Synopsis
In this work we explored how experience modulates
ROI-based fMRI functional connectivity patterns in two different meditators
groups: experts and novices. We recorded fMRI data during two styles of
meditation (focused attention (Samatha), and open monitoring (Vipassana)), in
two groups of subjects (Buddist Theravada Monks, and novices), and we
calculated the connectivity pattern between ROIs from the AAL90 atlas. We then
used a pattern classification approach to discriminate these
groups and find which connections and nodes are important to classify subject
experience. Regions having a role in decoding were those implicated in
self-awareness and attention control.Introduction
Meditation can be conceptualized as a
family of complex emotional and attentional regulatory practices, that can be
classified into two main styles: focused attention (FA) and open monitoring
(OM)
1,2. While FA meditation entails the capacities of monitoring
the focus of attention and detecting distraction
1,2, OM encompasses
an attentive set that is characterized by an open presence and a non judgemental
awareness of sensory, cognitive and affective fields of experience in the
present moment
1,2.
Here we want to describe how experience
modulates ROI-based fMRI functional connectivity patterns in two different
meditators groups: experts and novices. We used a pattern classification
approach
3,4 to discriminate these groups and find which connections
and nodes are important to classify subject experience.
Materials and Methods
Participants included 12 Theravada
Buddhist monks (males, mean age 37.9 years, SD 9.4 years) and 8 novice
meditators (males, mean age 32 years, SD 3 years).The novice participants were
given oral and written instructions on how to perform Samatha (FA) and
Vipassana (OM) meditation styles
5.
The FA–OM experimental paradigm
consisted of 6 min FA and 6 min OM meditation blocks, each preceded and
followed by a 3 min non-meditative resting state block, for three times;
conditions were switched by an auditory signal.
Data acquistion was performed on a
Siemens Magnetom Vision Scanner at 1.5 T, BOLD signal images were obtained
using T2*-weighted echo
planar (EPI) (TR = 4.087 s, voxel
size = 4x4x4 mm3) for a total of 860 images each containing 64x64x28
voxels.
Preprocessing consisted in a motion and time correction, temporal
filtering (high-pass filter of two cycles per time course) and linear and
non-linear trend removal; the first five scans of each run were discarded due
to T1 saturation effect.
Connectivity preprocessing consisted
in white matter, CSF regression, and temporal filtering (band-pass filter 0.08
– 0.009 Hz), then timecourses were extracted, for each meditation session, from
ROIs defined by AAL90 atlas
6 and the correlation matrix between ROIs
computed.
The value of the connections were
used as feature for the classifier while subjects were the samples, for each
subject we computed 3 matrices per meditation style. Subject number within
groups (i.e. Monks and Novices) were different, resulting in an unbalanced
dataset; we therefore decided to analyze the dataset using two different
approaches
7: down-sampling, and classifier weighting.
We divided the dataset in 4 folds
for each group and we down-sampled the larger class within each fold; this
procedure was repeated 100 times, each dataset was analyzed using a linear SVM,
with an ANOVA Univariate Feature Selection that picked 1% of most significant
features in a k-fold cross validation procedure with k = 2; permutation tests
(n=2000) were used to assess statistical significance.
Results
Decoding results were above chance for both
Vipassana (average accuracy across balanced datasets: 60.70%) and Samatha
(63.60%). We tested with a one sample t-test against chance level if the
accuracy distribution of each of 100 balanced datasets was significative (OM
p<0.001; FA p<0.001), moreover we tested the statistical significance of
the average accuracies for each style using permutation test
8 finding
significant values (OM p<0.05; FA p<0.05). We also found above chance
classification for accuracy obtained with classifier weighting (OM 61.67%
p<0.05 through permutation test; FA 66.67% p<0.01). Connections that
drive the classifier were found summing the number of times a connection is
chosen for each balanced dataset, then we selected only the connections chosen
with a value higher than mu+2std and we use the classifier weights as magnitude
of connection importance.
In Figure 1 and 2 are plotted the feature
choice frequency and the feature weights, our results suggests that Amygdala,
Putamen and Insula nodes play an important role to discriminate experts and
novice medidators during FA style. Several studies
9,10,11 have
shown that these regions are involved during attention control. Superior
Frontal Gyrus is an important node to discriminate these categories during OM
meditation, this node has been demonstrated to be highly involved in
self-awareness
12.
Conclusion
We showed how pattern classification and
functional connectivity could be used to detect fine-grained differences in
fMRI connectome of different meditators categories. Regions having a role in
decoding are those implicated in self-awareness and attention control.
Acknowledgements
We thank the monks of the Santacittarama monastery for their kind participation in the study.References
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