Chem. Senses 27: 629-634,
2002
© Oxford University Press 2002
Gustatory Evoked Cortical Activity in Humans Studied by Simultaneous EEG and MEG Recording
Human Perception and Cognition Group, Neuroscience Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1, Tsukuba 305-8566, Japan 1 Department of Physiology, Kumamoto University School of Medicine, Honjo 2-2-1, Kumamoto 862-0811, Japan
Correspondence to be sent to: Tatsu Kobayakawa, Human Perception and Cognition Group, Neuroscience Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1, Tsukuba 305-8566, Japan. e-mail: kobayakawa-tatsu{at}aist.go.jp
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Abstract |
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Evoked potentials are widely used in clinical medicine for objective evaluation of sensory disturbances. However, gustatory evoked potentials (GEPs) have not been extensively studied due to lack of agreement among investigators regarding the waveforms. In this study GEPs and gustatory magnetic fields (GEMfs) were simultaneously recorded from five subjects in response to 0.3 M NaCl in an attempt to establish GEP recording as an objective gustatometer. Each subject received a total of 240 stimulus presentations over six sessions. Three GEP components (P1, N1 and P2) were observed and correlated with their corresponding equivalent current dipoles (ECD1, ECD2 and ECD3, respectively). ECD1 was localized to area G in all subjects, P1 being the indicator of intact gustatory projection to area G. No significant GEP activity was detected during the time preceding P1, which suggests that there was no activity in cortical gyri other than that detected by magnetoencephalography. ECD2 and ECD3 were localized to various cortical structures, including the inferior insula and the superior temporal sulcus, indicating that N1 and P2 reflect higher order gustatory functions. The present results indicate that measurement of GEPs may be useful for objective evaluation of gustatory disturbance.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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It is well known from clinical observation that brain damage and aging often result in gustatory disorders (Lee et al., 1998
; Onoda
and Ikeda, 1999; Aglioti et
al., 2000). So far, however, an objective medical examination
of gustatory function has not been carried out. Electroencephalography (EEG)
is a useful and established tool for objective examination of functional
disturbances of various sensory modalities, but it has seldom been applied to
subjects with gustatory disturbances. This is because, in the past, the
difficulty involved in adequately controlling the necessary liquid stimuli led
to inconsistent measurements of gustatory evoked potentials (GEPs)
(Funakoshi and Kawamura, 1971;
Kobal, 1985;
Plattig et al., 1989;
Prescott, 1989). To date,
there has also been no agreement between investigators regarding the sequence
of potentials to be measured.
Among the requirements originally proposed by Evans et al.
(Evans et al., 1993)
regarding stimulation to record chemosensory evoked potentials, stimuli should
be in the form of a square wave with a rise time to 70% of the maximum
concentration of <50 ms. Kobayakawa et al.
(Kobayakawa et al.,
1996) developed a gustatory stimulator which fully meets this
requirement. When they previously located human gustatory areas by
magnetoencephalography (MEG), a recently introduced technique with high
temporal and spatial resolution, their stimulating system was found to be
useful for activating gustatory evoked magnetic fields (GEMfs) without
contamination due to tactile stimulation
(Kobayakawa et al.,
1996). However, due to the expensive running costs of the MEG
system and the requirement for a magnetically shielded room, the use of MEG
systems for gustatometry in clinical practice is practically unfeasible.
Meanwhile, EEG has the same superior temporal resolution as MEG, but with far
lower running costs. Repetitive presentations of gustatory stimuli are
necessary to obtain clear GEMfs or GEPs. Roughly 40 stimulus presentations,
taking 
30 min, are enough to obtain GEMfs, but several hundred are most
likely required to collect adequate GEPs, which would require the subjects to
be restrained for several hours. However, EEGs obtained from the same subject
over several days can be averaged together if the electrodes are carefully
placed at the same position for each set of recordings. Used in this way, EEG
is much more practical than MEG for use in the clinical field.
Previously, Kobayakawa et al.
(Kobayakawa et al.,
1996) located the primary gustatory area, area G, at the
transition between the parietal operculum and the insula of the cerebral
cortex in humans by estimating the location of the first equivalent current
dipole (ECD) from GEMfs. Because MEG is incapable of detecting electrical
current which is vertically oriented to the brain surface
(Gryzpan and Geselowitz, 1973),
while EEG signals reflect cortical activities of gyri and sulci, simultaneous
recording of EEG and MEG may reveal information about cortical evoked activity
that might be missed by MEG alone.
The aim of the present study was twofold: (i) to obtain fundamental data regarding GEPs in order to establish an objective gustatometer; (ii) to investigate whether GEPs yield information about possible cortical activities of gyri not detected by MEG. To successfully accomplish this, we needed to measure GEPs induced by an appropriate stimulator as well as to clarify the sources of activities of recorded GEP peaks through simultaneous measurement of GEMfs.
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Subjects
Five healthy subjects (three females and two males, aged 25-30 years, mean 27 years) participated in this experiment. Each subject was required to attend six identical experimental sessions. Each session was held on a different day or with an interval of at least 2 h if on the same day. The study was conducted in accordance with the revised version of the Helsinki declaration and was approved by the National Institute of Advanced Industrial Science and Technology, Japan.
Stimulation
A 0.3 M solution of NaCl was used as the gustatory stimulus. The methods of
gustatory stimulation have previously been described in detail
(Kobayakawa et al.,
1996). Briefly, for each subject a gustatory but non-tactile
stimulator with a rapid rise-time was used to deliver taste solutions to the
tongue. The deionized water and the gustatory solution, separated by a small
amount of air, were delivered via a Teflon tube at a rate of 140 ml/min.
Solenoid valves controlled by a PC switched between deionized water, gustatory
solution and air. Stimuli were presented through a hole (7 x 2.8 mm)
situated in the wall of the tube. The subject's tongue was firmly fixed to the
hole by the small amount of negative pressure generated by the flow of liquid
and air in the tube. The triggering time points were calculated off-line,
based on signals from two optical sensors placed as close as possible to the
subject's mouth.
Each 400 ms stimulus was presented 40 times with an inter-stimulus interval of 30 s during each session. The gustatory solution and water were kept at 36°C, approximately the same temperature as that of the tongue. The subjects were asked to show a perceived intensity of saltiness between 0 (not detectable) and 5 (very strong) using their fingers a few seconds after each stimulus presentation. This evaluation was used to confirm that gustatory adaptation did not occur. After every recording session, the subjects were asked about the stimulus quality and confirmed no difference in temperature between water and gustatory solution.
Recording
EEG measurements were recorded from five sites (Fz, Cz, Pz, T3 and T4), with reference to A1 + A2 based on the international 10-20 method, using a conventional EEG recording system. GEMfs were measured using a 64 channel whole head SQUID system (CTF System Inc., Canada). Both GEPs and GEMfs were recorded simultaneously. The signals were digitized at a sampling rate of 250 Hz and filtered on-line with a 100 Hz low-pass filter and off-line with a 40 Hz low-pass filter. For EEG signals, a 0.53 Hz high-pass filter was also used. A pre-triggering control period of 400 ms was used and the total duration of the recording per stimulus presentation was 2 s. The measurements of peak and mean amplitude were referred to a pre-stimulus baseline. Any records contaminated by eye movements were rejected from the averages. More than 80% of the 40 records were available every session.
Data processing
The recorded GEP signals were averaged every session (40 stimulus
presentations), and the averaged data were further averaged over six sessions
to obtain the peak latency and amplitude of all sessions for each subject. The
grand averaged GEPs were also obtained.
Several ECDs were selected for each GEP peak latency every session using
MEG data. ECDs were searched for within 50 ms around the peak latency of an
early GEP component and within 100 ms for later components, based on the
following requirements: (i) >80% fit with estimation; (ii) more than 50 ms
stability; (iii) suitable power between 10 and 100 nA/m. The estimated ECD
locations were superimposed on the corresponding axial, coronal and sagittal
MRI images. The cortical structures where ECDs were located were referred to
an atlas of the human brain (Mai et
al., 1997).
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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After averaging the GEPs recorded over six sessions, two positive peaks (P1 and P2) were observed for all five subjects and one negative peak (N1) was noted in four out of five. Figure 1a—e shows averaged waves recorded at Cz in five subjects and Figure 1f shows a grand averaged waveform from all subjects in order to display every peak.
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The averaged latency and amplitude of P1 were 126.67 ± 31.95 ms and 5.91 ± 4.31 µV, respectively (n = 5). Although the P1 component was not clearly detected in the wave averaged from 40 stimulus presentations, it became clear after averaging over six sessions (240 stimulus presentations) for every subject. The N1 component was revealed in four of the five subjects after averaging over six sessions, with an averaged latency and amplitude of 262.68 ± 23.86 ms and -3.01 ± 3.64 µV (n = 4). The P2 component could be detected after averaging signals over 40 stimulus presentations for every subject and became more prominent after averaging over six sessions. The averaged latency and amplitude of P2 were 431.57 ± 44.19 ms and 10.65 ± 6.90 µV, respectively (n = 5). This component displayed remarkably high amplitude and was quite long lived, lasting on the order of 200 ms. All components displayed the highest amplitude at Cz among the five recording sites in all subjects. GEPs from the left and right hemispheres were recorded only at T3 and T4. No differences in P1 latency (T3, 104.8 ± 8.67 ms; T4, 114.4 ± 4.56 ms; n = 5) or amplitude (T3, 0.69 ± 0.98 µV; T4, 0.41 ± 1.66 µV; n = 5) were observed between the two recording sites (P > 0.05, df = 4, Student's t-test n.s.).
To confirm that the aforementioned GEPs were uniquely evoked by NaCl, we examined a sequence of the potentials evoked by water and compared it with the GEPs induced by NaCl (Figure 2). No significant change in potentials was noticed with water stimulation following an air bubble.
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The ECDs around each GEP peak latency were examined (Figure 3Aa,Ab). ECD1 was successfully estimated in all 30 sessions (5 subjects x 6 sessions) near the P1 latency, and no activation was found before P1 and ECD1. The averaged latency of ECD1 was 125.87 ± 32.45 ms in both hemispheres (n = 5), very similar to the P1 latency (126.67 ± 31.95 ms, n = 5) (P > 0.05, df = 4, Student's t-test n.s.). ECD2 could be estimated in 20 sessions in the left hemisphere and in 19 sessions in the right hemisphere, with averaged latencies of 276.68 ± 45.85 (n = 4) and 277.1 ± 47.75 ms (n = 4), respectively. ECD3 could be estimated in 27 sessions in the left hemisphere and in 29 sessions in the right hemisphere, with averaged latencies of 439.36 ± 44.22 (n = 5) and 436.93 ± 44.22 ms (n = 5), respectively. The direction of the ECDs in area G, the region in which all of the ECDs were most commonly located, was in most cases upward (90% of sessions for ECD1, 67.46% for ECD2 and 95.84% for ECD3).
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Once the ECDs were pinpointed, they were then overlaid on coronal anatomical MR images, as in Figure 3B. The cortical activated regions associated with ECD1, ECD2 and ECD3 from all 30 sessions are shown in Table 1. ECD1 was successfully extrapolated (100% of 30 sessions in both hemispheres). It was localized to area G most frequently in all extrapolated areas (53.33% in the left hemisphere, 66.67% in the right hemisphere), followed by the inferior part of the pre-central sulcus and the central sulcus (30% in the left hemisphere, 16.67% in the right hemisphere) (Table 1).
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Compared with ECD1, the anatomical position corresponding to ECD2 was less successfully extrapolated (66.67% of 30 sessions in the left hemisphere, 63.33% in the right hemisphere) and tended to show more variation, being localized to the superior temporal sulcus more frequently (15.38% of 39 sessions in both hemispheres) in comparison with ECD1 (1.67% of 60 sessions in both hemispheres). ECD2 was most frequently localized to area G (35% of 20 sessions in the left hemisphere, 47.37% of 19 sessions in the right hemisphere) (Table 1).
The success rate for pinpointing the anatomical location of ECD3 was higher than for ECD2 (90% of 30 sessions in the left hemisphere, 96.67% in the right hemisphere). The percentage localization of ECD3 to area G (44.44% of 27 sessions in the left hemisphere, 41.38% of 29 sessions in the right hemisphere) was the highest among localized areas, but smaller than that for other ECDs. ECD3 was more frequently (21.43% of 56 sessions in both hemispheres) localized to the inferior insula and the superior temporal sulcus than ECD1 (6.67% of 60 sessions in both hemispheres) (Table 1).
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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In this study gustatory evoked activity in the cerebral cortex was examined by simultaneous EEG and MEG recording. Using a stimulator designed by Kobayakawa to meet Evan's original requirements (Kobayakawa et al., 1996
), we succeeded in identifying three GEP components. We also
clarified the source of each component based on the relevant ECDs estimated
from the GEMfs.
Although tactile stimulation of the tongue evokes potentials with a latency
similar to the P1 at the secondary somatosensory response
(Fitzsimons et al.,
1999), a small bubble of air applied at the stimulus point did not
elicit any potential change at the scalp. This indicates that the recorded
GEPs represent exclusively gustatory, and not tactile, responses to the
stimulus, confirming the specificity of the stimulator. The contribution of
tactile interference to the formation of GEMfs has also previously been denied
(Kobayakawa et al.,
1996).
GEP components
For all but one of the five subjects the P1, N1 and P2 components of the
gustatory evoked activity were noticeable after averaging the potentials (240
stimulus presentations in six sessions). The averaged latencies of the P1, N1
and P2 components were 126.67 ± 31.95 (n = 5), 262.68 ±
23.86 (n = 4) and 431.57 ± 44.19 ms (n = 5),
respectively, shorter than previously reported
(Funakoshi and Kawamura, 1971;
Plattig et al., 1989;
Prescott, 1989). The
difference can be ascribed to a slower rise time of the gustatory stimulation
used by the previous investigators. Kobal
(Kobal, 1985), using gaseous
stimuli, reported that the same three components, P1 (180 ms), N1 (260 ms) and
P2 (400 ms), had peaks comparable to those of the present study. In every
subject the amplitude of the potentials evoked was largest at Cz for all
components at the five recording sites, in agreement with Kobal
(Kobal, 1985). None of the
components of the GEPs differed in amplitude and latency between T3 and T4,
suggesting no prominent laterality of gustatory projection from the tip of the
tongue to the cortex.
Averaging the 40 records from any single session was not sufficient to
reveal the P1 component, which suggests that a larger number of stimulus
presentations is most likely required to raise the signal to noise ratio to
the point where the P1 component may be detected. The previous studies also
suffered from the same difficulty. Since a long interstimulus interval is
needed to prevent adaptation, the maximum possible number of stimulus
presentations per session is extremely limited in comparison with other
sensory modalities, e.g. audition and vision. Retaining a sufficiently long
experimental time for a session (20 min), we conducted six sessions at
one or two sessions per day and obtained a total of approximately 240 stimulus
presentations. With this amount of data, we were able to isolate the P1
component clearly for every subject. Thus, >200 stimulus presentations are
required to clearly obtain the earliest GEP component. Since this repetitive
stimulation method did not place subjects under restraint for >20 min per
session, it proved to be quite tolerable, an important clinical consideration.
Forty stimulus presentations were sufficient, on the other hand, to obtain a
clear P2 component. Hardest to detect was the N1 component. Even using the
data averaged over six sessions, it could only be detected in four of the five
subjects.
Comparison of GEPs and GEMfs
ECD1 was most commonly found to originate from area G, followed by the feet
of the pre- and post-central sulci and the central sulcus. This was consistent
with previously reported data (Faurion
et al., 1998;
Kobayakawa et al.,
1999). The primary responses of both GEPs and GEMfs were observed
to have similar latencies (P1, 126.67 ± 31.95 ms; ECD1, 125.87 ±
32.45 ms; n = 5) and no remarkable potential change was detected
before P1. ECD1 was located near the center of the anterior-posterior line in
both hemispheres and was directed mainly upward on either side of the
hemisphere. EEG potential changes due to ECD1 on both sides may be summated to
yield the maximum amplitude at Cz and the polarity of this maximum amplitude
was most often positive, consistent with the GEP data. This indicates that
both P1 and ECD1 are generated from the same source and that P1 reflects the
primary gustatory response.
Based on ECD2 and ECD3, the current generators for N1 and P2 were localized
to area G or the central sulcus in about a half of the sessions, less
frequently than P1. Typically, they were found to be located in a more diverse
group of regions than ECD1. Since several regions, including area G, were
sometimes activated simultaneously during N1 and P2, it is not clear how the
polarity of the N1 and P2 components were determined. Interestingly, ECD2 and
ECD3, unlike ECD1, were significantly more likely to be localized to the
right, rather than the left, superior temporal sulcus. Kettenmann et
al. (Kettenmann et al.,
1997) also noted late activation of the superior temporal sulcus
after olfactory stimulation and suggested that this region was one of the main
cortical areas involved in olfactory cognition. Activation of the inferior
insula was more commonly found to be associated with ECD3 than ECD1 or ECD2,
consistent with the reports of fMRI studies that this region was likely to be
involved in gustatory cognitive processing
(Faurion et al., 1999;
Cerf et al., 2001).
Thus, the N1 and P2 components, corresponding to ECD2 and ECD3, probably
reflect higher gustatory functions. In terms of clinical screening, detection
of the P1 component most likely confirms the intact projection of gustatory
information to the primary cortex, whereas the P2 component is possibly a good
marker for higher levels of gustatory functions.
Kobayakawa et al. (Kobayakawa
et al., 1996) placed the location of the human gustatory
area at the transition between the parietal operculum and the insula cortex,
which corresponds to area G in subhuman primates
(Ogawa et al., 1985).
However, MEG measurement is not capable of recording cortical activation of
gyri where the electric current generated is oriented vertically to the brain
surface (Gryzpan and Geselowitz,
1973). Thus, measurement of cortical activities by MEG only still
left the possibility that certain cortical gyri might have been activated by
gustatory stimulation with a shorter latency than area G. In contrast, EEG
signals accurately reflect the cortical activities of gyri and sulci. By
simultaneous recording of EEG and MEG, we obtained data for P1 and ECD1 which
demonstrated consistent latency and electromagnetic characters. Although we do
not deny that some cortical gyri are activated by gustatory stimulation with
the same latency as area G or the central sulcus, no significant changes in
the GEPs were found in the time period preceding the P1 component and ECD1.
Our results indicate that gustatory information first arrives in the cortex at
area G and the central sulcus, as detected by MEG, but probably not at the
crown of any cortical gyri, to produce the P1 component of the GEPs. It is not
clear whether any cortical gyri are activated during the surveyed time period
after the stimulation (say 800 ms), since the later components (N1 and P2)
probably result from the ECDs, as calculated from the GEMfs.
In the present study we used 0.3 M NaCl as a stimulus. Future work will be needed to examine whether the amplitudes and latencies of the GEP peaks are dependent on the concentration or the quality of the stimulus. Information from such studies, now in progress, are likely to lead to improvements in technique as well as to provide new information as to the nature of GEPs.
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Acknowledgments |
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This research was supported by the Industrial Technology Research Grant Program for NEDO (New Energy and Industrial Technology Development Organization). We would like to thank Ms Aiko Kitajima, who provided technical assistance.
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Accepted May 30, 2002
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