Kruskal-Wallis Test on the categorical variable group (N, PM, PMA) and the dependent variables of electrical threshold stimulus acquisition, mean intensity (CO2, electrical stimulus, puff), mean hedonics (CO2, electrical stimulus, puff), and mean performance (CO2, electrical stimulus, puff) was performed. Results revealed a statistically significant difference for the following variables: mean intensity rating of the CO2 stimulus (χ2(2, n = 51) = 11.4, p = 0.003; post-hoc PM > PMA & PM > N), mean hedonic rating of the CO2 stimulus (χ2(2, n = 51) = 12.8, p = 0.002, post-hoc PM < PMA), mean tracking task performance index during EEG acquisition following the CO2 stimulation (χ2(2, n = 51) = 9.11, p = 0.011 (PMA better performer)), mean tracking task performance index during EEG acquisition following the electrical stimulation (χ2(2, n = 48) = 7.52, p = 0.02 (PMA better performer)) and mean tracking task performance index during EEG acquisition following the mechanical stimulation (χ2(2, n = 47) = 7.40 p = 0.025 (PMA better performer)). No statistically significant differences were detected for mean electrical thresholds or intensity, and pleasantness of the stimulation applied.
Microstate segmentation and Source analysis
Mechanosensory condition
The segmentation, obtained from the 128-channel grand averages of the three groups, identified a total of 18 microstates, explaining 81.9% of the global variance (Fig. 2(iii) and (iv)). Identified stable topographies (microstates) were labelled with numbers from 1 to 18. For the control group (N) ten micro-states (namely 1,4,5,6,9,12,10,13,15,16) represented the processing of the mechanosensory stimulus after the onset. For the PM group the epoch was segmented in eight micro-states, namely 3,5,6,9,12,8,14,17 and for the PMA group in nine micro-states, namely 2,9,11,5,6,7,10,12,18. Microstates 5,6 and 9 are common to the three groups within the same time frame of appearance (tf1: from 78ms to 412ms and tf2: from c.a. 572ms to 792ms) and same order (respectively in tf1 5, 6, and 9 and in tf2 9 ). Groups N and PM share map 12 from c.a. 417ms to 472ms (tf3), while N and PMA share map 10 from ca. 466ms to 607ms (tf4). Finally, PM and PMA share map 12 from c.a.732ms to 880ms (tf5). Interestingly, the initial time range of the epoch, from 0ms to 123ms, and the final part of the epoch, from 466ms till the end, are represented by different microstates across groups suggesting distinct stimulus processing in the three groups. Comparing to the original data, microstate analysis clearly reflects it (Fig. 2 (I)) with the identified microstates (Fig. 2 (iii), and (iv)). Topographies in Fig. 2(i) similar to the microstates in Fig. 2(iii) and (iv) were highlighted with the same color frame.
Figure 2 approx. here
Fitting back to the single subject EEG-grand average. The fitted back procedure was based on the microstates included in the epoch segment that contained shared microstates. This corresponds to our segmentation of the mechanosensory condition to a time frame from 76- 1200ms and includes microstates 5, 6,7,8, 9, 10, 12, and 13. For them, we extracted the variables ‘mean segment duration’ (Duration) and ‘global explained variance’ (GEV) for each subject. Both are indicative of how much the corresponding microstate is present in the relative single subject epoch segment. In addition, we also extracted the ‘first onset’ (Fonset) of appearance as an effect of the disease condition (map 5, 6, 9 ) and the maximum of the global field power (max GFP) in map 9. Based on the segmentation on the GM, we have defined time frames (tf) of interest where to extract the variable at the single-subject level, these were tf1, from 76 to 428ms, for Duration and GEV, and Fonset of map 5, 6, and 9. In addition, in this time frame, we also extracted the maximal Global Field Power (max GFP) that reflects the strength of the signal for the specific condition. In tf2, from 416 to 609ms, we extracted Duration and GEV of maps 6, 7,8, 10, and 12. In tf3, from 566 to 793ms Duration, GEV, first onset, and max GFP on the second appearance of map 9 and in tf4, from 729 to 1200ms Duration, GEV of the second appearance of map 12, and the first appearance of map 13 were extracted.
A non-parametric Kruskal-Wallis one-way analysis of variance (one variable x 3 groups) was applied on the variable extracted from the fit back on the single subject above specified. The detailed statistical results are reported in Table 3. Mean values and SE of the variables are illustrated in Fig. 3 along with the time frame (tf) in the analysis.
Figure 3 approx. here
Results showed that map 9 in tf1 had a significant different mean duration, GEV, and max GFP across the groups. Post hoc tests revealed that Duration, GEV, and max GFP were significantly bigger for the PMA condition with respect to N and PM.
Map 7 within tf2 had a significantly longer duration and bigger GEV in the PMA group than N and PM groups.
Finally, map 12 was significantly longer and explained more variance for both group PM and PMA with respect to N, and map 13 had a longer duration and a bigger GEV for N with respect to PM and PMA.
These results suggested that a longer duration of map 9 and map 7 and higher signal strength for map 9 are specifically observed in PMA. Map 12 is a unique topography associated with the migraine condition in both aura and non-aura symptoms. Finally, map 13 represents an exclusive topography characteristic of the normal condition.
Source analysis for the mechanosensory condition. The source analysis of map 9 revealed maximal activity in the right cerebellum, lower activity in the left cerebellum, and the left temporal inferior and superior pole.
The source analysis of map 7 pointed to a maximum activity in the left and right Cerebellum and the left and right gyrus rectus.
Map 12 is generated by a source in the right cerebellum and the left inferior frontal triangular area. Finally, map 13 is generated by a source in the right postcentral gyrus and left temporal pole. Brain activities associated with the discussed maps are illustrated in Fig. 4.
Figure 4 approx. here
Chemosensory condition
The segmentation on the chemosensory condition Grand Means for the three groups identified a total of 18 stable topographies, explaining 80.3% of the global variance (Fig. 5(iii) and (iv)). The micro-states, originally named with numbers in crescent order, were renamed with alphabetic letters from a to t to avoid confusion with the maps identified in the mechanosensory segmentation. In detail, the chemosensory stimulus processing was described by eleven micro-states for the control (N), namely a, b, d, e, f, h, i, m, o, s, and t; six microstates for the PM group, namely c, f, g, p, q, and t and by height microstates for the PMA group, namely a, f, c, h, l, n, r, and t. The three groups shared map f from c.a.78ms to 287ms, although with notable different duration and onset, and map t from c.a. 636ms till the end of the epoch. N and PMA groups shared microstate a from the beginning of the epoch to c.a. 46ms and microstates h from c.a. 236ms to 441ms. Finally, PM and PMA groups shared map c that appears in different time frames, at the beginning of the epoch for PM and around 107ms for PMA. The micro-states analysis clearly reflects the original data, as is visible from Fig. 5(iii), (iv), and (i), where we also highlight the similar topographies with the same color code.
Figure 5 approx. here
Fitting back to the single subject EEG-grand average. To test the consistency of the group analysis at the single-subject level, we fitted back to the single-subject the shared micro-states among the groups, namely: a, c, f, h, and t, highlighted in Fig. 5(iii). For each micro-state, we extracted the following variables: Duration, GEV, and the first onset within a specific time frame (TF) of interest defined as TF0, within 0 to 223ms for map a and c, TF1 within 80 to 359ms, and TF2 361 to 559ms for map f, TF3, within 639 to 1199ms for map t and TF4 between 119 and 225ms, for map h. We additionally extracted the first onset of map f on TF1 and the first onset of map c on TF0.
A non-parametric Kruskal-Wallis one-way analysis of variance among the three groups was used to test statistical differences on the defined variables. Details of the statistical results are reported in Table 4; mean and SE across the groups are reported in Fig. 6 for each variable. The results showed a significant difference in the mean duration of map c within the TF0. The post hoc test showed that map c was shorter with a smaller GEV for the N-group, confirming the segmentation. Also, map f within the TF2 was shorter with a smaller GEV for the N-group. Figure 6 illustrates the mean values and SE of the tested variables. This suggests that a longer map c in the initial part of the epoch and a longer map f in TF2 indicates a migraine condition.
Table 4
Statistical test results for the variables extracted after the fitting back of the segmentation to the single subject grand average for the chemosensory condition. p, p-value of the statistical test, H, the value of the Kruskal-Wallis test, **/* indicates p < 0.025/0.05. § indicates a value that supposes a trend of significance.
Map – TF
|
mean duration
|
GEV
|
first onset
|
p (K-W)
|
a - TF0
|
0.8803 (0.255)
|
0.8405 (0.348)
|
-
|
c - TF0
|
0.0298 (7.026)*
|
0.0471 (6.109)*
|
0.1991 (3.228)
|
h - TF4
|
0.4700 (1.510)
|
0.6571 (1.134)
|
-
|
f - TF1
|
0.4675 (1.521)
|
0.4675 (1.521)
|
0.8945 (0.223)
|
f - TF2
|
0.0490 (6.032)*
|
0.0516 (5.927)*
|
-
|
t - TF3
|
0.0655 (5.452)
|
0.2652 (2.655)
|
-
|
Figure 6 approx. here
Source analysis for the chemosensory condition: The source analysis of map c revealed activity in the right precuneus, left temporal Mid area, and the frontal superior gyrus. The source analysis of map f indicated activity in the temporal pole mid-left, the cerebellum Crus 1 left, and the precentral gyrus right. Brain areas involved in the maps discussed above are illustrated in Fig. 7.
Figure 7 approx. here
Table 5 summarizes the maps of interest, the correspondent maximum of brain activities in MNI coordinates, and the relative brain areas nomenclature.
Table 5
Source localization of the relevant brain topographies for migraine condition, migraine with aura and control.
|
|
MNI (mm)
|
Brain area
|
|
Map
|
x
|
y
|
z
|
Brain lobe
|
Brain sub-area
|
Brodnmann area
|
general migraine
|
c
|
15.11
|
-57.41
|
39.28
|
Parietal lobe
|
Precuneous R
|
7
|
-63.45
|
-3.02
|
-15.11
|
Temporal lobe
|
Superior temporal gyrus L
|
22/21
|
15.11
|
81.58
|
-3.02
|
Frontal lobe
|
Frontal pole R
|
10
|
f
|
-51.36
|
9.06
|
-33.23
|
Temporal lobe
|
Temporal pole L
|
21/20
|
-21.15
|
-63.45
|
-33.23
|
Cerebellum
|
Posterio lobe
|
Crus 1
|
51.36
|
-9.06
|
45.32
|
Frontal lobe
|
Precentral gyrus R
|
4
|
12
|
9.06
|
-33.23
|
-57.41
|
Cerebellum
|
Posterio lobe R
|
9
|
-51.36
|
27.19
|
15.11
|
Frontal lobe
|
Inferior frontal gyrus opercular part.
|
45/46
|
migraine with aura
|
7
|
-33.23
|
-45.32
|
-57.41
|
Cerebellum
|
Posterior lobe L
|
8
|
45.32
|
-45.32
|
-45.32
|
Cerebellum
|
Posterior lobe R
|
Crus 2
|
3.02
|
63.45
|
-21.15
|
Frontal lobe
|
Fusiform gyrus R
|
11
|
-9.06
|
69.49
|
-21.15
|
Frontal lobe
|
Lingual gyrus L
|
11
|
9
|
15
|
-63
|
-39
|
Cerebellum
|
Posterior lobe L
|
8
|
-15
|
-63
|
-39
|
Cerebellum
|
Posterior lobe L
|
8
|
-51.36
|
15.11
|
-39.28
|
Temporal lobe
|
Inferior temporal gyrus L
|
38/20
|
Control
|
13
|
57.41
|
-9.06
|
39.28
|
Parietal lobe
|
Postcentral gyrus R
|
3
|
-45.32
|
21.15
|
-39.28
|
Temporal lobe
|
Temporal Pole L
|
20
|
Correlation analysis with clinical data
General migraine condition.
Our results showed a relationship between the general migraine disorder (with and without aura) and map c and f within the chemosensory condition and map 12 within the mechanosensory condition. To test the relationship between the eCSD signal of the corresponding elicited brain activities and the clinical data available for the PM and PMA groups, namely BDI, MIDAS, HIT-6, ‘Years with migraine’ and ‘migraine days last 3 months’ we performed a Spearman rho correlation analysis. There was a strong positive correlation between eCSD in the right precuneus, left temporal pole, and right cerebellum and the variable ‘Years with migraine’, with respective values r = 0.61, p = 0.039; r = 0.60, p = 0.043; and r = 0.62, p = 0.34, indicating a high level of eCSD associated with longer years of persistent migraine disease.
Further, medium to low correlation have been found for patient reported measures as HIT-6 (left temporal pole: r=-0.43(p = 0.02); right cerebellum: r=-0.35(p = 0.05)). Since other patient reported measures as Midas did not show any correlation, we view these results with caution, also due to subjectivity and inter-individual variability in a rather small group of patients.
Details of the correlations are reported in Fig. 8 and Table 6.
Table 6
Spearman rho Correlation analysis and significance between the eCSD for the max brain activity in map c (precuneus), f (temporal pole) and 12 (cerebellum) and the clinical scores BDI, MIDAS, HIT-6, Years with migraine, Migraine days last three months.
|
Spearman r
|
BDI
|
MIDAS
|
HIT-6
|
Years with migraine
|
Migraine days last 3 months
|
Precuneus
|
r
|
-0.03
|
-0.15
|
-0.17
|
0.61
|
-0.03
|
P
|
0.88
|
0.44
|
0.38
|
0.04 *
|
0.91
|
Temporal Pole
|
r
|
-0.16
|
0.11
|
-0.43
|
0.60
|
-0.18
|
P
|
0.41
|
0.55
|
0.02 *
|
0.04 *
|
0.57
|
Cerebellum
|
r
|
-0.22
|
0.01
|
-0.36
|
0.62
|
-0.21
|
P
|
0.26
|
0.94
|
0.05
|
0.03 *
|
0.51
|
Figure 8 approx. here
Migraine condition with aura.
Segmentation analysis and the fitting back to the single subjects of the variable defining the microstates have indicated map 7 and the strength and duration of map 9 as specific topographies for the migraine condition with aura. To confirm these results, we have computed the results of the inverse solution and the related eCSD in the sources for each group. A Kruskal-Wallis test revealed a statistically significant difference in eCSD signal extracted from the ROI with maximum brain activity sources of map 9 (right cerebellar cortex) across the three different groups, N, PM, and PMA, H = 8.496, p = 0.0143. The PMA group recorded a higher mean eCSD of 1.163 than the other two groups N and PM, which recorded a mean eCSD of 0.9348 and 0.9215. The post-hoc test confirmed the significant differences with an alpha level < 0.05 and multiple comparison correction. This confirms an increase in the strength of the eCSD signal in the right cerebellar cortex during the tf1 for the PMA group.
Data are illustrated in the box plot of Fig. 9.
Figure 9 approx. here