Research in Biomedicine and Health, School of Medicine, University of Split, Split, Croatia
b
c
a r t i c l e
i n f o
Article history:
Accepted 24 January 2014
Available online 11 February 2014
Keywords:
Premotor cortex
Inferior frontal gyrus
Broca area
Primary motor cortex for laryngeal muscles
Motor speech areas
Navigated transcranial magnetic
stimulation
Electrical stimulation of motor speech areas
Neurophysiologic markers
h i g h l i g h t s
This is a unique study describing methods for eliciting neurophysiologic markers of primary motor
cortex (M1) for laryngeal muscles and premotor cortex of inferior frontal gyrus.
The neurophysiologic markers were elicited by: (a) navigated transcranial magnetic stimulation
(nTMS) in a group of healthy subjects, (b) direct cortical stimulation (DCS) of exposed cortex during
craniotomy.
These neurophysiologic markers indicate functional anatomy of M1 for laryngeal muscles and premotor cortex in the caudal opercular part of inferior frontal gyrus.
a b s t r a c t
Objective: The aim of this study was to identify neurophysiologic markers generated by primary motor
and premotor cortex for laryngeal muscles, recorded from laryngeal muscle.
Methods: Ten right-handed healthy subjects underwent navigated transcranial magnetic stimulation
(nTMS) and 18 patients underwent direct cortical stimulation (DCS) over the left hemisphere, while
recording neurophysiologic markers, short latency response (SLR) and long latency response (LLR) from
cricothyroid muscle. Both healthy subjects and patients were engaged in the visual object-naming task. In
healthy subjects, the stimulation was time-locked at 10–300 ms after picture presentation while in the
patients it was at zero time.
Results: The latency of SLR in healthy subjects was 12.66 ± 1.09 ms and in patients 12.67 ± 1.23 ms. The
latency of LLR in healthy subjects was 58.5 ± 5.9 ms, while in patients 54.25 ± 3.69 ms. SLR elicited by the
stimulation of M1 for laryngeal muscles corresponded to induced dysarthria, while LLR elicited by stimulation of the premotor cortex in the caudal opercular part of inferior frontal gyrus, recorded from laryngeal muscle, corresponded to speech arrest in patients and speech arrest and/or language disturbances in
healthy subjects.
Conclusion: In both groups, SLR indicated location of M1 for laryngeal muscles, and LLR location of premotor cortex in the caudal opercular part of inferior frontal gyrus, recorded from laryngeal muscle, while
stimulation of these areas in the dominant hemisphere induced transient speech disruptions.
Significance: Described methodology can be used in preoperative mapping, and it is expected to facilitate
surgical planning and intraoperative mapping, preserving these areas from injuries.
Ó 2014 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights
reserved
⇑ Corresponding author at: Laboratory for Human and Experimental Neurophysiology (LAHEN), School of Medicine, University of Split, Šoltanska 2, 21000 Split, Croatia.
Tel.: +385 91 754 1385.
E-mail address: [email protected] (V. Deletis).
1
These authors equally contributed.
http://dx.doi.org/10.1016/j.clinph.2014.01.023
1388-2457/Ó 2014 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved
V. Deletis et al. / Clinical Neurophysiology 125 (2014) 1912–1922
1. Introduction
2. Methodology
Mapping of primary motor cortex and Broca’s area during
awake craniotomy is not always the best option, especially in
children and uncooperative patients; it would be important to
find a neurophysiologic methodology for intraoperative mapping
and monitoring anatomic and functional integrity of these cortical areas. The development of a methodology for preoperative
identification using navigated transcranial magnetic stimulation
(nTMS) would facilitate surgical planning and intraoperative
mapping.
The primary motor cortex (M1) for laryngeal muscles has a role
in execution and motor speech control, while the posterior inferior
frontal gyrus, namely Broca’s area, is regarded as an important motor speech cortical area having a role in all stages of word encoding
and their unification (Sahin et al., 2009), as well as sending coded
‘‘commands’’ to M1.
After electrical stimulation of Broca’s area, postsynaptic potentials of high amplitudes are recorded in the lateral part of the M1
(Greenlee et al., 2004). These results indirectly indicate a functional
connection of Broca’s area and M1 for face, mouth, pharynx, and
larynx. It has been proposed that the critical parts of M1 needed
to control vocalization are closely and uniquely associated with
the laryngeal muscles (Corballis, 2003). The direct functional connectivity of M1 for laryngeal muscles was demonstrated in our
studies (Deletis et al., 2008, 2009, 2011; Espadaler et al. 2012). In
these studies, we have developed methodologies for stimulating
M1 for laryngeal muscles and recording corticobulbar motorevoked potentials (MEPs) from vocal and cricothyroid muscles.
Corticobulbar MEPs can be regarded as a synonym for short latency
response (SLR) representing a neurophysiologic marker of M1 for
laryngeal muscles. It has been also reported that long latency response (LLR) can be recorded from laryngeal muscles after magnetic
stimulation of the frontal cortex (Amassian et al., 1988; Ertekin
et al., 2001). We also recorded LLRs intraoperatively but we neither
studied this systematically nor investigated or speculated about its
exact origin (Amassian et al., 1988; Ertekin et al., 2001; Deletis
et al., 2008, 2011).
The standard intraoperative neurophysiologic method for mapping of Broca’s area and M1 for orofacial, pharyngeal, and laryngeal muscles consists of electrical stimulation of these areas and
inducing transient speech disruptions: (a) speech arrest and/or
language disturbances while stimulating Broca’s area and (b) dysarthria while stimulating M1 for orofacial, pharyngeal, and laryngeal muscles. Electrical stimulation with 50–60 Hz is used during
surgery or through the subdural grid electrodes. In addition to 50Hz stimulation for 1–3 s, speech arrest was also elicited with a
short train of stimuli technique (5 pulses, 3-ms interstimulus
interval) (Axelson et al., 2009). Furthermore, high-frequency
repetitive transcranial magnetic stimulation with a repetition rate
of 2–25 Hz was also used, but so far, only three groups of authors
reported induced speech arrest (Epstein et al., 1996, 1999;
Pascual-Leone et al., 1991). Recently, Picht et al. (2013) showed
a good overall correlation between repetitive navigated (nTMS)
and direct cortical stimulation (DCS) during awake surgery
for the identification of language-related areas in patients with
left-hemisphere lesions.
We hypothesized that time-locked electrical activity is recorded
in laryngeal muscles after the stimulation of M1 for laryngeal muscles and of premotor cortex in the caudal opercular part of inferior
frontal gyrus.
Therefore, the goal of this study was to investigate the relationship between cortical spots generating neurophysiologic markers
recorded in laryngeal muscle and the functional role of these cortical areas by clinically producing transient speech disruptions.
2.1. Healthy subjects/patients
1913
Ten right-handed healthy subjects, three male and seven
female, average age 31 ± 13.32 (range 22–66 years), and 18 righthanded patients, 10 male and eight female, average age
46.2 ± 13.69, range 27–68 years, were included in the study. The
Edinburgh Inventory Questionnaire test (Oldfield, 1971) was used
for assessment of handedness. All healthy subjects and patients
signed informed consent forms to participate in the study. Healthy
subjects received a small honorarium. The study was approved by
the Ethical Committees of School of Medicine, University of Split,
Croatia, and University Hospital Bellvitge, Barcelona, Spain.
2.2. Healthy subjects group
2.2.1. Magnetic resonance imaging (MRI) and navigated transcranial
magnetic stimulation (nTMS)
Magnetic resonance imaging (MRI) of the head for each subject
was performed with Siemens Magnetom Avanto, Tim (76 18)
strength 1.5 T. MRI images were recorded by specific MRI requirements for nTMS-NBS (Navigated Brain Stimulation), Nexstim System 4 (Helsinki, Finland), including the head, visible ears, and
nose. MRI images are integrated in the nTMS machine with a
three-dimensional navigation system display of the subjects’ brain.
Sophisticated real-time data processing allows the precise display
of the induced electric field (E-field) within the brain tissue. Targeting tools available on-screen are the following: grid for systematic brain mapping, targeting tool for optimal coil placement,
aiming tool for precise repetition of given stimuli, and automated
stimulation (location controlled). The subject wears an optical
head tracker and by using a pointer, 12 points are registered on
the subject’s scalp. An air-cooled eight-shaped figure coil was used,
generating a biphasic pulse of 289 ls pulse length. The maximum
E-field is 172 V/m below the Nexstim Focal coil in the spherical
conductor model representing the human head.
The nTMS mapping procedure is as follows:
1. Eliciting MEP resting threshold for hand muscle, abductor
pollicis brevis (APB);
2. Mapping of the very lateral part of M1 and recording SLR in
cricothyroid muscle during the visual object-naming task;
3. Mapping of the inferior frontal gyrus and recording LLR in
cricothyroid muscle during the visual object-naming task;
4. Stimulation of cortical spot which elicited SLR to produce
transient speech disruption during the visual object-naming
task;
5. Stimulation of cortical spot which elicited LLR to produce
transient speech disruptions during the visual object-naming task.
Mapping of the M1 for hand muscle is the standard method in
nTMS studies dealing with mapping of M1 (Epstein et al., 1999;
Schmidt et al., 2009; Julkunen et al., 2011) and its vicinity (Jennum
et al., 1994; Michelucci et al., 1994; Epstein et al., 1999; Stewart
et al., 2001), and as a standard intraoperative procedure preceding
mapping of primary motor cortex and Broca’s area (Duffau, 2008;
Kim et al., 2009; Lubrano et al., 2010). Mapping over the left M1
for APB was determined by the ‘‘omega knob’’ on axial MRI images,
or a ‘‘hook structure’’ at the sagittal MRI (Yousry et al., 1997). The
central sulcus was also used as a landmark while moving the coil in
the anterior–posterior direction in order to map the hot spot for
M1 for APB. The MEP resting threshold was defined as the lowest
stimulus intensity for eliciting at least five MEPs in the APB muscle
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V. Deletis et al. / Clinical Neurophysiology 125 (2014) 1912–1922
with an amplitude of at least 50 lV in a series of 10 consecutive
trails. Mapping over the left M1 for laryngeal muscle was
performed after mapping of M1 for APB muscle. The threshold
and location of M1 for laryngeal muscle was determined by recording corticobulbar MEPs from the cricothyroid muscle. Mapping
over the left M1 for cricothyroid muscle was performed using into
account our recently reported data on Euclidian distance of
25.19 ± 6.51 mm between the cortical representation of M1 for
APB and M1 for cricothyroid muscle, with cricothyroid muscle lateral to APB (Espadaler et al., 2012). The resting threshold of corticobulbar MEP was defined as the lowest stimulus intensity for
eliciting at least five corticobulbar MEPs in cricothyroid muscle
with an amplitude of 50 lV in a series of 10 consecutive trails of
TMS.
For mapping of the inferior part of frontal gyrus, the guideline
topography of Broca’s area was used according to Ebeling et al.
(1989). We also used as a guideline the distance between M1 for
cricothyroid muscle and of inferior frontal gyrus generating transient speech disruptions area of 17.23 ± 4.73 mm (Rogić et al.,
2014). For mapping of the inferior frontal gyrus, the stimulation
intensity was adjusted using the resting threshold for corticobulbar MEPs in cricothyroid muscle. If this intensity was unpleasant
to the subject, the stimulus intensity was reduced in steps of 1–
2% of maximum stimulator output.
2.2.2. Stimulation parameters and speech task procedure
Patterned bursts of magnetic stimuli were used, described in
our previous study (Rogić et al., 2014). Briefly, two paradigms of
patterned repetitive stimulation were used:
The ‘‘first paradigm’’ consisted of four bursts of five stimuli
each, 6 ms apart, burst repetition rate of 4 Hz.
The ‘‘second paradigm’’ consisted of 16 bursts of four stimuli
each, 6 ms apart, burst repetition rate of 12 Hz.
All healthy subjects underwent two testing sessions separated
by 4 months.
First testing session. The first paradigm of stimulation was used
for eliciting: (a) a neurophysiologic marker of M1 for laryngeal
muscles obtained by mapping of the lateral part of M1 and recording of SLR from cricothyroid muscle and (b) a neurophysiologic
marker of premotor cortex of inferior frontal gyrus, recorded from
laryngeal muscle, obtained by mapping of the caudal opercular
part of inferior frontal gyrus and recording of LLR from cricothyroid
muscle. Due to the slight jittering in LLR responses, the mean
values of five consecutive latencies of LLRs have been calculated.
Second testing session. For testing the functional role of cortical
spots generating neurophysiologic markers in cricothyroid muscle,
the second paradigm of stimulation was used in order to elicit
transient speech disruptions while stimulating the cortical spot
which elicited neurophysiologic markers.
Stimulation was applied while subjects were engaged in the
visual object-naming task. The methodology for visually presented
objects (VPOs)/pictures were taken from a normative study by
Brodeur et al. (2010). Fifty pictures were presented during measurement in a randomized manner in one session. The pictures
were presented on a computer monitor (LG 2200 ‘‘LCD’’ with
1920 1080 resolution) using the Presentation program (Neurobehavioral Systems, Albany, CA, USA). The picture was presented
on a white background while the edges of the screen were black
for the photo sensor to indicate the start and end of the presented
picture. VPO was presented for 3000 ms, followed by 2390 ms of
blank screen. The time from VPO until the presentation of the next
VPO was 5390 ms. The Presentation program triggered nTMS 0–
350 ms after VPO. In the first testing session in eight subjects,
the beginning of stimulation was 300 ms after VPO, in one subject
350 ms, and in another 150 ms after VPO. In the second testing session, the Presentation program triggered nTMS at zero time with
the VPO.
In the first testing session, neurophysiologic markers were
recorded from cricothyroid muscle, while in the second testing session, stimulation of identical cortical spots generating markers
were repeated to elicit transient speech disruptions, without placing electrodes in cricothyroid muscle. One subject (Table 1, No. 1)
was an exception in whom in both testing sessions the electrodes
were placed in cricothyroid muscle to record neurophysiologic
markers and speech-related activity. In this subject, the second
paradigm of stimulation was used to elicit neurophysiologic markers and transient speech disruptions.
According to Abel et al. (2009), errors can happen incidentally in
healthy subjects during the visual object-naming task; therefore,
before the nTMS stimulation session started, each subject participated in a visual object-naming task in two control sessions. In
the second control session, we discarded ‘‘problematic’’ pictures,
and did not use them during the nTMS session.
Control measurement with nTMS was performed in four subjects
with the coil set away from the subject’s head during the visual object-naming task in the first testing session. This measurement was
performed to exclude possible influence of micro-reflexes, especially of sonomotor origin (Bickford et al. 1964; Bickford, 1966).
Table 1
SLR and LLR parameters in a group of healthy subjects and the type of transient speech disruption by stimulating cortical spot which elicited SLR in cricothyroid muscle.
First testing session
Subjects
Gender
Age
SLR
Latency
(ms)
Intensity
(%)
1
2
3
4
5
6
7
8
9
10
Mean ± SD
Range
F
M
F
M
F
F
M
F
F
F
29
66
27
39
27
26
30
21
23
22
31 ± 13.32
22–66
11.57
13.57
12.53
11.65
12.54
14.96
11.9
11.98
13.7
12.2
12.66 ± 1.09
11.57–14.96
38
45
34
42
34
40
40
34
37
39
38.3 ± 3.7
34–45
Second testing session
Intensity
(%)
Dysarthria
Dysarthria
Dysarthria
Dysarthria
Dysarthria
Dysarthria
Dysarthria
26
37
23
30
41
40
43
a
a
Dysarthria
Dysarthria
35
29
33.8 ± 7.08
23–43
No = transient language disruption elicited number of times.
a
Subject did not participate in the second testing session of the study.
First testing session
Second testing session
LLR
Latency
(ms)
Intensity
(%)
Speech
arrest (No)
Language
disturbances (No)
Intensity
(%)
64.0
61.1
58.9
57.6
51.4
50.4
67.7
51.5
58.7
63.7
58.5 ± 5.9
50.4–67.7
31
45
36
40
30
40
40
32
36
37
36.7 ± 4.7
30–45
3
0
1
1
1
1
2
8
1
4
1
3
3
0
26
37
23
30
41
40
43
a
a
a
3
0
3
2
35
29
33.8 ± 7.08
23–43
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V. Deletis et al. / Clinical Neurophysiology 125 (2014) 1912–1922
2.2.3. Recordings
The recordings of neurophysiologic markers from cricothyroid
muscles were done using a five-channel EMG amplifier integrated
with E-field navigation and TMS stimulation of Nexstim NBS system (Nexstim NBS System 4, Helsinki, Finland). The channels are:
(a) EMG from APB muscle, (b) EMG from cricothyroid muscle, (c)
beginning and end of VPO, (d) stimulator trigger, and e) microphone recordings.
Surface electrodes Neuroline 720 (Ambu Inc, Glen Burnie, MD,
USA) were placed over the right APB muscle. The method for the
percutaneous placement of electrodes into cricothyroid muscle
has been described in our previous studies (Deletis et al., 2011;
Espadaler et al., 2012). Briefly described, for recording SLR and
LLR from right cricothyroid muscle, two hook-wire electrodes were
used, each consisting of a Teflon-coated stainless steel wire, 76 lm
in diameter passing through 27-gauge needles (0.4 mm) and of
13 mm length (hook-wire electrodes, specially modified, Viasys
Healthcare WI, MA, USA). The recording wires have a stripped Teflon isolation of 2 mm at their tip and curved to form the hook for
anchoring them.
In the second testing session, modified methods of video and
audio recordings (Panasonic HDC-SDT750) (Lioumis et al., 2012)
were taken for off-line analysis.
2.3. Patient group
2.3.1. Intraoperative DCS
The patients presented with pathology in the left hemisphere
(Table 2), located in or nearby the eloquent cortical areas. A monopolar handheld probe was used to stimulate the exposed cortex
with a stimulation paradigm of bursts (monopolar stimulator
instrument dissector 1.2 mm curved 30°, INOMED, Germany),
while a bipolar electrode for 50 Hz stimulation. The electrodes
were placed in the cricothyroid muscle and recordings were made
during all mapping procedures for M1 for laryngeal muscles and
premotor cortex of inferior frontal gyrus.
2.3.2. DCS procedure
The procedure includes the following:
1. Mapping of somatosensory cortex by using phase reversal median somatosensory-evoked potentials
2. Mapping of M1 for hand, leg, and facial muscles to produce
muscle contractions
3. Mapping of the inferior frontal gyrus in order to produce speech
arrest
4. Positive spots from mapping step 2 were repeated for eliciting
MEPs in hand, leg, and facial muscles – very lateral part of M1
was also mapped to elicit SLR in cricothyroid muscle
5. Positive spots from mapping step 3 were repeated for eliciting
LLR of premotor cortex in the caudal opercular part of inferior
frontal gyrus, recorded from laryngeal muscle.
2.3.3. Stimulation parameters and speech task procedure
All patients underwent electrical stimulation of the exposed
cortex during awake craniotomy using the following stimulating
parameters:
a) Stimulation paradigm of 50 Hz, biphasic pulses of 4 ms
duration, 3 s of train, intensity up to 15 mA for mapping
of M1 for upper (hand muscles) and lower extremities
(leg muscles) and facial muscles in order to produce muscle
contractions were applied. The same paradigm of stimulation was applied in order to elicit speech arrest while
mapping the inferior frontal gyrus. During surgery after
identification of the Rolandic cortex, a series of line-drawing images were presented on the computer screen to the
patients as a visual object-naming task. Images were
time-locked at zero time with the electrical stimulation.
Before the surgery, patients were acquainted with the
visual object-naming task and problematic pictures were
excluded from the intraoperative testing. The patient was
instructed to name the object in combination with the
beginning sentence: ‘‘This is a...’’ during cortical stimulation. All cortical sites were stimulated three times as a
standard procedure during intraoperative mapping of these
cortical areas. The cortical spot was considered ‘‘positive’’ if
DCS elicited speech arrest in at least two of three times
after testing of identical point.
b) Stimulation paradigm of bursts consists of four monophasic
pulses of 0.5 ms duration each, 4 ms apart, burst repetition
rate of 4 Hz, intensity up to 15 mA for eliciting SLR, and
LLR recorded from cricothyroid muscle. Stimulation was
applied while subjects were in rest (not engaged in the
visual object-naming task). Due to the slight jittering in
LLR responses, the mean values of five consecutive latencies
of LLRs have been calculated.
Table 2
Latencies of SLR and LLR recorded in cricothyroid muscle in a group of patients.
Patients
Gender
Age
Pathology
SLR latency (ms)
LLR latency (ms)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Mean ± SD
Range
M
M
F
F
M
M
M
M
F
F
F
F
M
M
F
M
F
M
59
67
68
32
43
35
60
38
26
39
55
27
59
43
34
61
38
48
46.22 ± 13.69
27–68
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
Left
10.49
12.2
12.6
12.1
11
12.35
12.95
14.23
12.15
15.7
14.35
13.45
12.9
13.45
12.9
11.6
11.65
11.6
12.67 ± 1.23
10.49–15.7
55.2
53
56.76
60.35
51.45
58.7
59.05
58.7
50.15
50.05
52.45
55.1
50.05
50.15
53.95
50.15
58.95
52.45
54.25 ± 3.69
50.05–60.35
frontal GBM
frontal M1
temporal GBM
frontal glioma
temporal GBM
frontal cavernoma
frontal M1
frontal cavernoma
parietal glioma
fronto-temporal astrocitoma
frontal oligodendroglioma
frontal MAV
frontal GBM
fronto-temporal astrocitoma
temporal cavernoma
front-opercular glioma
temporal-insular astrocitoma
parietal cavernoma
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V. Deletis et al. / Clinical Neurophysiology 125 (2014) 1912–1922
In the patient group, the electrodes were placed in the cricothyroid muscle and recordings were made during all mapping
procedures.
2.3.4. Recordings
A pair of twisted subdermal needles (Neuroline-Ambu,
0.500 27 G) were placed in: orbicularis oris, extensor digitorum
communis and APB, tibialis anterior, and abductor hallucis. The
method for the percutaneous placement of electrodes into cricothyroid muscle was identical to that described in the group of
healthy subjects. In the patient group, we used a modified Synergy
(Viasys Healthcare) 10-channel EMG-evoked potential unit.
The team consisted of a neurophysiologist and neuropsychologist observing and documenting elicited transient speech
disruptions.
2.4. Anesthesia
All patients underwent a standard technique of asleep–awake–
asleep anesthesia, with conscious sedation using propofol and
remifentanil, and assisted ventilation with laryngeal mask (LMA).
After craniotomy was performed, the patients were gradually
awakened by weaning their sedation and LMA was removed, and
reinserted after completion of the mapping procedure.
3. Results
3.1. Short latency response
In all healthy subjects and patients, SLR was successfully recorded from cricothyroid muscle while stimulating the very lateral
Fig. 1. Upper: Subject during naming visually presented object (VPO) and examiner holding the coil on the dominant hemisphere with stimulation localized over the
premotor cortex in the caudal opercular part of inferior frontal gyrus. (Legend: 1 = microphone connected to EMG amplifier, 2 = monitor with visual object presented to the
subject with the attached photo sensor, 3 = microphone connected with the video camera, 4 = magnetic coil for nTMS, 5 = monitor with MRI for precise determination of
stimulation site, 6 = cloned monitor 5 for video shooting, and 7 = cloned monitor 2 for video shooting). Lower: Neurophysiologic markers indicating functional anatomy of
primary motor cortex for laryngeal muscles and premotor cortex in the caudal opercular part of inferior frontal gyrus shown for one healthy subject (No. 3). Left: SLR of M1 for
laryngeal muscle; (A) repeatability of SLRs recorded from cricothyroid muscle, (B) superimposed SLRs. Right: LLR of opercular part of Broca’s area, (C) repeatability of LLRs
recorded from cricothyroid muscle, and (D) superimposed LLRs. The beginning of recording is set on the zero time which corresponds to time of 300 ms after VPO. In the
middle: 3D MRI of subject’s brain with the localization of the stimulation.
V. Deletis et al. / Clinical Neurophysiology 125 (2014) 1912–1922
1917
Fig. 2. Upper: Intraoperative mapping during awake craniotomy. The patient and the surgeon are separated with surgical drapes and can communicate with each other. The
patient is engaged in the visual object-naming task. Lower: The results of cortical mapping. Stimulation of the cortical spot marked with red marker 4 elicited SLR, while spot
marked with yellow marker 1 elicited LLR in cricothyroid muscle. From the same spot where LLR was elicited, speech arrest was induced for Spanish. (T = tumor location; Blue
markers 1–7 = somatosensory cortex; Red markers 1–4 = primary motor cortex; English, Catalan, and Spanish flags = cortical spots for speech arrest for these languages; from
these cortical spots LLR was not elicited in the cricothyroid muscle). (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
part of M1. The latency of SLR elicited by magnetic stimulation in
healthy subjects was 12.66 ± 1.09 ms (Table 1 and Fig. 1), and by
electrical stimulation in patients 12.67 ± 1.23 ms (Table 2, Fig. 2).
The functional role of this area was determined by stimulation of
cortical spot which elicited SLR and clinically producing dysarthria
(Table 1 and Table 2), accompanied by contractions of facial and
masticatory muscles.
Long Latency Response. In all healthy subjects and patients,
LLR was successfully recorded from cricothyroid muscle while
mapping the caudal opercular part of inferior frontal gyrus. The
latency of LLR elicited by magnetic stimulation in healthy subjects
was 58.5 ± 5.9 ms (Table 1, Fig. 1), while by electrical stimulation
in patients it was 54.25 ± 3.69 ms (Table 2, Fig. 2).
The functional role of this area was tested by stimulation of cortical spot that elicited LLR and clinically produced transient speech
disruptions (Table 1 and Table 2). Intraoperatively, speech arrest
was elicited in all patients, while in healthy subjects speech arrest
and/or language disturbances were elicited (Table 1).
Speech arrest was elicited in seven healthy subjects (77.8%). In
those seven healthy subjects in whom speech arrest was induced
once, twice, or more times, language disturbances (semantic errors)
were also induced stimulating the same cortical spot. In two subjects (22.2%) (No. 2 and No. 10), language disturbances (semantic er- (grapes)/,
rors) (Table 1) were elicited (kruška (pear)/instead/grožde
etc.). In healthy subjects, speech arrest was characterized as a complete cessation of speech, confirmed by absent microphone recordings together with an off-line analysis of video recordings showing
no facial and masticatory muscle contractions. Semantic errors
(subjects gave semantically similar word for the object) were also
confirmed by an off-line analysis of video recordings.
In healthy subjects, when M1 for cricothyroid muscle and inferior frontal gyrus were mapped, the electrical activity was recorded
in cricothyroid muscle in the pre-speech (pre-EMG period) and
speech execution period (contractions related to the speech). The
SLRs and LLRs are elicited in cricothyroid muscle and can be detected in the period before the pre-speech phase. By stimulating
the same LLR cortical spot which elicited speech arrest, semantic
paraphasias could also be elicited with LLR recordings before
pre-speech, and with electrical activity in pre-speech and speech
execution period. When semantic paraphasias were elicited from
other parts of inferior frontal gyrus (not LLR cortical spot), LLR
was not recorded in cricothyroid muscle; only electrical activity
in the pre-speech and speech execution phase was recorded in
cricothyroid muscle.
In one healthy subject (Table 1, No. 1), in whom speech arrest
was elicited while simultaneously recording activity from cricothyroid muscle, LLRs were present in EMG, but voice was absent in
microphone recordings (Fig. 3). The subject did not give any response for object presenting/vješalica (coat hanger)/which was
confirmed with absent activity recorded with the microphone.
The off-line video analysis also showed absent activity in facial
and masticatory muscles. The latency of LLRs, indicated with the
red stars in Fig. 3 was 57.6 ± 0.71 ms.
LLR was elicited in all healthy subjects 423.50 ± 115.31 ms from
the beginning of the visual object presentation. The speech onset
reaction time was 684.07 ± 91.48 ms. The repeatability of eliciting
LLR when mapping the caudal opercular part of inferior frontal
gyrus is shown for one healthy subject No. 3 in Fig. 4.
In the patient group, M1 was mapped first applying electrical
stimulation with 50 Hz in order to elicit tonic contractions in the
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V. Deletis et al. / Clinical Neurophysiology 125 (2014) 1912–1922
Fig. 3. Eliciting LLRs in cricothyroid muscle during speech arrest. Upper: Absent activity in the microphone recordings during speech arrest, while LLRs were elicited in
cricothyroid muscle (within the frame). Lower: Recordings in square (indicated with the red star) represent repeatability of four consecutive LLR, superimposed at the bottom.
Scale in seconds, while amplitude in mV. Activity at the beginning of each trace represents the tail from the previous response. CTHY = cricothyroid muscle; VON = visual
object naming; arrow indicates beginning of visual presented object; MIC = microphone recordings; STIM = stimulation; artifacts from 16 bursts of stimuli are shown across
all channels.
orofacial muscles during rest and dysarthric speech. With the same
stimulation paradigm, inferior frontal cortex was mapped, and
only a specific area in the caudal opercular part of inferior frontal
gyrus, which elicited speech arrest, also elicited LLR from cricothyroid muscle (Fig. 2). Stimulation of larger and more rostral parts of
the opercular inferior frontal gyrus elicited speech arrest or
paraphasia without eliciting LLR in cricothyroid muscle (Figs. 2
and 5).
opercular part of inferior frontal gyrus for laryngeal muscles,
applying a specific paradigm of stimulation. To elicit SLR and LLR,
a patterned burst of magnetic stimuli were used in healthy subjects (Rogić et al., 2014), while a short train of electrical stimuli
was used in patients. The functional role of these areas was tested
by stimulation of cortical spot that elicited SLR and LLR and clinically producing transient speech disruptions, respectively.
SLR can be regarded as a neurophysiologic marker of M1 for laryngeal muscles, according to the following evidence:
4. Discussion
For the first time, we have shown that it is possible to elicit neurophysiologic markers of M1 and premotor cortex in the caudal
1) SLRs are elicited from the vocal and cricothyroid muscles
during electrical and magnetic stimulation of the M1 for laryngeal muscles in a group of patients and healthy subjects
V. Deletis et al. / Clinical Neurophysiology 125 (2014) 1912–1922
1919
Fig. 4. Eliciting LLR when mapping premotor cortex in the caudal opercular part of inferior frontal in healthy subject No. 3. Pink star indicates LLR elicited in cricothyroid
muscle before speech execution. The green star indicates the beginning of the microphone recordings of speech. The first channel: stimulation packages; the second channel:
recordings of electrical activity from cricothyroid muscle; and the third channel: microphone recordings.
(Deletis et al., 2008, 2009, 2011; Espadaler et al., 2012). SLR
corresponds/equals to corticobulbar MEP.
2) The results of this study have shown that in all healthy subjects and patients, nTMS and DCS of cortical spot, which elicited SLR, clinically produced dysarthria.
3) M1 for laryngeal muscles is anatomically represented at the
most lateral part of M1. Recently, we have reported that the
distance between the cortical representation of M1 for APB
and M1 for cricothyroid muscle was 25.19 ± 6.51 mm, with
cricothyroid muscle lateral to APB (Espadaler et al., 2012).
During speech disruptions induced by stimulation of SLR cortical spot, dysarthric speech was observed accompanied with contractions of the orofacial and masticatory muscles visible on the
video recordings. Contractions of orofacial and masticatory muscles were presumably produced due to the spread of the current
from primary motor cortex for laryngeal muscles to the nearby primary motor cortical representations for other cranial muscles.
While the results demonstrate the SLR spot’s role in speech execution, it probably also has functional roles in other voluntary laryngeal movements that we did not test, such as coughing, throat
clearing, swallowing, crying, moaning, or humming.
The following are evidence for LLR being the most likely neurophysiologic marker of premotor cortex for laryngeal muscles:
1) The results of this study have shown that DCS and nTMS of
the stimulated cortical spot, which elicited LLR, clinically produced speech arrest in all patients and speech arrest and/or
language disturbances in healthy subjects. Anatomically, this
cortical spot corresponds to a small region of the caudal opercular inferior frontal gyrus detectable on an MRI of healthy
subjects and in the patients by intraoperative inspection.
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V. Deletis et al. / Clinical Neurophysiology 125 (2014) 1912–1922
Fig. 5. Mapping results for M1 and premotor cortex in the caudal opercular part of
inferior frontal gyrus for single patient. (1) Markers indicate mapping results for
premotor cortex in the caudal opercular part of inferior frontal gyrus. Green marks
indicate produced language disturbances (semantic and phonological paraphasias).
Spanish flags/yellow marks are spots where speech arrest could be elicited. Only
spot with Spanish flag 1 (yellow mark 1) elicited LLR in cricothyroid muscle; (2) red
marks 1–6 primary motor cortex; (3) blue marks 1–7 primary somatosensory
cortex; (4) Results for mapping temporal area (specific language disturbances); and
(5) T = tumor location. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
2) After electrical stimulation of Broca’s area, postsynaptic
potentials of high amplitude were recorded in the lateral
part of the M1 (Greenlee et al., 2004). These data can be
regarded as indirect evidence of an anatomical connection
between the premotor cortex in the caudal opercular part
of inferior frontal gyrus and M1 for laryngeal muscle. There
are also possible contributions of other pathways to LLR origin, other than through M1.
LLR was recorded in cricothyroid muscle when speech arrest
was elicited (Fig. 3) in one healthy subject since electrodes were
inserted in the second session only in that subject. By stimulating
the same LLR cortical spot which elicited speech arrest, semantic
paraphasias could also be elicited with LLR recordings before
pre-speech, and with electrical activity in pre-speech and speech
execution period. Stimulus spread to nearby Broca’s area language
cortex likely explains these observations. When semantic paraphasias were elicited by stimulating other parts of the dominant
opercular inferior frontal gyrus (not LLR cortical spot), LLR was
not recorded in cricothyroid muscle; only electrical activity in
pre-speech and speech execution phase was recorded in cricothyroid muscle. Dysphasia due to Broca’s area language cortex disruption likely explains this kind of speech arrest without elicited LLR.
Furthermore, even though we did not test the nondominant hemisphere, it is possible that cortical spot eliciting LLR by stimulation
of nondominant hemisphere is likely part of the relays for emotive
expression (e.g., prosody and singing). What functional role the LLR
spot of either hemisphere may play in other voluntary laryngeal
movements, such as humming, etc., is unknown because we tested
only speech.
In the patient group, electrical stimulation with 50 Hz was first
applied in order to elicit contractions in the orofacial muscles during rest and dysarthric speech when mapping M1, and to elicit
speech arrest and/or language disturbances when mapping the
dominant inferior frontal gyrus. Dysarthric speech was presumably
accompanied with dysphonic/aphonic speech as well.
Speech arrest and/or phonological or semantic paraphasias
were elicited by mapping specific parts of premotor cortex in the
caudal opercular part of inferior frontal gyrus, and tonic
contractions could be elicited from cricothyroid muscle. After that,
bursts consisting of four stimuli each, 4 ms apart, burst repetition
rate of 4 Hz, were applied to the cortical spots which have previously elicited speech arrest. Cortical spots which elicit semantic
or phonological paraphasias and/or speech arrest without LLR
recording in cricothyroid muscle when mapping the dominant
inferior frontal gyrus can be regarded as Broca’s area language cortex (Figs. 2 and 5). We suggest that dysphasia/aphasia might be
likely contributing mechanism to semantic or phonological paraphasias and/or speech arrest without LLR recording in cricothyroid
muscle.
The spot in the caudal opercular part of dominant inferior frontal gyrus generating LLR and speech arrest after stimulation can be
considered only as premotor cortex for laryngeal muscle, but not as
motor language cortex. This spot cannot be a safe resection margin,
but an orientation spot for close proximity to the language producer Broca’s area.
We believe that we are dealing with small neuronal pools
where language as a mental process gets transformed into a coded
message to the M1 in order to activate muscles involved in speech.
It is a matter of fact that no methodology, so far, to look at the electrical activity of laryngeal muscles during speech arrest has been
published.
During mapping procedures for eliciting neurophysiologic
markers, only a very limited area, approximately 5 5 mm of
stimulated cerebral cortices, by either nTMS or DCS (Deletis
et al., 2008), generates specific neurophysiologic markers recordable in the laryngeal muscles. Therefore, we consider these markers as anatomical and functional indicators of primary motor and
premotor cortex for laryngeal muscles. In our previous studies
(Deletis et al., 2008, 2011), we have also showed the possibility
to elicit SLR and LLR in laryngeal muscles by transcranial electrical
stimulation (TES) of the right hemisphere with the TES montage C3
versus Cz and C4 versus Cz. TES stimulating parameters were a
short train of electric stimuli consisting of three to five stimuli having a duration of 0.5 ms each. These stimuli were spaced 2 ms apart
with a train repetition rate of 2 Hz and intensity up to 120 mA.
Thus, neither spot can be specific to language or speech and in
order to use the LLR as a guide to nearby Broca’s area language cortex, one must know through other means that the dominant hemisphere is being stimulated
As we have mentioned earlier in the methodological part, we
made an effort to exclude micro-reflexes of Bickford (Bickford
et al., 1964; Bickford, 1966) as a potential source of SLR or LLR in
the nTMS measurement of healthy subjects. The sonomotor micro-reflex can be activated via corticoreticular neurons as has been
already shown (Fisher et al., 2012). We excluded micro-reflexes by
moving the coil away from the head during the stimulation. By this
approach, SLR and LLR were not recorded. Furthermore, if SLR and
LLR are micro-reflexes due to TMS, or activation of the auditory
system by click produced by magnetic coil, micro-reflex would
be activated by the coil away from the head. We have shown that
is not the case. We have presented further evidence that SLR and
LLR are cortical origins and specific for the particular region in
our data. SLR can only be elicited if M1 for laryngeal muscles is
stimulated and LLR can only be elicited if premotor cortex in the
caudal opercular part of inferior frontal gyrus is stimulated.
Our explanation of the mechanism of SLR and LLR generation
during speech execution is as follows: when the coded signal from
the premotor cortex in the caudal opercular part of inferior frontal
gyrus reaches the M1 motoneurons of the muscles involved in
speech, it gets transmitted further via the corticobulbar pathway
to the motor neurons of the cranial nerves in the brain stem. From
the motoneurons of the cranial nerves and via cranial motor nerves,
targeted muscles get activated. If M1 for laryngeal muscles and
premotor cortex in the caudal opercular part of inferior frontal
V. Deletis et al. / Clinical Neurophysiology 125 (2014) 1912–1922
gyrus are stimulated during their ‘‘working state,’’ when their excitability is increased, we can easily induce synchronized activity of
their neurons, and record this activity in the laryngeal muscles as
SLR or LLR, depending on the neural structure being stimulated.
Increased excitability of Broca’s area during motor speech processing has been shown in a temporal domain of 300 ms during the
execution of specific speech tasks. This was shown by an invasive
study of recording EPSPs from Broca’s area in epileptic patients
(Sahin et al., 2009), and noninvasively (Schuhmann et al., 2009,
2012; Rogić et al., 2014). In this study, for most of the healthy
subjects the delay of 300 ms for triggering of nTMS after picture
presentation has been shown to be optimal for eliciting SLR and LLR.
Presumably, more synapses are involved when stimulating
premotor cortex in the caudal opercular part of inferior frontal
gyrus, compared to M1 for laryngeal muscles; therefore, the latency
of LLR is longer than for SLR. Not only latency difference but also
jittering is more pronounced in LLR than SLR. After analysis of
response latencies of LLR elicited by magnetic stimulation in
healthy subjects and by electric stimulation in patients, we found
out that LLR have longer latencies when elicited by magnetic versus
electric stimulation. The LLR latencies elicited by magnetic versus
electric stimulation were 58.5 ± 5.9 and 54.25 ± 3.69 ms. Furthermore, it was easier to elicit both SLR and LLR electrically than
magnetically. This was the reason why we needed to introduce a
time-locked visual object-naming task when applying nTMS in
healthy subjects to facilitate eliciting of SLR and LLR. Intraoperatively, patients were in the rest state when eliciting SLR and LLR.
4. Conclusion
In all healthy subjects and patients, neurophysiologic markers
of specific cortical areas are identified: (a) a neurophysiologic marker of M1 for laryngeal muscles obtained by mapping of the lateral
part of M1 and recording of SLR from cricothyroid muscle and (b) a
neurophysiologic marker of premotor cortex for laryngeal muscles
obtained by mapping of the caudal opercular part of inferior frontal
gyrus and recording of LLR from cricothyroid muscle. Stimulation
of the SLR spot clinically produced dysarthria (and probably also
dysphonia). Stimulation of the dominant hemisphere’s LLR spot
produced speech arrest likely by disrupting phonation and, in some
healthy controls, produced language disturbances likely by
unavoidably also exciting nearby Broca’s area language cortex.
LLR cortical spot in the dominant hemisphere may be a useful
guide to the nearby proximity of Broca’s area language cortex. It
might also help approximate the likely nearby location of Broca’s
area language cortex for patients unable to tolerate language mapping under local anesthesia. Further studies are needed on the
same group of patients undergoing nTMS and DCS in order to replicate the data obtained in this study. The nondominant hemisphere should also be tested on the group of healthy subjects
and patients, since it is possible that nondominant SLR or LLR
might disrupt speech execution, which is not specific to the dominant hemisphere.
In addition, the possible functional role of the SLR and LLR spots
in executing voluntary phonations and laryngeal movements other
than speech could be evaluated by specific testing.
5. Significance
Novel methodology for testing the anatomic and functional
integrity of primary motor and premotor cortex for laryngeal muscles can be used in preoperative and intraoperative mapping. Given
the close proximity of laryngeal premotor cortex to Broca’s language cortex in the dominant hemisphere, the methodology could
1921
facilitate or compliment surgical planning and intraoperative mapping intended to help avoid motor speech deficits.
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